INTERNET-DRAFT Tom Yu draft-ietf-krb-wg-rfc1510ter-00.txt MIT Expires: 25 July 2005 21 January 2005 The Kerberos Network Authentication Service (Version 5) Status of This Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract This document describes version 5 of the Kerberos network authentication protocol. It describes a framework to allow for extensions to be made to the protocol without creating interoperability problems. Key Words for Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119. Yu Expires: Jul 2005 [Page 1] Internet-Draft rfc1510ter-00 21 Jan 2005 Table of Contents Status of This Memo .............................................. 1 Copyright Notice ................................................. 1 Abstract ......................................................... 1 Key Words for Requirements ....................................... 1 Table of Contents ................................................ 2 1. Introduction ................................................. 5 1.1. Kerberos Protocol Overview ................................. 5 1.2. Document Organization ...................................... 6 2. Compatibility Considerations ................................. 6 2.1. Extensibility .............................................. 7 2.2. Compatibility with RFC 1510 ................................ 7 2.3. Backwards Compatibility .................................... 7 2.4. 1.4.2. Sending Extensible Messages ......................... 8 2.5. Criticality ................................................ 8 2.6. Authenticating Cleartext Portions of Messages .............. 9 2.7. Capability Negotiation ..................................... 10 3. Use of ASN.1 in Kerberos ..................................... 10 3.1. Module Header .............................................. 11 3.2. Top-Level Type ............................................. 11 3.3. Previously Unused ASN.1 Notation (informative) ............. 12 3.3.1. Parameterized Types ...................................... 12 3.3.2. COMPONENTS OF Notation ................................... 12 3.3.3. Constraints .............................................. 12 3.4. New Types .................................................. 13 4. Basic Types .................................................. 14 4.1. Constrained Integer Types .................................. 14 4.2. KerberosTime ............................................... 15 4.3. KerberosString ............................................. 15 4.4. Language Tags .............................................. 16 4.5. KerberosFlags .............................................. 16 4.6. Typed Holes ................................................ 16 4.7. HostAddress and HostAddresses .............................. 17 4.7.1. Internet (IPv4) Addresses ................................ 17 4.7.2. Internet (IPv6) Addresses ................................ 17 4.7.3. DECnet Phase IV addresses ................................ 18 4.7.4. Netbios addresses ........................................ 18 4.7.5. Directional Addresses .................................... 18 5. Principals ................................................... 19 5.1. Name Types ................................................. 19 5.2. Principal Type Definition .................................. 19 5.3. Principal Name Reuse ....................................... 20 5.4. Realm Names ................................................ 20 5.5. Printable Representations of Principal Names ............... 21 5.6. Ticket-Granting Service Principal .......................... 21 5.6.1. Cross-Realm TGS Principals ............................... 21 6. Types Relating to Encryption ................................. 21 6.1. Assigned Numbers for Encryption ............................ 22 6.1.1. EType .................................................... 22 6.1.2. Key Usages ............................................... 22 Yu Expires: Jul 2005 [Page 2] Internet-Draft rfc1510ter-00 21 Jan 2005 6.2. Which Key to Use ........................................... 23 6.3. EncryptionKey .............................................. 24 6.4. EncryptedData .............................................. 24 6.5. Checksums .................................................. 25 6.5.1. ChecksumOf ............................................... 26 6.5.2. Signed ................................................... 27 7. Tickets ...................................................... 27 7.1. Timestamps ................................................. 28 7.2. Ticket Flags ............................................... 28 7.2.1. Flags Relating to Initial Ticket Acquisition ............. 29 7.2.2. Invalid Tickets .......................................... 29 7.2.3. OK as Delegate ........................................... 30 7.2.4. Renewable Tickets ........................................ 30 7.2.5. Postdated Tickets ........................................ 31 7.2.6. Proxiable and Proxy Tickets .............................. 32 7.2.7. Forwarded and Forwardable Tickets ........................ 33 7.3. Transited Realms ........................................... 34 7.4. Authorization Data ......................................... 34 7.4.1. AD-IF-RELEVANT ........................................... 35 7.4.2. AD-KDCIssued ............................................. 35 7.4.3. AD-AND-OR ................................................ 37 7.4.4. AD-MANDATORY-FOR-KDC ..................................... 37 7.5. Encrypted Part of Ticket ................................... 37 7.6. Cleartext Part of Ticket ................................... 38 8. Credential Acquisition ....................................... 40 8.1. KDC-REQ .................................................... 40 8.2. PA-DATA .................................................... 46 8.3. KDC-REQ Processing ......................................... 46 8.3.1. Handling Replays ......................................... 46 8.3.2. Request Validation ....................................... 47 8.3.2.1. AS-REQ Authentication .................................. 47 8.3.2.2. TGS-REQ Authentication ................................. 47 8.3.2.3. Principal Validation ................................... 47 8.3.2.4. Checking For Revoked or Invalid Tickets ................ 48 8.3.3. Timestamp Handling ....................................... 48 8.3.3.1. AS-REQ Timestamp Processing ............................ 49 8.3.3.2. TGS-REQ Timestamp Processing ........................... 49 8.3.4. Handling Transited Realms ................................ 50 8.3.5. Address Processing ....................................... 50 8.3.6. Ticket Flag Processing ................................... 51 8.3.7. Key Selection ............................................ 52 8.3.7.1. Reply Key and Session Key Selection .................... 52 8.3.7.2. Ticket Key Selection ................................... 52 8.4. KDC-REP .................................................... 52 8.5. Reply Validation ........................................... 55 8.6. IP Transports .............................................. 55 8.6.1. UDP/IP transport ......................................... 55 8.6.2. TCP/IP transport ......................................... 56 8.6.3. KDC Discovery on IP Networks ............................. 57 8.6.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names ..... 57 8.6.3.2. DNS SRV records for KDC location ....................... 58 Yu Expires: Jul 2005 [Page 3] Internet-Draft rfc1510ter-00 21 Jan 2005 8.6.3.3. KDC Discovery for Domain Style Realm Names on IP Networks ............................................ 58 9. Errors ....................................................... 58 10. Session Key Exchange ........................................ 61 10.1. AP-REQ .................................................... 61 10.2. AP-REP .................................................... 63 11. Session Key Use ............................................. 65 11.1. KRB-SAFE .................................................. 65 11.2. KRB-PRIV .................................................. 65 11.3. KRB-CRED .................................................. 66 12. Security Considerations ..................................... 67 12.1. Time Synchronization ...................................... 67 12.2. Replays ................................................... 67 12.3. Principal Name Reuse ...................................... 67 12.4. Password Guessing ......................................... 67 12.5. Forward Secrecy ........................................... 67 12.6. Authorization ............................................. 68 12.7. Login Authentication ...................................... 68 13. IANA Considerations ......................................... 68 14. Acknowledgments ............................................. 69 Appendices ....................................................... 69 A. ASN.1 Module (Normative) ..................................... 69 B. Kerberos and Character Encodings (Informative) ...............103 C. Kerberos History (Informative) ...............................104 D. Notational Differences from [KCLAR] ..........................105 Normative References .............................................106 Informative References ...........................................106 Author's Address .................................................108 Full Copyright Statement .........................................108 Yu Expires: Jul 2005 [Page 4] Internet-Draft rfc1510ter-00 21 Jan 2005 1. Introduction The Kerberos network authentication protocol is a trusted-third-party protocol utilizing symmetric-key cryptography. It assumes that all communications between parties in the protocol may be arbitrarily tampered with or monitored, and that the security of the overall system depends only on the effectiveness of the cryptographic techniques and the secrecy of the cryptographic keys used. The Kerberos protocol authenticates an application client's identity to an application server, and likewise authenticates the application server's identity to the application client. These assurances are made possible by the client and the server sharing secrets with the trusted third party: the Kerberos server, also known as the Key Distribution Center (KDC). In addition, the protocol establishes an ephemeral shared secret (the session key) between the client and the server, allowing the protection of further communications between them. The Kerberos protocol, as originally specified, provides insufficient means for extending the protocol in a backwards-compatible way. This deficiency has caused problems for interoperability. This document describes a framework which enables backwards-compatible extensions to the Kerberos protocol. 1.1. Kerberos Protocol Overview Kerberos comprises three main sub-protocols: credentials acquisition, session key exchange, and session key usage. A client acquires credentials by asking the KDC for a credential for a service; the KDC issues the credential, which contains a ticket and a session key. The ticket, containing the client's identity, timestamps, expiration time, and a session key, is a encrypted in a key known to the application server. The KDC encrypts the credential using a key known to the client, and transmits the credential to the client. There are two means of requesting credentials: the Authentication Service (AS) exchange, and the Ticket-Granting Service (TGS) exchange. In the typical AS exchange, a client uses a password- derived key to decrypt the response. In the TGS exchange, the KDC behaves as an application server; the client authenticates to the TGS by using a Ticket-Granting Ticket (TGT). The client usually obtains the TGT by using the AS exchange. Session key exchange consists of the client transmitting the ticket to the application server, accompanied by an authenticator. The authenticator contains a timestamp and additional data encrypted using the ticket's session key. The application server decrypts the ticket, extracting the session key. The application server then uses the session key to decrypt the authenticator. Upon successful decryption of the authenticator, the application server knows that the data in the authenticator were sent by the client named in the Yu Expires: Jul 2005 [Page 5] Internet-Draft rfc1510ter-00 21 Jan 2005 associated ticket. Additionally, since authenticators expire more quickly than tickets, the application server has some assurance that the transaction is not a replay. The application server may send an encrypted acknowledgment to the client, verifying its identity to the client. Once session key exchange has occurred, the client and server may use the established session key to protect further traffic. This protection may consist of protection of integrity only, or of protection of confidentiality and integrity. Additional measures exist for a client to securely forward credentials to a server. The entire scheme depends on loosely synchronized clocks. Synchronization of the clock on the KDC with the application server clock allows the application server to accurately determine whether a credential is expired. Likewise, synchronization of the clock on the client with the application server clock prevents replay attacks utilizing the same credential. Careful design of the application protocol may allow replay prevention without requiring client-server clock synchronization. After establishing a session key, application client and the application server can exchange Kerberos protocol messages that use the session key to protect the integrity or confidentiality of communications between the client and the server. Additionally, the client may forward credentials to the application server. The credentials acquisition protocol takes place over specific, defined transports (UDP and TCP). Application protocols define which transport to use for the session key establishment protocol and for messages using the session key; the application may choose to perform its own encapsulation of the Kerberos messages, for example. 1.2. Document Organization The remainder of this document begins by describing the general frameworks for protocol extensibility, including whether to interpret unknown extensions as critical. It then defines the protocol messages and exchanges. The definition of the Kerberos protocol uses Abstract Syntax Notation One (ASN.1) [X680], which specifies notation for describing the abstract content of protocol messages. This document defines a number of base types using ASN.1; these base types subsequently appear in multiple types which define actual protocol messages. Definitions of principal names and of tickets, which are central to the protocol, also appear preceding the protocol message definitions. Yu Expires: Jul 2005 [Page 6] Internet-Draft rfc1510ter-00 21 Jan 2005 2. Compatibility Considerations 2.1. Extensibility In the past, significant interoperability problems have resulted from conflicting assumptions about how the Kerberos protocol can be extended. As the deployed base of Kerberos grows, the ability to extend the Kerberos protocol becomes more important. In order to ensure that vendors and the IETF can extend the protocol while maintaining backwards compatibility, this document outlines a framework for extending Kerberos. Kerberos provides two general mechanisms for protocol extensibility. Many protocol messages, including some defined in RFC 1510, contain typed holes--sub-messages containing an octet string along with an integer that identifies how to interpret the octet string. The integer identifiers are registered centrally, but can be used both for vendor extensions and for extensions standardized in the IETF. This document adds typed holes to a number of messages which previously lacked typed holes. Many new messages defined in this document also contain ASN.1 extension markers. These markers allow future revisions of this document to add additional elements to messages, for cases where typed holes are inadequate for some reason. Because tag numbers and position in a sequence need to be coordinated in order to maintain interoperability, implementations MUST NOT include ASN.1 extensions except when those extensions are specified by IETF standards-track documents. 2.2. Compatibility with RFC 1510 Implementations of RFC 1510 did not use extensible ASN.1 types. Sending additional fields not in RFC 1510 to these implementations results in undefined behavior. Examples of this behavior are known to include discarding messages with no error indications. Where messages have been changed since RFC 1510, ASN.1 CHOICE types are used; one alternative of the CHOICE provides a message which is wire-encoding compatible with RFC 1510, and the other alternative provides the new, extensible message. Implementations sending new messages MUST ensure that the recipient supports these new messages. Along with each extensible message is a guideline for when that message MAY be used. IF that guideline is followed, then the recipient is guaranteed to understand the message. 2.3. Backwards Compatibility This document describes two sets (for the most part) of ASN.1 types. The first set of types is wire-encoding compatible with RFC 1510 and Yu Expires: Jul 2005 [Page 7] Internet-Draft rfc1510ter-00 21 Jan 2005 [KCLAR]. The second set of types is the set of types enabling extensibility. This second set may be referred to as "extensibility-enabled types". [ need to make this consistent throughout? ] A major difference between the new extensibility-enabled types and the types for RFC 1510 compatibility is that the extensibility- enabled types allow for the use of UTF-8 encodings in various character strings in the protocol. Each party in the protocol must have some knowledge of the capabilities of the other parties in the protocol. There are methods for establishing this knowledge without necessarily requiring explicit configuration. An extensibility-enabled client can detect whether a KDC supports the extensibility-enabled types by requesting an extensibility-enabled reply. If the KDC replies with an extensibility-enabled reply, the client knows that the KDC supports extensibility. If the KDC issues an extensibility-enabled ticket, the client knows that the service named in the ticket is extensibility-enabled. 2.4. 1.4.2. Sending Extensible Messages Care must be taken to make sure that old implementations can understand messages sent to them even if they do not understand an extension that is used. Unless the sender knows the extension is supported, the extension cannot change the semantics of the core message or previously defined extensions. For example, an extension including key information necessary to decrypt the encrypted part of a KDC-REP could only be used in situations where the recipient was known to support the extension. Thus when designing such extensions it is important to provide a way for the recipient to notify the sender of support for the extension. For example in the case of an extension that changes the KDC-REP reply key, the client could indicate support for the extension by including a padata element in the AS-REQ sequence. The KDC should only use the extension if this padata element is present in the AS- REQ. Even if policy requires the use of the extension, it is better to return an error indicating that the extension is required than to use the extension when the recipient may not support it; debugging why implementations do not interoperate is easier when errors are returned. 2.5. Criticality Recipients of unknown message extensions (including typed holes, new flags, and ASN.1 extension elements) should preserve the encoding of the extension but otherwise ignore the presence of the extension; i.e., unknown extensions SHOULD be treated as non-critical. If a copy of the message is used later--for example, when a Ticket received in a KDC-REP is later used in an AP-REQ--then the unknown Yu Expires: Jul 2005 [Page 8] Internet-Draft rfc1510ter-00 21 Jan 2005 extensions MUST be included. An implementation SHOULD NOT reject a request merely because it does not understand some element of the request. As a related consequence, implementations SHOULD handle communicating with other implementations which do not implement some requested options. This may require designers of options to provide means to determine whether an option has been rejected, not understood, or (perhaps maliciously) deleted or modified in transit. There is one exception to non-criticality described above: if an unknown authorization data element is received by a server either in an AP-REQ or in a Ticket contained in an AP-REQ, then the authentication SHOULD fail. Authorization data is intended to restrict the use of a ticket. If the service cannot determine whether the restriction applies to that service then a security weakness may result if authentication succeeds. Authorization elements meant to be truly optional can be enclosed in the AD-IF- RELEVANT element. Many RFC 1510 implementations ignore unknown authorization data elements. Depending on these implementations to honor authorization data restrictions may create a security weakness. 2.6. Authenticating Cleartext Portions of Messages Various denial of service attacks and downgrade attacks against Kerberos are possible unless plaintexts are somehow protected against modification. An early design goal of Kerberos Version 5 was to avoid encrypting more of the authentication exchange that was required. (Version 4 doubly-encrypted the encrypted part of a ticket in a KDC reply, for example.) This minimization of encryption reduces the load on the KDC and busy servers. Also, during the initial design of Version 5, the existence of legal restrictions on the export of cryptography made it desirable to minimize of the number of uses of encryption in the protocol. Unfortunately, performing this minimization created numerous instances of unauthenticated security-relevant plaintext fields. The extensible variants of the messages described in this document wrap the actual message in an ASN.1 sequence containing a keyed checksum of the contents of the message. Guidelines in [XXX] section 3 specify when the checksum MUST be included and what key MUST be used. Guidelines on when to include a checksum are never ambiguous: a particular PDU is never correct both with and without a checksum. With the exception of the KRB-ERROR message, receiving implementations MUST reject messages where a checksum is included and not expected or where a checksum is expected but not included. The receiving implementation does not always have sufficient information to know whether a KRB-ERROR should contain a checksum. Even so, KRB-ERROR messages with invalid checksums MUST be rejected and Yu Expires: Jul 2005 [Page 9] Internet-Draft rfc1510ter-00 21 Jan 2005 implementations MAY consider the presence or absence of a checksum when evaluating whether to trust the error. This authenticated cleartext protection is provided only in the extensible variants of the messages; it is never used when communicating with an RFC 1510 implementation. 2.7. Capability Negotiation Kerberos is a three-party protocol. Each of the three parties involved needs a means of detecting the capabilities supported by the others. Two of the parties, the KDC and the application server, do not communicate directly in the Kerberos protocol. Communicating capabilities from the KDC to the application server requires using a ticket as an intermediary. The main capability requiring negotiation is the support of the extensibility framework described in this document. Negotiation of this capability while remaining compatible with RFC 1510 implementations is possible. The main complication is that the client needs to know whether the application server supports the extensibility framework prior to sending any message to the application server. This can be accomplished if the KDC has knowledge of whether an application server supports the extensibility framework. Client software advertizes its capabilities when requesting credentials from the KDC. If the KDC recognizes the capabilities, it acknowledges this fact to the client in its reply. In addition, if the KDC has knowledge that the application server supports certain capabilities, it also communicates this knowledge to the client in its reply. The KDC can encode its own capabilities in the ticket so that the application server may discover these capabilities. The client advertizes its capabilities to the application server when it initiates authentication to the application server. 3. Use of ASN.1 in Kerberos Kerberos uses the ASN.1 Distinguished Encoding Rules (DER) [X690]. Even though ASN.1 theoretically allows the description of protocol messages to be independent of the encoding rules used to encode the messages, Kerberos messages MUST be encoded with DER. Subtleties in the semantics of the notation, such as whether tags carry any semantic content to the application, may cause the use of other ASN.1 encoding rules to be problematic. Implementors not using existing ASN.1 tools (e.g., compilers or support libraries) are cautioned to thoroughly read and understand the actual ASN.1 specification to ensure correct implementation behavior. There is more complexity in the notation than is immediately obvious, and some tutorials and guides to ASN.1 are Yu Expires: Jul 2005 [Page 10] Internet-Draft rfc1510ter-00 21 Jan 2005 misleading or erroneous. Recommended tutorials and guides include [Dub00], [Lar99], though there is still no substitute for reading the actual ASN.1 specification. 3.1. Module Header The type definitions in this document assume an ASN.1 module definition of the following form: KerberosV5Spec3 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec3(4) } DEFINITIONS EXPLICIT TAGS ::= BEGIN -- Rest of definitions here END This specifies that the tagging context for the module will be explicit and that automatic tagging is not done. Some other publications [RFC1510] [RFC1964] erroneously specify an object identifier (OID) having an incorrect value of "5" for the "dod" component of the OID. In the case of RFC 1964, use of the "correct" OID value would result in a change in the wire protocol; therefore, the RFC 1964 OID remains unchanged for now. 3.2. Top-Level Type The ASN.1 type "KRB-PDU" is a CHOICE over all the types (Protocol Data Units or PDUs) which an application may directly reference. Applications SHOULD NOT transmit any types other than those which are alternatives of the KRB-PDU CHOICE. Yu Expires: Jul 2005 [Page 11] Internet-Draft rfc1510ter-00 21 Jan 2005 -- top-level type -- -- Applications should not directly reference any types -- other than KRB-PDU and its component types. -- KRB-PDU ::= CHOICE { ticket Ticket, as-req AS-REQ, as-rep AS-REP, tgs-req TGS-REQ, tgs-rep TGS-REP, ap-req AP-REQ, ap-rep AP-REP, krb-safe KRB-SAFE, krb-priv KRB-PRIV, krb-cred KRB-CRED, tgt-req TGT-REQ, krb-error KRB-ERROR, ... } 3.3. Previously Unused ASN.1 Notation (informative) Some aspects of ASN.1 notation used in this document were not used in [KCLAR], and may be unfamiliar to some readers. This subsection is informative; for normative definitions of the notation, see the actual ASN.1 specifications [X680], [X682], [X683]. 3.3.1. Parameterized Types This document uses ASN.1 parameterized types [X683] to make definitions of types more readable. For some types, some or all of the parameters are advisory, i.e., they are not encoded in any form for transmission in a protocol message. These advisory parameters can describe implementation behavior associated with the type. 3.3.2. COMPONENTS OF Notation The "COMPONENTS OF" notation, used to define the RFC 1510 compatibility types, extracts all of the component types of an ASN.1 SEQUENCE type. The extension marker (the "..." notation) and any extension components are not extracted by "COMPONENTS OF". The "COMPONENTS OF" notation permits concise definition of multiple types which have many components in common with each other. 3.3.3. Constraints This document uses ASN.1 constraints, including the "UserDefinedConstraint" notation [X682]. Some constraints can be handled automatically by tools that can parse them. Uses of the Yu Expires: Jul 2005 [Page 12] Internet-Draft rfc1510ter-00 21 Jan 2005 "UserDefinedConstraint" notation (the "CONSTRAINED BY" notation) will typically need to have behavior manually coded; the notation provides a formalized way of conveying intended implementation behavior. The "WITH COMPONENTS" constraint notation allows constraints to apply to component types of a SEQUENCE type. This constraint notation effectively allows constraints to "reach into" a type to constrain its component types. 3.4. New Types This document defines a number of ASN.1 types which are new since [KCLAR]. The names of these types will typically have a suffix like "Ext", indicating that the types are intended to support extensibility. Types original to RFC 1510 and [KCLAR] have been renamed to have a suffix like "1510". The "Ext" and "1510" types often contain a number of common elements; these common elements have a suffix like "Common". The "1510" types have various ASN.1 constraints applied to them; the constraints limit the possible values of the "1510" types to those permitted by RFC 1510 or by [KCLAR]. The "Ext" types may have different constraints, typically to force string values to be sent as UTF-8. For example, consider -- example "common" type Foo-Common ::= SEQUENCE { a [0] INTEGER, b [1] OCTET STRING, ..., c [2] INTEGER, ... } -- example "RFC 1510 compatibility" type Foo-1510 ::= SEQUENCE { -- the COMPONENTS OF notation drops the extension marker -- and all extension additions. COMPONENTS OF Foo-Common } -- example "extensibility-enabled" type Foo-Ext ::= Foo-Common where "Foo-Common" is a common type used to define both the "1510" and "Ext" versions of a type. "Foo-1510" is the RFC 1510 version of the type, while "Foo-Ext" is the extensible version. "Foo-1510" does not contain the extension marker, nor does it contain the extension component "c". The type "Foo-1510" is equivalent to "Foo-1510- Equiv", shown below. Yu Expires: Jul 2005 [Page 13] Internet-Draft rfc1510ter-00 21 Jan 2005 -- example type equivalent to Foo-1510 Foo-1510-Equiv ::= SEQUENCE { a [0] INTEGER, b [1] OCTET STRING } 4. Basic Types These "basic" Kerberos ASN.1 types appear in multiple other Kerberos types. 4.1. Constrained Integer Types In RFC 1510, many types contained references to the unconstrained INTEGER type. Since an unconstrained INTEGER can contain almost any possible abstract integer value, most of the unconstrained references to INTEGER in RFC 1510 were constrained to 32 bits or smaller in [KCLAR]. -- signed values representable in 32 bits -- -- These are often used as assigned numbers for various things. Int32 ::= INTEGER (-2147483648..2147483647) The "Int32" type often contains an assigned number identifying the contents of a typed hole. Unless otherwise stated, non-negative values are registered, and negative values are available for local use. -- unsigned 32 bit values UInt32 ::= INTEGER (0..4294967295) The "UInt32" type is used in some places where an unsigned 32-bit integer is needed. -- unsigned 64 bit values UInt64 ::= INTEGER (0..18446744073709551615) The "UInt64" type is used in places where 32 bits of precision may provide inadequate security. -- sequence numbers SeqNum ::= UInt64 Sequence numbers were constrained to 32 bits in [KCLAR], but this document allows for 64-bit sequence numbers. -- nonces Nonce ::= UInt64 Yu Expires: Jul 2005 [Page 14] Internet-Draft rfc1510ter-00 21 Jan 2005 Likewise, nonces were constrained to 32 bits in [KCLAR], but expanded to 64 bits here. -- microseconds Microseconds ::= INTEGER (0..999999) The "microseconds" type is intended to carry the microseconds part of a time value. 4.2. KerberosTime KerberosTime ::= GeneralizedTime (CONSTRAINED BY { -- MUST NOT include fractional seconds }) The timestamps used in Kerberos are encoded as GeneralizedTimes. A KerberosTime value MUST NOT include any fractional portions of the seconds. As required by the DER, it further MUST NOT include any separators, and it specifies the UTC time zone (Z). Example: The only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6 November 1985 is "19851106210627Z". 4.3. KerberosString -- used for names and for error messages KerberosString ::= CHOICE { ia5 GeneralString (IA5String), utf8 UTF8String, ... -- no extension may be sent -- to an rfc1510 implementation -- } The KerberosString type is used for character strings in various places in the Kerberos protocol. For compatibility with RFC 1510, GeneralString values constrained to IA5String (US-ASCII) are permitted in messages exchanged with RFC 1510 implementations. The new protocol messages contain strings encoded as UTF-8, and these strings MUST be normalized using [SASLPREP]. KerberosString values are present in principal names and in error messages. Control characters SHOULD NOT be used in principal names, and used with caution in error messages. -- IA5 choice only; useful for constraints KerberosStringIA5 ::= KerberosString (WITH COMPONENTS { ia5 PRESENT }) -- IA5 excluded; useful for constraints KerberosStringExt ::= KerberosString (WITH COMPONENTS { ia5 ABSENT }) KerberosStringIA5 requires the use of the "ia5" alternative, while Yu Expires: Jul 2005 [Page 15] Internet-Draft rfc1510ter-00 21 Jan 2005 KerberosStringExt forbids it. These types appear in ASN.1 constraints on messages. For detailed background regarding the history of character string use in Kerberos, as well as discussion of some compatibility issues, see Appendix B. 4.4. Language Tags -- used for language tags LangTag ::= PrintableString (FROM ("A".."Z" | "a".."z" | "0".."9" | "-")) The "LangTag" type is used to specify language tags for localization purposes, using the [RFC3066] format. 4.5. KerberosFlags For several message types, a specific constrained bit string type, KerberosFlags, is used. KerberosFlags { NamedBits } ::= BIT STRING (SIZE (32..MAX)) (CONSTRAINED BY { -- MUST be a valid value of -- NamedBits -- but if the value to be sent would be truncated to shorter -- than 32 bits according to DER, the value MUST be padded -- with trailing zero bits to 32 bits. Otherwise, no -- trailing zero bits may be present. }) The actual bit string type encoded in Kerberos messages does not use named bits. The advisory parameter of the KerberosFlags type names a bit string type defined using named bits, whose value is encoded as if it were a bit string with unnamed bits. This practice is necessary because the DER require trailing zero bits to be removed when encoding bit strings defined using named bits. Existing implementations of Kerberos send exactly 32 bits rather than truncating, so the size constraint requires the transmission of at least 32 bits. Trailing zero bits beyond the first 32 bits are truncated. 4.6. Typed Holes -- Typed hole identifiers TH-id ::= CHOICE { int32 Int32, rel-oid RELATIVE-OID } Yu Expires: Jul 2005 [Page 16] Internet-Draft rfc1510ter-00 21 Jan 2005 The "TH-id" type is a generalized means to identify the contents of a typed hole. The "int32" alternative may be used for simple integer assignments, in the same manner as "Int32", while the "rel-oid" alternative may be used for hierarchical delegation. 4.7. HostAddress and HostAddresses AddrType ::= Int32 HostAddress ::= SEQUENCE { addr-type [0] AddrType, address [1] OCTET STRING } -- NOTE: HostAddresses is always used as an OPTIONAL field and -- should not be a zero-length SEQUENCE OF. -- -- The extensible messages explicitly constrain this to be -- non-empty. HostAddresses ::= SEQUENCE OF HostAddress addr-type This field specifies the type of address that follows. address This field encodes a single address of the type identified by "addr-type". All negative values for the host address type are reserved for local use. All non-negative values are reserved for officially assigned type fields and interpretations. addr-type | meaning __________|______________ 2 | IPv4 3 | directional 12 | DECnet 20 | NetBIOS 24 | IPv6 4.7.1. Internet (IPv4) Addresses Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in MSB order (most significant byte first). The IPv4 loopback address SHOULD NOT appear in a Kerberos PDU. The type of IPv4 addresses is two (2). Yu Expires: Jul 2005 [Page 17] Internet-Draft rfc1510ter-00 21 Jan 2005 4.7.2. Internet (IPv6) Addresses IPv6 addresses [RFC2373] are 128-bit (16-octet) quantities, encoded in MSB order (most significant byte first). The type of IPv6 addresses is twenty-four (24). The following addresses MUST NOT appear in any Kerberos PDU: * the Unspecified Address * the Loopback Address * Link-Local addresses This restriction applies to the inclusion in the address fields of Kerberos PDUs, but not to the address fields of packets that might carry such PDUs. The restriction is necessary because the use of an address with non-global scope could allow the acceptance of a message sent from a node that may have the same address, but which is not the host intended by the entity that added the restriction. If the link-local address type needs to be used for communication, then the address restriction in tickets must not be used (i.e. addressless tickets must be used). IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2. 4.7.3. DECnet Phase IV addresses DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The type of DECnet Phase IV addresses is twelve (12). 4.7.4. Netbios addresses Netbios addresses are 16-octet addresses typically composed of 1 to 15 alphanumeric characters and padded with the US-ASCII SPC character (code 32). The 16th octet MUST be the US-ASCII NUL character (code 0). The type of Netbios addresses is twenty (20). 4.7.5. Directional Addresses In many environments, including the sender address in KRB-SAFE and KRB-PRIV messages is undesirable because the addresses may be changed in transport by network address translators. However, if these addresses are removed, the messages may be subject to a reflection attack in which a message is reflected back to its originator. The directional address type provides a way to avoid transport addresses and reflection attacks. Directional addresses are encoded as four byte unsigned integers in network byte order. If the message is originated by the party sending the original AP-REQ message, then an address of 0 SHOULD be used. If the message is originated by the party to whom that AP-REQ was sent, then the address 1 SHOULD be Yu Expires: Jul 2005 [Page 18] Internet-Draft rfc1510ter-00 21 Jan 2005 used. Applications involving multiple parties can specify the use of other addresses. Directional addresses MUST only be used for the sender address field in the KRB-SAFE or KRB-PRIV messages. They MUST NOT be used as a ticket address or in a AP-REQ message. This address type SHOULD only be used in situations where the sending party knows that the receiving party supports the address type. This generally means that directional addresses may only be used when the application protocol requires their support. Directional addresses are type (3). 5. Principals Principals are participants in the Kerberos protocol. A "realm" consists of principals in one administrative domain, served by one KDC (or one replicated set of KDCs). Each principal name has an arbitrary number of components, though typical principal names will only have one or two components. A principal name is meant to be readable by and meaningful to humans, especially in a realm lacking a centrally adminstered authorization infrastructure. 5.1. Name Types Each PrincipalName has NameType indicating what sort of name it is. The name-type SHOULD be treated as a hint. Ignoring the name type, no two names can be the same (i.e., at least one of the components, or the realm, must be different). -- assigned numbers for name types (used in principal names) NameType ::= Int32 -- Name type not known nt-unknown NameType ::= 0 -- Just the name of the principal as in DCE, or for users nt-principal NameType ::= 1 -- Service and other unique instance (krbtgt) nt-srv-inst NameType ::= 2 -- Service with host name as instance (telnet, rcommands) nt-srv-hst NameType ::= 3 -- Service with host as remaining components nt-srv-xhst NameType ::= 4 -- Unique ID nt-uid NameType ::= 5 -- Encoded X.509 Distingished name [RFC 2253] nt-x500-principal NameType ::= 6 -- Name in form of SMTP email name (e.g. user@foo.com) nt-smtp-name NameType ::= 7 -- Enterprise name - may be mapped to principal name nt-enterprise NameType ::= 10 Yu Expires: Jul 2005 [Page 19] Internet-Draft rfc1510ter-00 21 Jan 2005 5.2. Principal Type Definition PrincipalName ::= SEQUENCE { name-type [0] NameType, -- May have zero elements, or individual elements may be -- zero-length, but this is NOT RECOMMENDED. name-string [1] SEQUENCE OF KerberosString } name-type hint of the type of name that follows name-string The "name-string" encodes a sequence of components that form a name, each component encoded as a KerberosString. Taken together, a PrincipalName and a Realm form a principal identifier. Most PrincipalNames will have only a few components (typically one or two). 5.3. Principal Name Reuse Realm administrators SHOULD use extreme caution when considering reusing a principal name. A service administrator might explicitly enter principal names into a local access control list (ACL) for the service. If such local ACLs exist independently of a centrally administered authorization infrastructure, realm administrators SHOULD NOT reuse principal names until confirming that all extant ACL entries referencing that principal name have been updated. Failure to perform this check can result in a security vulnerability, as a new principal can inadvertently inherit unauthorized privileges upon receiving a reused principal name. An organization whose Kerberos- authenticated services all use a centrally-adminstered authorization infrastructure may not need to take these precautions regarding principal name reuse. 5.4. Realm Names Realm ::= KerberosString -- IA5 only RealmIA5 ::= Realm (KerberosStringIA5) -- IA5 excluded RealmExt ::= Realm (KerberosStringExt) Kerberos realm names are KerberosStrings. Realms MUST NOT contain a character with the code 0 (the US-ASCII NUL). Most realms will usually consist of several components separated by periods (.), in the style of Internet Domain Names, or separated by slashes (/) in Yu Expires: Jul 2005 [Page 20] Internet-Draft rfc1510ter-00 21 Jan 2005 the style of X.500 names. 5.5. Printable Representations of Principal Names [ perhaps non-normative? ] The printable form of a principal name consists of the concatenation of components of the PrincipalName value using the slash character (/), followed by an at-sign (@), followed by the realm name. Any occurrence of a backslash (\), slash (/) or at-sign (@) in the PrincipalName value is quoted by a backslash. 5.6. Ticket-Granting Service Principal The PrincipalName value corresponding to a ticket-granting service has two components: the first component is the string "krbtgt", and the second component is the realm name of the TGS which will accept a ticket-granting ticket having this service principal name. The realm name of service always indicates which realm issued the ticket. A ticket-granting ticket issued by "A.EXAMPLE.COM" which is valid for obtaining tickets in the same realm would have the following ASN.1 values for its "realm" and "sname" components, respectively: -- Example Realm and PrincipalName for a TGS tgtRealm1 Realm ::= ia5 : "A.EXAMPLE.COM" tgtPrinc1 PrincipalName ::= { name-type nt-srv-inst, name-string { ia5 : "krbtgt", ia5 : "A.EXAMPLE.COM" } } Its printable representation would be written as "krbtgt/A.EXAMPLE.COM@A.EXAMPLE.COM". 5.6.1. Cross-Realm TGS Principals It is possible for a principal in one realm to authenticate to a service in another realm. A KDC can issue a cross-realm ticket- granting ticket to allow one of its principals to authenticate to a service in a foreign realm. For example, the TGS principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" is a principal that permits a client principal in the realm A.EXAMPLE.COM to authenticate to a service in the realm B.EXAMPLE.COM. When the KDC for B.EXAMPLE.COM issues a ticket to a client originating in A.EXAMPLE.COM, the client's realm name remains "A.EXAMPLE.COM", even though the service principal will have the realm "B.EXAMPLE.COM". Yu Expires: Jul 2005 [Page 21] Internet-Draft rfc1510ter-00 21 Jan 2005 6. Types Relating to Encryption Many Kerberos protocol messages contain encrypted encodings of various data types. Some Kerberos protocol messages also contain checksums (signatures) of encodings of various types. 6.1. Assigned Numbers for Encryption Encryption algorithm identifiers and key usages both have assigned numbers, described in [KCRYPTO]. 6.1.1. EType EType is the integer type for assigned numbers for encryption algorithms. Defined in [KCRYPTO]. -- Assigned numbers denoting encryption mechanisms. EType ::= Int32 -- assigned numbers for encryption schemes et-des-cbc-crc EType ::= 1 et-des-cbc-md4 EType ::= 2 et-des-cbc-md5 EType ::= 3 -- [reserved] 4 et-des3-cbc-md5 EType ::= 5 -- [reserved] 6 et-des3-cbc-sha1 EType ::= 7 et-dsaWithSHA1-CmsOID EType ::= 9 et-md5WithRSAEncryption-CmsOID EType ::= 10 et-sha1WithRSAEncryption-CmsOID EType ::= 11 et-rc2CBC-EnvOID EType ::= 12 et-rsaEncryption-EnvOID EType ::= 13 et-rsaES-OAEP-ENV-OID EType ::= 14 et-des-ede3-cbc-Env-OID EType ::= 15 et-des3-cbc-sha1-kd EType ::= 16 -- AES et-aes128-cts-hmac-sha1-96 EType ::= 17 -- AES et-aes256-cts-hmac-sha1-96 EType ::= 18 -- Microsoft et-rc4-hmac EType ::= 23 -- Microsoft et-rc4-hmac-exp EType ::= 24 -- opaque; PacketCable et-subkey-keymaterial EType ::= 65 6.1.2. Key Usages KeyUsage is the integer type for assigned numbers for key usages. Key usage values are inputs to the encryption and decryption Yu Expires: Jul 2005 [Page 22] Internet-Draft rfc1510ter-00 21 Jan 2005 functions described in [KCRYPTO]. -- Assigned numbers denoting key usages. KeyUsage ::= UInt32 -- -- Actual identifier names are provisional and subject to -- change. -- ku-pa-enc-ts KeyUsage ::= 1 ku-Ticket KeyUsage ::= 2 ku-EncASRepPart KeyUsage ::= 3 ku-TGSReqAuthData-sesskey KeyUsage ::= 4 ku-TGSReqAuthData-subkey KeyUsage ::= 5 ku-pa-TGSReq-cksum KeyUsage ::= 6 ku-pa-TGSReq-authenticator KeyUsage ::= 7 ku-EncTGSRepPart-sesskey KeyUsage ::= 8 ku-EncTGSRepPart-subkey KeyUsage ::= 9 ku-Authenticator-cksum KeyUsage ::= 10 ku-APReq-authenticator KeyUsage ::= 11 ku-EncAPRepPart KeyUsage ::= 12 ku-EncKrbPrivPart KeyUsage ::= 13 ku-EncKrbCredPart KeyUsage ::= 14 ku-KrbSafe-cksum KeyUsage ::= 15 ku-ad-KDCIssued-cksum KeyUsage ::= 19 -- The following numbers are provisional... -- conflicts may exist elsewhere. ku-Ticket-cksum KeyUsage ::= 25 ku-ASReq-cksum KeyUsage ::= 26 ku-TGSReq-cksum KeyUsage ::= 27 ku-ASRep-cksum KeyUsage ::= 28 ku-TGSRep-cksum KeyUsage ::= 29 ku-APReq-cksum KeyUsage ::= 30 ku-APRep-cksum KeyUsage ::= 31 ku-KrbCred-cksum KeyUsage ::= 32 ku-KrbError-cksum KeyUsage ::= 33 ku-KDCRep-cksum KeyUsage ::= 34 6.2. Which Key to Use Yu Expires: Jul 2005 [Page 23] Internet-Draft rfc1510ter-00 21 Jan 2005 -- KeyToUse identifies which key is to be used to encrypt or -- sign a given value. -- -- Values of KeyToUse are never actually transmitted over the -- wire, or even used directly by the implementation in any -- way, as key usages are; it exists primarily to identify -- which key gets used for what purpose. Thus, the specific -- numeric values associated with this type are irrelevant. KeyToUse ::= ENUMERATED { -- unspecified key-unspecified, -- server long term key key-server, -- client long term key key-client, -- key selected by KDC for encryption of a KDC-REP key-kdc-rep, -- session key from ticket key-session, -- subsession key negotiated via AP-REQ/AP-REP key-subsession, ... } 6.3. EncryptionKey The "EncryptionKey" type holds an encryption key. EncryptionKey ::= SEQUENCE { keytype [0] EType, keyvalue [1] OCTET STRING } keytype This "EType" identifies the encryption algorithm, described in [KCRYPTO]. keyvalue Contains the actual key. 6.4. EncryptedData The "EncryptedData" type contains the encryption of another data type. The recipient uses fields within EncryptedData to determine which key to use for decryption. Yu Expires: Jul 2005 [Page 24] Internet-Draft rfc1510ter-00 21 Jan 2005 -- "Type" specifies which ASN.1 type is encrypted to the -- ciphertext in the EncryptedData. "Keys" specifies a set of -- keys of which one key may be used to encrypt the data. -- "KeyUsages" specifies a set of key usages, one of which may -- be used to encrypt. -- -- None of the parameters is transmitted over the wire. EncryptedData { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { etype [0] EType, kvno [1] UInt32 OPTIONAL, cipher [2] OCTET STRING (CONSTRAINED BY { -- must be encryption of -- OCTET STRING (CONTAINING Type), -- with one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }), ... } KeyUsages Advisory parameter indicating which key usage to use when encrypting the ciphertext. If "KeyUsages" indicate multiple "KeyUsage" values, the detailed description of the containing message will indicate which key to use under which conditions. Type Advisory parameter indicating the ASN.1 type whose DER encoding is the plaintext encrypted into the EncryptedData. Keys Advisory parameter indicating which key to use to perform the encryption. If "Keys" indicate multiple "KeyToUse" values, the detailed description of the containing message will indicate which key to use under which conditions. KeyUsages Advisory parameter indicating which "KeyUsage" value is used to encrypt. If "KeyUsages" indicates multiple "KeyUsage" values, the detailed description of the containing message will indicate which key usage to use under which conditions. 6.5. Checksums Several types contain checksums (actually signatures) of data. Yu Expires: Jul 2005 [Page 25] Internet-Draft rfc1510ter-00 21 Jan 2005 CksumType ::= Int32 -- The parameters specify which key to use to produce the -- signature, as well as which key usage to use. The -- parameters are not actually sent over the wire. Checksum { KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksumtype [0] CksumType, checksum [1] OCTET STRING (CONSTRAINED BY { -- signed using one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }) } CksumType Integer type for assigned numbers for signature algorithms. Defined in [KCRYPTO] Keys As in EncryptedData KeyUsages As in EncryptedData cksumtype Signature algorithm used to produce the signature. checksum The actual checksum. 6.5.1. ChecksumOf ChecksumOf is similar to "Checksum", but specifies which type is signed. -- a Checksum that must contain the checksum -- of a particular type ChecksumOf { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS { ..., checksum (CONSTRAINED BY { -- must be checksum of -- OCTET STRING (CONTAINING Type) }) }) Yu Expires: Jul 2005 [Page 26] Internet-Draft rfc1510ter-00 21 Jan 2005 Type Indicates the ASN.1 type whose DER encoding is signed. 6.5.2. Signed Signed is similar to "ChecksumOf", but contains an actual instance of the type being signed in addition to the signature. -- parameterized type for wrapping authenticated plaintext Signed { InnerType, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksum [0] ChecksumOf { InnerType, Keys, KeyUsages } OPTIONAL, inner [1] InnerType, ... } 7. Tickets [ A large number of items described here are duplicated in the sections describing KDC-REQ processing. Should find a way to avoid this duplication. ] A ticket binds a principal name to a session key. The important fields of a ticket are in the encrypted part. The components in common between the RFC 1510 and the extensible versions are EncTicketPartCommon ::= SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL, ... } crealm This field contains the client's realm. cname This field contains the client's name. Yu Expires: Jul 2005 [Page 27] Internet-Draft rfc1510ter-00 21 Jan 2005 caddr This field lists the network addresses (if absent, all addresses are permitted) from which the ticket is valid. Descriptions of the other fields appear in the following sections. 7.1. Timestamps Three of the ticket timestamps may be requested from the KDC. The timestamps may differ from those requested, depending on site policy. authtime The time at which the client authenticated, as recorded by the KDC. starttime The earliest time when the ticket is valid. If not present, the ticket is valid starting at the authtime. This is requested as the "from" field of KDC-REQ-BODY-COMMON. endtime This time is requested in the "till" field of KDC-REQ-BODY- COMMON. Contains the time after which the ticket will not be honored (its expiration time). Note that individual services MAY place their own limits on the life of a ticket and MAY reject tickets which have not yet expired. As such, this is really an upper bound on the expiration time for the ticket. renew-till This time is requested in the "rtime" field of KDC-REQ-BODY- COMMON. It is only present in tickets that have the "renewable" flag set in the flags field. It indicates the maximum endtime that may be included in a renewal. It can be thought of as the absolute expiration time for the ticket, including all renewals. 7.2. Ticket Flags A number of flags may be set in the ticket, further defining some of its capabilities. Some of these flags map to flags in a KDC request. Yu Expires: Jul 2005 [Page 28] Internet-Draft rfc1510ter-00 21 Jan 2005 TicketFlags ::= KerberosFlags { TicketFlagsBits } TicketFlagsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), may-postdate (5), postdated (6), invalid (7), renewable (8), initial (9), pre-authent (10), hw-authent (11), transited-policy-checked (12), ok-as-delegate (13), anonymous (14), cksummed-ticket (15) } 7.2.1. Flags Relating to Initial Ticket Acquisition [ adapted KCLAR 2.1. ] Several flags indicate the details of how the initial ticket was acquired. initial The "initial" flag indicates that a ticket was issued using the AS protocol, rather than issued based on a ticket-granting ticket. Application servers (e.g., a password-changing program) requiring a client's definite knowledge of its secret key can insist that this flag be set in any tickets they accept, thus being assured that the client's key was recently presented to the application client. pre-authent The "pre-authent" flag indicates that some sort of pre- authentication was used during the AS exchange. hw-authent The "hw-authent" flag indicates that some sort of hardware-based pre-authentication occurred during the AS exchange. Both the "pre-authent" and the "hw-authent" flags may be present with or without the "initial" flag; such tickets with the "initial" flag clear are ones which are derived from initial tickets with the "pre- authent" or "hw-authent" flags set. Yu Expires: Jul 2005 [Page 29] Internet-Draft rfc1510ter-00 21 Jan 2005 7.2.2. Invalid Tickets [ KCLAR 2.2. ] The "invalid" flag indicates that a ticket is invalid. Application servers MUST reject tickets which have this flag set. A postdated ticket will be issued in this form. Invalid tickets MUST be validated by the KDC before use, by presenting them to the KDC in a TGS request with the "validate" option specified. The KDC will only validate tickets after their starttime has passed. The validation is required so that postdated tickets which have been stolen before their starttime can be rendered permanently invalid (through a hot- list mechanism -- see Section 8.3.2.4). 7.2.3. OK as Delegate [ KCLAR 2.8. ] The "ok-as-delegate" flag provides a way for a KDC to communicate local realm policy to a client regarding whether the service for which the ticket is issued is trusted to accept delegated credentials. For some applications, a client may need to delegate credentials to a service to act on its behalf in contacting other services. The ability of a client to obtain a service ticket for a service conveys no information to the client about whether the service should be trusted to accept delegated credentials. The copy of the ticket flags visible to the client may have the "ok- as-delegate" flag set to indicate to the client that the service specified in the ticket has been determined by policy of the realm to be a suitable recipient of delegation. A client can use the presence of this flag to help it make a decision whether to delegate credentials (either grant a proxy or a forwarded ticket-granting ticket) to this service. It is acceptable to ignore the value of this flag. When setting this flag, an administrator should consider the security and placement of the server on which the service will run, as well as whether the service requires the use of delegated credentials. 7.2.4. Renewable Tickets [ adapted KCLAR 2.3. ] The "renewable" flag indicates whether the ticket may be renewed. Renewable tickets can be used to mitigate the consequences of ticket theft. Applications may desire to hold credentials which can be valid for long periods of time. However, this can expose the credentials to potential theft for equally long periods, and those stolen credentials would be valid until the expiration time of the ticket(s). Simply using short-lived tickets and obtaining new ones Yu Expires: Jul 2005 [Page 30] Internet-Draft rfc1510ter-00 21 Jan 2005 periodically would require the application to have long-term access to the client's secret key, which is an even greater risk. Renewable tickets have two "expiration times": the first is when the current instance of the ticket expires, and the second is the latest permissible value for an individual expiration time. An application client must periodically present an unexpired renewable ticket to the KDC, setting the "renew" option in the KDC request. The KDC will issue a new ticket with a new session key and a later expiration time. All other fields of the ticket are left unmodified by the renewal process. When the latest permissible expiration time arrives, the ticket expires permanently. At each renewal, the KDC MAY consult a hot-list to determine if the ticket had been reported stolen since its last renewal; it will refuse to renew such stolen tickets, and thus the usable lifetime of stolen tickets is reduced. The "renewable" flag in a ticket is normally only interpreted by the ticket-granting service. It can usually be ignored by application servers. However, some particularly careful application servers MAY disallow renewable tickets. If a renewable ticket is not renewed by its expiration time, the KDC will not renew the ticket. The "renewable" flag is clear by default, but a client can request it be set by setting the "renewable" option in the AS-REQ message. If it is set, then the "renew-till" field in the ticket contains the time after which the ticket may not be renewed. 7.2.5. Postdated Tickets postdated indicates a ticket which has been postdated may-postdate indicates that postdated tickets may be issued based on this ticket [ KCLAR 2.4. ] Applications may occasionally need to obtain tickets for use much later, e.g., a batch submission system would need tickets to be valid at the time the batch job is serviced. However, it is dangerous to hold valid tickets in a batch queue, since they will be on-line longer and more prone to theft. Postdated tickets provide a way to obtain these tickets from the KDC at job submission time, but to leave them "dormant" until they are activated and validated by a further request of the KDC. If a ticket theft were reported in the interim, the KDC would refuse to validate the ticket, and the thief would be foiled. Yu Expires: Jul 2005 [Page 31] Internet-Draft rfc1510ter-00 21 Jan 2005 The "may-postdate" flag in a ticket is normally only interpreted by the TGS. It can be ignored by application servers. This flag MUST be set in a ticket-granting ticket in order for the KDC to issue a postdated ticket based on the presented ticket. It is reset by default; it MAY be requested by a client by setting the "allow- postdate" option in the AS-REQ [?also TGS-REQ?] message. This flag does not allow a client to obtain a postdated ticket-granting ticket; postdated ticket-granting tickets can only by obtained by requesting the postdating in the AS-REQ message. The life (endtime minus starttime) of a postdated ticket will be the remaining life of the ticket-granting ticket at the time of the request, unless the "renewable" option is also set, in which case it can be the full life (endtime minus starttime) of the ticket-granting ticket. The KDC MAY limit how far in the future a ticket may be postdated. The "postdated" flag indicates that a ticket has been postdated. The application server can check the authtime field in the ticket to see when the original authentication occurred. Some services MAY choose to reject postdated tickets, or they may only accept them within a certain period after the original authentication. When the KDC issues a "postdated" ticket, it will also be marked as "invalid", so that the application client MUST present the ticket to the KDC for validation before use. 7.2.6. Proxiable and Proxy Tickets proxy indicates a proxy ticket proxiable indicates that proxy tickets may be issued based on this ticket [ KCLAR 2.5. ] It may be necessary for a principal to allow a service to perform an operation on its behalf. The service must be able to take on the identity of the client, but only for a particular purpose. A principal can allow a service to take on the principal's identity for a particular purpose by granting it a proxy. The process of granting a proxy using the "proxy" and "proxiable" flags is used to provide credentials for use with specific services. Though conceptually also a proxy, users wishing to delegate their identity in a form usable for all purposes MUST use the ticket forwarding mechanism described in the next section to forward a ticket-granting ticket. The "proxiable" flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. When set, this flag tells the ticket-granting server that it is OK to issue a new ticket (but not a ticket-granting ticket) with a Yu Expires: Jul 2005 [Page 32] Internet-Draft rfc1510ter-00 21 Jan 2005 different network address based on this ticket. This flag is set if requested by the client on initial authentication. By default, the client will request that it be set when requesting a ticket-granting ticket, and reset when requesting any other ticket. This flag allows a client to pass a proxy to a server to perform a remote request on its behalf (e.g. a print service client can give the print server a proxy to access the client's files on a particular file server in order to satisfy a print request). In order to complicate the use of stolen credentials, Kerberos tickets may contain a set of network addresses from which they are valid. When granting a proxy, the client MUST specify the new network address from which the proxy is to be used, or indicate that the proxy is to be issued for use from any address. The "proxy" flag is set in a ticket by the TGS when it issues a proxy ticket. Application servers MAY check this flag and at their option they MAY require additional authentication from the agent presenting the proxy in order to provide an audit trail. 7.2.7. Forwarded and Forwardable Tickets forwarded indicates a forwarded ticket forwardable indicates that forwarded tickets may be issued based on this ticket [ KCLAR 2.6. ] Authentication forwarding is an instance of a proxy where the service that is granted is complete use of the client's identity. An example where it might be used is when a user logs in to a remote system and wants authentication to work from that system as if the login were local. The "forwardable" flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. The "forwardable" flag has an interpretation similar to that of the "proxiable" flag, except ticket-granting tickets may also be issued with different network addresses. This flag is reset by default, but users MAY request that it be set by setting the "forwardable" option in the AS request when they request their initial ticket-granting ticket. This flag allows for authentication forwarding without requiring the user to enter a password again. If the flag is not set, then authentication forwarding is not permitted, but the same result can still be achieved if the user engages in the AS exchange specifying Yu Expires: Jul 2005 [Page 33] Internet-Draft rfc1510ter-00 21 Jan 2005 the requested network addresses and supplies a password. The "forwarded" flag is set by the TGS when a client presents a ticket with the "forwardable" flag set and requests a forwarded ticket by specifying the "forwarded" KDC option and supplying a set of addresses for the new ticket. It is also set in all tickets issued based on tickets with the "forwarded" flag set. Application servers may choose to process "forwarded" tickets differently than non-forwarded tickets. If addressless tickets are forwarded from one system to another, clients SHOULD still use this option to obtain a new TGT in order to have different session keys on the different systems. 7.3. Transited Realms [ KCLAR 2.7., plus new stuff ] 7.4. Authorization Data [ KCLAR 5.2.6. ] ADType ::= TH-id AuthorizationData ::= SEQUENCE OF SEQUENCE { ad-type [0] ADType, ad-data [1] OCTET STRING } ad-type This field identifies the contents of the ad-data. All negative values are reserved for local use. Non-negative values are reserved for registered use. ad-data This field contains authorization data to be interpreted according to the value of the corresponding ad-type field. Each sequence of ADType and OCTET STRING is referred to as an authorization element. Elements MAY be application specific, however, there is a common set of recursive elements that should be understood by all implementations. These elements contain other AuthorizationData, and the interpretation of the encapsulating element determines which enclosed elements must be interpreted, and which may be ignored. Depending on the meaning of the encapsulating element, the encapsulated AuthorizationData may be ignored, interpreted as issued directly by the KDC, or be stored in a separate plaintext part of the ticket. The types of the encapsulating elements are specified as Yu Expires: Jul 2005 [Page 34] Internet-Draft rfc1510ter-00 21 Jan 2005 part of the Kerberos protocol because behavior based on these container elements should be understood across implementations, while other elements need only be understood by the applications which they affect. Authorization data elements are considered critical if present in a ticket or authenticator. Unless encapsulated in a known authorization data element modifying the criticality of the elements it contains, an application server MUST reject the authentication of a client whose AP-REQ or ticket contains an unrecognized authorization data element. Authorization data is intended to restrict the use of a ticket. If the service cannot determine whether it is the target of a restriction, a security weakness may exist if the ticket can be used for that service. Authorization elements that are truly optional can be enclosed in AD-IF-RELEVANT element. ad-type | contents of ad-data ________|_______________________________________ 1 | DER encoding of AD-IF-RELEVANT 4 | DER encoding of AD-KDCIssued 5 | DER encoding of AD-AND-OR 8 | DER encoding of AD-MANDATORY-FOR-KDC 7.4.1. AD-IF-RELEVANT ad-if-relevant ADType ::= int32 : 1 -- Encapsulates another AuthorizationData. -- Intended for application servers; receiving application servers -- MAY ignore. AD-IF-RELEVANT ::= AuthorizationData AD elements encapsulated within the if-relevant element are intended for interpretation only by application servers that understand the particular ad-type of the embedded element. Application servers that do not understand the type of an element embedded within the if- relevant element MAY ignore the uninterpretable element. This element promotes interoperability across implementations which may have local extensions for authorization. The ad-type for AD-IF-RELEVANT is (1). 7.4.2. AD-KDCIssued Yu Expires: Jul 2005 [Page 35] Internet-Draft rfc1510ter-00 21 Jan 2005 -- KDC-issued privilege attributes ad-kdcissued ADType ::= int32 : 4 AD-KDCIssued ::= SEQUENCE { ad-checksum [0] ChecksumOf { AuthorizationData, { key-session }, { ku-ad-KDCIssued-cksum }}, i-realm [1] Realm OPTIONAL, i-sname [2] PrincipalName OPTIONAL, elements [3] AuthorizationData } ad-checksum A cryptographic checksum computed over the DER encoding of the AuthorizationData in the "elements" field, keyed with the session key. Its checksumtype is the mandatory checksum type for the encryption type of the session key, and its key usage value is 19. i-realm, i-sname The name of the issuing principal if different from the KDC itself. This field would be used when the KDC can verify the authenticity of elements signed by the issuing principal and it allows this KDC to notify the application server of the validity of those elements. elements AuthorizationData issued by the KDC. The KDC-issued ad-data field is intended to provide a means for Kerberos credentials to embed within themselves privilege attributes and other mechanisms for positive authorization, amplifying the privileges of the principal beyond what it would have if using credentials without such an authorization-data element. This amplification of privileges cannot be provided without this element because the definition of the authorization-data field allows elements to be added at will by the bearer of a TGT at the time that they request service tickets and elements may also be added to a delegated ticket by inclusion in the authenticator. For KDC-issued elements this is prevented because the elements are signed by the KDC by including a checksum encrypted using the server's key (the same key used to encrypt the ticket -- or a key derived from that key). AuthorizationData encapsulated with in the AD-KDCIssued element MUST be ignored by the application server if this "signature" is not present. Further, AuthorizationData encapsulated within this element from a ticket-granting ticket MAY be interpreted by the KDC, and used as a basis according to policy for including new signed elements within derivative tickets, but they Yu Expires: Jul 2005 [Page 36] Internet-Draft rfc1510ter-00 21 Jan 2005 will not be copied to a derivative ticket directly. If they are copied directly to a derivative ticket by a KDC that is not aware of this element, the signature will not be correct for the application ticket elements, and the field will be ignored by the application server. This element and the elements it encapsulates MAY be safely ignored by applications, application servers, and KDCs that do not implement this element. The ad-type for AD-KDC-ISSUED is (4). 7.4.3. AD-AND-OR ad-and-or ADType ::= int32 : 5 AD-AND-OR ::= SEQUENCE { condition-count [0] INTEGER, elements [1] AuthorizationData } When restrictive AD elements are encapsulated within the and-or element, the and-or element is considered satisfied if and only if at least the number of encapsulated elements specified in condition- count are satisfied. Therefore, this element MAY be used to implement an "or" operation by setting the condition-count field to 1, and it MAY specify an "and" operation by setting the condition count to the number of embedded elements. Application servers that do not implement this element MUST reject tickets that contain authorization data elements of this type. The ad-type for AD-AND-OR is (5). 7.4.4. AD-MANDATORY-FOR-KDC -- KDCs MUST interpret any AuthorizationData wrapped in this. ad-mandatory-for-kdc ADType ::= int32 : 8 AD-MANDATORY-FOR-KDC ::= AuthorizationData AD elements encapsulated within the mandatory-for-kdc element are to be interpreted by the KDC. KDCs that do not understand the type of an element embedded within the mandatory-for-kdc element MUST reject the request. The ad-type for AD-MANDATORY-FOR-KDC is (8). 7.5. Encrypted Part of Ticket The complete definition of the encrypted part is Yu Expires: Jul 2005 [Page 37] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Encrypted part of ticket EncTicketPart ::= CHOICE { rfc1510 [APPLICATION 3] EncTicketPart1510, ext [APPLICATION 5] EncTicketPartExt } with the common portion being EncTicketPartCommon ::= SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL, ... } The encrypted part of the backwards-compatibility form of a ticket is: EncTicketPart1510 ::= SEQUENCE { COMPONENTS OF EncTicketPartCommon } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings crealm (RealmIA5), cname (PrincipalNameIA5) }) The encrypted part of the extensible form of a ticket is: EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings crealm (RealmExt), cname (PrincipalNameExt), -- explicitly constrain caddr to be non-empty if present caddr (SIZE (1..MAX)), -- forbid empty authorization-data encodings authorization-data (SIZE (1..MAX)) }) Yu Expires: Jul 2005 [Page 38] Internet-Draft rfc1510ter-00 21 Jan 2005 7.6. Cleartext Part of Ticket The complete definition of Ticket is: Ticket ::= CHOICE { rfc1510 [APPLICATION 1] Ticket1510, ext [APPLICATION 4] Signed { TicketExt, { key-server }, { ku-Ticket-cksum } } } with the common portion being: -- takes a parameter specifying which type gets encrypted TicketCommon { EncPart } ::= SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData { EncPart, { key-server }, { ku-Ticket } }, ..., extensions [4] TicketExtensions OPTIONAL, ... } The "sname" field provides the name of the target service principal in cleartext, as a hint to aid the server in choosing the correct decryption key. The backwards-compatibility form of Ticket is: Ticket1510 ::= SEQUENCE { COMPONENTS OF TicketCommon { EncTicketPart1510 } } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings realm (RealmIA5), sname (PrincipalNameIA5) }) The extensible form of Ticket is: Yu Expires: Jul 2005 [Page 39] Internet-Draft rfc1510ter-00 21 Jan 2005 -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. TicketExt ::= [APPLICATION 4] TicketCommon { EncTicketPartExt } (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings realm (RealmExt), sname (PrincipalNameExt) }) TicketExtensions, which may only be present in the extensible form of Ticket, are a cleartext typed hole for extension use. AuthorizationData already provides an encrypted typed hole. TEType ::= TH-id -- ticket extensions: for TicketExt only TicketExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { te-type [0] TEType, te-data [1] OCTET STRING } A client will only receive an extensible Ticket if the application server supports extensibility. 8. Credential Acquisition There are two exchanges that can be used for acquiring credentials: the AS exchange and the TGS exchange. These exchanges have many similarities, and this document describes them in parallel, noting which behaviors are specific to one type of exchange. The AS request (AS-REQ) and TGS request (TGS-REQ) are both forms of KDC requests (KDC-REQ). Likewise, the AS reply (AS-REP) and TGS reply (TGS-REP) are forms of KDC replies (KDC-REP). The credentials acquisition protocol operates over specific transports. Additionally, specific methods exist to permit a client to discover the KDC host with which to communicate. 8.1. KDC-REQ The KDC-REQ has a large number of fields in common between the RFC 1510 and the extensible versions. The KDC-REQ type itself is never directly encoded; it is always a part of a AS-REQ or a TGS-REQ. Yu Expires: Jul 2005 [Page 40] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REQ-COMMON ::= SEQUENCE { -- NOTE: first tag is [1], not [0] pvno [1] INTEGER (5), msg-type [2] INTEGER (10 -- AS-REQ.rfc1510 -- | 12 -- TGS-REQ.rfc1510 -- | 6 -- AS-REQ.ext -- | 8 -- TGS-REQ.ext -- ), padata [3] SEQUENCE OF PA-DATA OPTIONAL -- NOTE: not empty } KDC-REQ-BODY-COMMON ::= SEQUENCE { kdc-options [0] KDCOptions, cname [1] PrincipalName OPTIONAL -- Used only in AS-REQ --, realm [2] Realm -- Server's realm; also client's in AS-REQ --, sname [3] PrincipalName OPTIONAL, from [4] KerberosTime OPTIONAL, till [5] KerberosTime OPTIONAL -- was required in rfc1510; -- still required for compat versions -- of messages --, rtime [6] KerberosTime OPTIONAL, nonce [7] Nonce, etype [8] SEQUENCE OF EType -- in preference order --, addresses [9] HostAddresses OPTIONAL, enc-authorization-data [10] EncryptedData { AuthorizationData, { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } } OPTIONAL, additional-tickets [11] SEQUENCE OF Ticket OPTIONAL -- NOTE: not empty --, ... lang-tags [5] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } Many fields of KDC-REQ-BODY-COMMON correspond directly to fields of an EncTicketPartCommon. The KDC copies most of them unchanged, provided that the requested values meet site policy. Yu Expires: Jul 2005 [Page 41] Internet-Draft rfc1510ter-00 21 Jan 2005 kdc-options These flags do not correspond directly to "flags" in EncTicketPartCommon. cname This field is copied to the "cname" field in EncTicketPartCommon. The "cname" field is required in an AS- REQ; the client places its own name here. In a TGS-REQ, the "cname" in the ticket in the AP-REQ takes precedence. realm This field is the server's realm, and also holds the client's realm in an AS-REQ. sname The "sname" field indicates the server's name. It may be absent in a TGS-REQ which requests user-to-user authentication, in which case the "sname" of the issued ticket will be taken from the included additional ticket. The "from", "till", and "rtime" fields correspond to the "starttime", "endtime", and "renew-till" fields of EncTicketPartCommon. addresses This field corresponds to the "caddr" field of EncTicketPartCommon. enc-authorization-data For TGS-REQ, this field contains authorization data encrypted using either the TGT session key or the AP-REQ subsession key; the KDC may copy these into the "authorization-data" field of EncTicketPartCommon if policy permits. lang-tags Only present in the extensible messages. Specifies the set of languages which the client is willing to accept in error messages. KDC options used in a KDC-REQ are: Yu Expires: Jul 2005 [Page 42] Internet-Draft rfc1510ter-00 21 Jan 2005 KDCOptions ::= KerberosFlags { KDCOptionsBits } KDCOptionsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), allow-postdate (5), postdated (6), unused7 (7), renewable (8), unused9 (9), unused10 (10), unused11 (11), unused12 (12), unused13 (13), requestanonymous (14), canonicalize (15), disable-transited-check (26), renewable-ok (27), enc-tkt-in-skey (28), renew (30), validate (31) -- XXX need "need ticket1" flag? } Different options apply to different phases of KDC-REQ processing. The backwards-compatibility form of a KDC-REQ is: KDC-REQ-1510 ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-1510 } (WITH COMPONENTS { ..., msg-type (10 | 12) }) The extensible form of a KDC-REQ is: -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. KDC-REQ-EXT ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-EXT, ... } (WITH COMPONENTS { ..., msg-type (6 | 8), padata (SIZE (1..MAX)) }) The backwards-compatibility form of a KDC-REQ-BODY is: Yu Expires: Jul 2005 [Page 43] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REQ-BODY-1510 ::= SEQUENCE { COMPONENTS OF KDC-REQ-BODY-COMMON } (WITH COMPONENTS { ..., cname (PrincipalNameIA5), realm (RealmIA5), sname (PrincipalNameIA5), till PRESENT, nonce (UInt32) }) The extensible form of a KDC-REQ-BODY is: KDC-REQ-BODY-EXT ::= KDC-REQ-BODY-COMMON (WITH COMPONENTS { ..., cname (PrincipalNameExt), realm (RealmExt), sname (PrincipalNameExt), addresses (SIZE (1..MAX)), enc-authorization-data (EncryptedData { AuthorizationData (SIZE (1..MAX)), { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } }), additional-tickets (SIZE (1..MAX)) }) The AS-REQ is: Yu Expires: Jul 2005 [Page 44] Internet-Draft rfc1510ter-00 21 Jan 2005 AS-REQ ::= CHOICE { rfc1510 [APPLICATION 10] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (10), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }), ext [APPLICATION 6] Signed { -- APPLICATION tag goes inside Signed{} as well as -- outside, to prevent possible substitution attacks. [APPLICATION 6] KDC-REQ-EXT, -- not sure this is correct key to use; do we even want -- to sign AS-REQ? { key-client }, { ku-ASReq-cksum } } (WITH COMPONENTS { ..., msg-type (6), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }) } A client SHOULD NOT send the extensible AS-REQ alternative to a KDC if the client does not know that the KDC supports the extensibility framework. A client SHOULD send the extensible AS-REQ alternative in a PA-AS-REQ PA-DATA. A KDC supporting extensibility will treat the AS-REQ contained within the PA-AS-REQ as the actual AS-REQ. [ XXX what if their contents conflict? ] The TGS-REQ is: Yu Expires: Jul 2005 [Page 45] Internet-Draft rfc1510ter-00 21 Jan 2005 TGS-REQ ::= CHOICE { rfc1510 [APPLICATION 12] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (12), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }), ext [APPLICATION 8] Signed { -- APPLICATION tag goes inside Signed{} as well as -- outside, to prevent possible substitution attacks. [APPLICATION 8] KDC-REQ-EXT, { key-session }, { ku-TGSReq-cksum } } (WITH COMPONENTS { ..., msg-type (8), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }) } 8.2. PA-DATA PA-DATA have multiple uses in the Kerberos protocol. They may pre- authenticate an AS-REQ; they may also modify several of the encryption keys used in a KDC-REP. PA-DATA may also provide "hints" to the client about which long-term key (usually password-derived) to use. PA-DATA may also include "hints" about which pre-authentication mechanisms to use, or include data for input into a pre- authentication mechanism. [ XXX enumerate standard padata here ] 8.3. KDC-REQ Processing Processing of a KDC-REQ proceeds through several steps. An implementation need not perform these steps exactly as described, as long as it behaves as if the steps were performed as described. The KDC performs replay handling upon receiving the request; it then validates the request, adjusts timestamps, and selects the keys used in the reply. It copies data from the request into the issued ticket, adjusting the values to conform with its policies. The KDC then transmits the reply to the client. 8.3.1. Handling Replays Because Kerberos can run over unreliable transports such as UDP, the KDC MUST be prepared to retransmit responses in case they are lost. If a KDC receives a request identical to one it has recently Yu Expires: Jul 2005 [Page 46] Internet-Draft rfc1510ter-00 21 Jan 2005 successfully processed, the KDC MUST respond with a KDC-REP message rather than a replay error. In order to reduce the amount of ciphertext given to a potential attacker, KDCs MAY send the same response generated when the request was first handled. KDCs MUST obey this replay behavior even if the actual transport in use is reliable. If the AP-REQ which authenticates a TGS-REQ is a replay, and the entire request is not identical to a recently successfully processed request, the KDC SHOULD return "krb-ap-err-repeat", as is appropriate for AP-REQ processing. 8.3.2. Request Validation 8.3.2.1. AS-REQ Authentication Site policy determines whether a given client principal is required to provide some pre-authentication prior to receiving an AS-REP. Since the default reply key is typically the client's long-term password-based key, an attacker may easily request known plaintext (in the form of an AS-REP) upon which to mount a dictionary attack. Pre-authentication can limit the possibility of such an attack. If site policy requires pre-authentication for a client principal, and no pre-authentication is provided, the KDC returns the error "kdc-err-preauth-required". Accompanying this error are "e-data" which include hints telling the client which pre-authentication mechanisms to use, or data for input to pre-authentication mechanisms (e.g., input to challenge-response systems). If pre-authentication fails, the KDC returns the error "kdc-err-preauth-failed". [ may need additional changes based on Sam's preauth draft ] 8.3.2.2. TGS-REQ Authentication A TGS-REQ has an accompanying AP-REQ, which is included in the "pa- tgs-req". The KDC MUST validate the checksum in the Authenticator of the AP-REQ, which is computed over the KDC-REQ-BODY-1510 or KDC-REQ- BODY-EXT (for TGS-REQ-1510 or TGS-REQ-EXT, respectively) of the request. [ padata not signed by authenticator! ] Any error from the AP-REQ validation process SHOULD be returned in a KRB-ERROR message. The service principal in the ticket of the AP-REQ may be a ticket- granting service principal, or a normal application service principal. A ticket which is not a ticket-granting ticket MUST NOT be used to issue a ticket for any service other than the one named in the ticket. In this case, the "renew", "validate", or "proxy" [?also forwarded?] option must be set in the request. 8.3.2.3. Principal Validation If the client principal in an AS-REQ is unknown, the KDC returns the error "kdc-err-c-principal-unknown". If the server principal in a KDC-REQ is unknown, the KDC returns the error "kdc-err-s-principal- Yu Expires: Jul 2005 [Page 47] Internet-Draft rfc1510ter-00 21 Jan 2005 unknown". 8.3.2.4. Checking For Revoked or Invalid Tickets [ KCLAR 3.3.3.1 ] Whenever a request is made to the ticket-granting server, the presented ticket(s) is(are) checked against a hot-list of tickets which have been canceled. This hot-list might be implemented by storing a range of issue timestamps for "suspect tickets"; if a presented ticket had an authtime in that range, it would be rejected. In this way, a stolen ticket-granting ticket or renewable ticket cannot be used to gain additional tickets (renewals or otherwise) once the theft has been reported to the KDC for the realm in which the server resides. Any normal ticket obtained before it was reported stolen will still be valid (because they require no interaction with the KDC), but only until their normal expiration time. If TGTs have been issued for cross-realm authentication, use of the cross-realm TGT will not be affected unless the hot-list is propagated to the KDCs for the realms for which such cross-realm tickets were issued. If a TGS-REQ ticket has its "invalid" flag set, the KDC MUST NOT issue any ticket unless the TGS-REQ requests the "validate" option. 8.3.3. Timestamp Handling [ some aspects of timestamp handling, especially regarding postdating and renewal, are difficult to read in KCLAR... needs closer examination here ] Processing of "starttime" (requested in the "from" field) differs depending on whether the "postdated" option is set in the request. If the "postdated" option is not set, and the requested "starttime" is in the future beyond the window of acceptable clock skew, the KDC returns the error "kdc-err-cannot-postdate". If the "postdated" option is not set, and the requested "starttime" is absent or does not indicate a time in the future beyond the acceptable clock skew, the KDC sets the "starttime" to the KDC's current time. The "postdated" option MUST NOT be honored if the ticket is being requested by TGS-REQ and the "may-postdate" is not set in the TGT. Otherwise, if the "postdated" option is set, and site policy permits, the KDC sets the "starttime" as requested, and sets the "invalid" flag in the new ticket. The "till" field is required in the RFC 1510 version of the KDC-REQ. If the "till" field is equal to "19700101000000Z" (midnight, January 1, 1970), the KDC SHOULD behave as if the "till" field were absent. The KDC MUST NOT issue a ticket whose "starttime", "endtime", or "renew-till" time is later than the "renew-till" time of the ticket Yu Expires: Jul 2005 [Page 48] Internet-Draft rfc1510ter-00 21 Jan 2005 from which it is derived. 8.3.3.1. AS-REQ Timestamp Processing In the AS exchange, the "authtime" of a ticket is set to the local time at the KDC. The "endtime" of the ticket will be set to the earlier of the requested "till" time and a time determined by local policy, possibly determined using factors specific to the realm or principal. For example, the expiration time MAY be set to the earliest of the following: * the expiration time ("till" value) requested * the ticket's start time plus the maximum allowable lifetime associated with the client principal from the authentication server's database * the ticket's start time plus the maximum allowable lifetime associated with the server principal * the ticket's start time plus the maximum lifetime set by the policy of the local realm If the requested expiration time minus the start time (as determined above) is less than a site-determined minimum lifetime, an error message with code "kdc-err-never-valid" is returned. If the requested expiration time for the ticket exceeds what was determined as above, and if the "renewable-ok" option was requested, then the "renewable" flag is set in the new ticket, and the "renew-till" value is set as if the "renewable" option were requested. If the "renewable" option has been requested or if the "renewable-ok" option has been set and a renewable ticket is to be issued, then the "renew-till" field MAY be set to the earliest of: * its requested value * the start time of the ticket plus the minimum of the two maximum renewable lifetimes associated with the principals' database entries * the start time of the ticket plus the maximum renewable lifetime set by the policy of the local realm 8.3.3.2. TGS-REQ Timestamp Processing In the TGS exchange, the KDC sets the "authtime" to that of the ticket in the AP-REQ authenticating the TGS-REQ. [?application server can print a ticket for itself with a spoofed authtime. Yu Expires: Jul 2005 [Page 49] Internet-Draft rfc1510ter-00 21 Jan 2005 security issues for hot-list?] [ MIT implementation may change authtime of renewed tickets; needs check... ] If the TGS-REQ has a TGT as the ticket in its AP-REQ, and the TGS-REQ requests an "endtime" (in the "till" field), then the "endtime" of the new ticket is set to the minimum of * the requested "endtime" value, * the "endtime" in the TGT, and * an "endtime" determined by site policy on ticket lifetimes. If the new ticket is a renewal, the "endtime" of the new ticket is bounded by the minimum of * the requested "endtime" value, * the value of the "renew-till" value of the old, * the "starttime" of the new ticket plus the lifetime (endtime minus starttime) of the old ticket, i.e., the lifetime of the new ticket may not exceed that of the ticket being renewed [ adapted from KCLAR 3.3.3. ], and * an "endtime" determined by site policy on ticket lifetimes. When handling a TGS-REQ, a KDC MUST NOT issue a postdated ticket with a "starttime", "endtime", or "renew-till" time later than the "renew-till" time of the TGT. 8.3.4. Handling Transited Realms The KDC checks the ticket in a TGS-REQ against site policy, unless the "disable-transited-check" option is set in the TGS-REQ. If site policy permits the transit path in the TGS-REQ ticket, the KDC sets the "transited-policy-checked" flag in the issued ticket. If the "disable-transited-check" option is set, the issued ticket will have the "transited-policy-checked" flag cleared. 8.3.5. Address Processing The requested "addresses" in the KDC-REQ are copied into the issued ticket. If the "addresses" field is absent or empty in a TGS-REQ, the KDC copies addresses from the ticket in the TGS-REQ into the issued ticket, unless the either "forwarded" or "proxy" option is set. If the "forwarded" option is set, and the ticket in the TGS-REQ has its "forwardable" flag set, the KDC copies the addresses from the TGS-REQ, not the from TGS-REQ ticket, into the issued ticket. The KDC behaves similarly if the "proxy" option is set in the TGS-REQ and the "proxiable" flag is set in the ticket. The "proxy" option will not be honored on requests for additional ticket-granting tickets. Yu Expires: Jul 2005 [Page 50] Internet-Draft rfc1510ter-00 21 Jan 2005 8.3.6. Ticket Flag Processing Many kdc-options request that the KDC set a corresponding flag in the issued ticket. kdc-options marked with an asterisk (*) in the following table do not directly request the corresponding ticket flag and therefore require special handling. kdc-option | ticket flag affected ________________________|__________________________ forwardable | forwardable forwarded | forwarded proxiable | proxiable proxy | proxy allow-postdate | may-postdate postdated | postdated renewable | renewable requestanonymous | anonymous canonicalize | - disable-transited-check*| transited-policy-checked renewable-ok* | renewable enc-tkt-in-skey | - renew | - validate* | invalid forwarded The KDC sets the "forwarded" flag in the issued ticket if the "forwarded" option is set in the TGS-REQ and the "forwardable" flag is set in the TGS-REQ ticket. proxy The KDC sets the "proxy" flag in the issued ticket if the "proxy" option is set in the TGS-REQ and the "proxiable" flag is set in the TGS-REQ ticket. disable-transited-check The handling of the "disable-transited-check" kdc-option is described in Section 8.3.4. renewable-ok The handling of the "renewable-ok" kdc-option is described in Section 8.3.3.1. enc-tkt-in-skey This flag modifies ticket key selection to use the session key of an additional ticket included in the TGS-REQ, for the purpose of user-to-user authentication. Yu Expires: Jul 2005 [Page 51] Internet-Draft rfc1510ter-00 21 Jan 2005 validate If the "validate" kdc-option is set in a TGS-REQ, and the "starttime" has passed, the KDC will clear the "invalid" bit on the ticket before re-issuing it. 8.3.7. Key Selection Three keys are involved in creating a KDC-REP. The reply key encrypts the encrypted part of the KDC-REP. The session key is stored in the encrypted part of the ticket, and is also present in the encrypted part of the KDC-REP so that the client can retrieve it. The ticket key is used to encrypt the ticket. These keys all have initial values for a given exchange; pre-authentication and other extension mechanisms may change the value used for any of these keys. [ again, may need changes based on Sam's preauth draft ] 8.3.7.1. Reply Key and Session Key Selection The set of encryption types which the client will understand appears in the "etype" field of KDC-REQ-BODY-COMMON. The KDC limits the set of possible reply keys and the set of session key encryption types based on the "etype" field. For the AS exchange, the reply key is initially a long-term key of the client, limited to those encryption types listed in the "etype" field. The KDC SHOULD use the first valid strong "etype" for which an encryption key is available. For the TGS exchange, the reply key is initially the subsession key of the Authenticator. If the Authenticator subsesion key is absent, the reply key is initially the session key of the ticket used to authenticate the TGS-REQ. The session key is initially randomly generated, and has an encryption type which both the client and the server will understand. Typically, the KDC has prior knowledge of which encryption types the server will understand. It selects the first valid strong "etype" listed the request which the server also will understand. 8.3.7.2. Ticket Key Selection The ticket key is initially the long-term key of the service. The "enc-tkt-in-skey" option requests user-to-user authentication, where the ticket encryption key of the issued ticket is set equal to the session key of the additional ticket in the request. 8.4. KDC-REP The important parts of the KDC-REP are encrypted. Yu Expires: Jul 2005 [Page 52] Internet-Draft rfc1510ter-00 21 Jan 2005 EncASRepPart1510 ::= [APPLICATION 25] EncKDCRepPart1510 EncTGSRepPart1510 ::= [APPLICATION 26] EncKDCRepPart1510 EncASRepPartExt ::= [APPLICATION 32] EncKDCRepPartExt EncTGSRepPartExt ::= [APPLICATION 33] EncKDCRepPartExt EncKDCRepPartCom ::= SEQUENCE { key [0] EncryptionKey, last-req [1] LastReq, nonce [2] Nonce, key-expiration [3] KerberosTime OPTIONAL, flags [4] TicketFlags, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, srealm [9] Realm, sname [10] PrincipalName, caddr [11] HostAddresses OPTIONAL, ... } EncKDCRepPart1510 ::= SEQUENCE { COMPONENTS OF EncKDCRepPartCom } (WITH COMPONENTS { ..., srealm (RealmIA5), sname (PrincipalNameIA5), nonce UInt32 }) EncKdcRepPartExt ::= EncKDCRepPartCom (WITH COMPONENTS { ..., srealm (RealmExt), sname (PrincipalNameExt) }) Most of the fields of EncKDCRepPartCom are duplicates of the corresponding fields in the returned ticket. Yu Expires: Jul 2005 [Page 53] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (11 -- AS-REP.rfc1510 -- | 13 -- TGS.rfc1510 -- | 7 -- AS-REP.ext -- | 9 -- TGS-REP.ext -- ), padata [2] SEQUENCE OF PA-DATA OPTIONAL, crealm [3] Realm, cname [4] PrincipalName, ticket [5] Ticket, enc-part [6] EncryptedData { EncPart, { key-reply }, -- maybe reach into EncryptedData in AS-REP/TGS-REP -- definitions to apply constraints on key usages? { ku-EncASRepPart -- if AS-REP -- | ku-EncTGSRepPart-subkey -- if TGS-REP and -- using Authenticator -- session key -- | ku-EncTGSRepPart-sesskey -- if TGS-REP and using -- subsession key -- } }, ..., -- In extensible version, KDC signs original request -- to avoid replay attacks against client. req-cksum [7] ChecksumOf { CHOICE { as-req AS-REQ, tgs-req TGS-REQ }, { key-reply }, { ku-KDCRep-cksum }} OPTIONAL, lang-tag [8] LangTag OPTIONAL, ... } KDC-REP-1510 { EncPart } ::= SEQUENCE { COMPONENTS OF KDC-REP-COMMON { EncPart } } (WITH COMPONENTS { ..., msg-type (11 | 13), crealm (RealmIA5), cname (PrincipalNameIA5) }) Yu Expires: Jul 2005 [Page 54] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REP-EXT { EncPart } ::= KDC-REP-COMMON { EncPart } (WITH COMPONENTS { ..., msg-type (7 | 9), crealm (RealmExt), cname (PrincipalNameExt) }) req-cksum Signature of the original request using the reply key, to avoid replay attacks against the client, among other things. Only present in the extensible version of KDC-REP. AS-REP ::= CHOICE { rfc1510 [APPLICATION 11] KDC-REP-1510 { EncASRepPart1510 } (WITH COMPONENTS { ..., msg-type (11) }), ext [APPLICATION 7] Signed { [APPLICATION 7] KDC-REP-EXT { EncASRepPartExt }, { key-reply }, { ku-ASRep-cksum } } (WITH COMPONENTS { ..., msg-type (7) }) } TGS-REP ::= CHOICE { rfc1510 [APPLICATION 13] KDC-REP-1510 { EncTGSRepPart1510 } (WITH COMPONENTS { ..., msg-type (13) }), ext [APPLICATION 9] Signed { [APPLICATION 9] KDC-REP-EXT { EncTGSRepPartExt }, { key-reply }, { ku-TGSRep-cksum } } (WITH COMPONENTS { ..., msg-type (9) }) } The extensible versions of AS-REQ and TGS-REQ are signed with the reply key, to prevent an attacker from performing a delayed denial- of-service attack by substituting the ticket. 8.5. Reply Validation [ signature verification ] 8.6. IP Transports [ KCLAR 7.2 ] Kerberos defines two IP transport mechanisms for the credentials acquisition protocol: UDP/IP and TCP/IP. Yu Expires: Jul 2005 [Page 55] Internet-Draft rfc1510ter-00 21 Jan 2005 8.6.1. UDP/IP transport Kerberos servers (KDCs) supporting IP transports MUST accept UDP requests and SHOULD listen for such requests on port 88 (decimal) unless specifically configured to listen on an alternative UDP port. Alternate ports MAY be used when running multiple KDCs for multiple realms on the same host. Kerberos clients supporting IP transports SHOULD support the sending of UDP requests. Clients SHOULD use KDC discovery (Section 8.6.3) to identify the IP address and port to which they will send their request. When contacting a KDC for a KRB_KDC_REQ request using UDP/IP transport, the client shall send a UDP datagram containing only an encoding of the request to the KDC. The KDC will respond with a reply datagram containing only an encoding of the reply message (either a KRB-ERROR or a KDC-REP) to the sending port at the sender's IP address. The response to a request made through UDP/IP transport MUST also use UDP/IP transport. If the response can not be handled using UDP (for example because it is too large), the KDC MUST return "krb- err-response-too-big", forcing the client to retry the request using the TCP transport. 8.6.2. TCP/IP transport Kerberos servers (KDCs) supporting IP transports MUST accept TCP requests and SHOULD listen for such requests on port 88 (decimal) unless specifically configured to listen on an alternate TCP port. Alternate ports MAY be used when running multiple KDCs for multiple realms on the same host. Clients MUST support the sending of TCP requests, but MAY choose to initially try a request using the UDP transport. Clients SHOULD use KDC discovery (Section 8.6.3) to identify the IP address and port to which they will send their request. Implementation note: Some extensions to the Kerberos protocol will not succeed if any client or KDC not supporting the TCP transport is involved. Implementations of RFC 1510 were not required to support TCP/IP transports. When the KDC-REQ message is sent to the KDC over a TCP stream, the response (KDC-REP or KRB-ERROR message) MUST be returned to the client on the same TCP stream that was established for the request. The KDC MAY close the TCP stream after sending a response, but MAY leave the stream open for a reasonable period of time if it expects a followup. Care must be taken in managing TCP/IP connections on the KDC to prevent denial of service attacks based on the number of open TCP/IP connections. Yu Expires: Jul 2005 [Page 56] Internet-Draft rfc1510ter-00 21 Jan 2005 The client MUST be prepared to have the stream closed by the KDC at anytime after the receipt of a response. A stream closure SHOULD NOT be treated as a fatal error. Instead, if multiple exchanges are required (e.g., certain forms of pre-authentication) the client may need to establish a new connection when it is ready to send subsequent messages. A client MAY close the stream after receiving a response, and SHOULD close the stream if it does not expect to send followup messages. A client MAY send multiple requests before receiving responses, though it must be prepared to handle the connection being closed after the first response. Each request (KDC-REQ) and response (KDC-REP or KRB-ERROR) sent over the TCP stream is preceded by the length of the request as 4 octets in network byte order. The high bit of the length is reserved for future expansion and MUST currently be set to zero. If a KDC that does not understand how to interpret a set high bit of the length encoding receives a request with the high order bit of the length set, it MUST return a KRB-ERROR message with the error "krb-err- field-toolong" and MUST close the TCP stream. If multiple requests are sent over a single TCP connection, and the KDC sends multiple responses, the KDC is not required to send the responses in the order of the corresponding requests. This may permit some implementations to send each response as soon as it is ready even if earlier requests are still being processed (for example, waiting for a response from an external device or database). 8.6.3. KDC Discovery on IP Networks Kerberos client implementations MUST provide a means for the client to determine the location of the Kerberos Key Distribution Centers (KDCs). Traditionally, Kerberos implementations have stored such configuration information in a file on each client machine. Experience has shown this method of storing configuration information presents problems with out-of-date information and scaling problems, especially when using cross-realm authentication. This section describes a method for using the Domain Name System [RFC 1035] for storing KDC location information. 8.6.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names In Kerberos, realm names are case sensitive. While it is strongly encouraged that all realm names be all upper case this recommendation has not been adopted by all sites. Some sites use all lower case names and other use mixed case. DNS, on the other hand, is case insensitive for queries. Since the realm names "MYREALM", "myrealm", and "MyRealm" are all different, but resolve the same in the domain name system, it is necessary that only one of the possible combinations of upper and lower case characters be used in realm Yu Expires: Jul 2005 [Page 57] Internet-Draft rfc1510ter-00 21 Jan 2005 names. 8.6.3.2. DNS SRV records for KDC location KDC location information is to be stored using the DNS SRV RR [RFC 2782]. The format of this RR is as follows: _Service._Proto.Realm TTL Class SRV Priority Weight Port Target The Service name for Kerberos is always "kerberos". The Proto can be one of "udp", "tcp". If these SRV records are to be used, both "udp" and "tcp" records MUST be specified for all KDC deployments. The Realm is the Kerberos realm that this record corresponds to. The realm MUST be a domain style realm name. TTL, Class, SRV, Priority, Weight, and Target have the standard meaning as defined in RFC 2782. As per RFC 2782 the Port number used for "_udp" and "_tcp" SRV records SHOULD be the value assigned to "kerberos" by the Internet Assigned Number Authority: 88 (decimal) unless the KDC is configured to listen on an alternate TCP port. Implementation note: Many existing client implementations do not support KDC Discovery and are configured to send requests to the IANA assigned port (88 decimal), so it is strongly recommended that KDCs be configured to listen on that port. 8.6.3.3. KDC Discovery for Domain Style Realm Names on IP Networks These are DNS records for a Kerberos realm EXAMPLE.COM. It has two Kerberos servers, kdc1.example.com and kdc2.example.com. Queries should be directed to kdc1.example.com first as per the specified priority. Weights are not used in these sample records. _kerberos._udp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com. _kerberos._udp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com. _kerberos._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com. _kerberos._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com. 9. Errors The KRB-ERROR message is returned by the KDC if an error occurs during credentials acquisition. It may also be returned by an application server if an error occurs during authentication. Yu Expires: Jul 2005 [Page 58] Internet-Draft rfc1510ter-00 21 Jan 2005 ErrCode ::= Int32 KRB-ERROR ::= CHOICE { rfc1510 [APPLICATION 30] KRB-ERROR-1510, ext [APPLICATION 23] Signed { KRB-ERROR-EXT, { ku-KrbError-cksum } } } The extensible KRB-ERROR is only signed if there has been a key negotiated with its recipient. KRB-ERROR messages sent in response to AS-REQ messages will probably not be signed unless a preauthentication mechanism has negotiated a key. (Signing using a client's long-term key can expose ciphertext to dictionary attacks.) The parts of a KRB-ERROR common to both the backwards-compatibility and the extensibile messages are KRB-ERROR-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (30 | 23), ctime [2] KerberosTime OPTIONAL, cusec [3] Microseconds OPTIONAL, stime [4] KerberosTime, susec [5] Microseconds, error-code [6] ErrCode, crealm [7] Realm OPTIONAL, cname [8] PrincipalName OPTIONAL, realm [9] Realm -- Correct realm --, sname [10] PrincipalName -- Correct name --, e-text [11] KerberosString OPTIONAL, e-data [12] OCTET STRING OPTIONAL, ..., typed-data [13] TYPED-DATA OPTIONAL, nonce [14] Nonce OPTIONAL, lang-tag [15] LangTag OPTIONAL, ... } KRB-ERROR-1510 ::= SEQUENCE { COMPONENTS OF KRB-ERROR-COMMON } (WITH COMPONENTS { ..., msg-type (30) }) KRB-ERROR-EXT ::= [APPLICATION 23] KRB-ERROR-COMMON (WITH COMPONENTS { ..., msg-type (23) }) Yu Expires: Jul 2005 [Page 59] Internet-Draft rfc1510ter-00 21 Jan 2005 ctime, cusec Client's time, if known from a KDC-REQ or AP-REQ. stime, susec KDC or application server's current time. error-code Numeric error code designating the error. crealm, cname Client's realm and name, if known. realm, sname server's realm and name. [ XXX what if these aren't known? ] e-text Human-readable text providing additional details for the error. e-data This field contains additional data about the error for use by the client to help it recover from or handle the error. If the "error-code" is "kdc-err-preauth-required", then the e-data field will contain an encoding of a sequence of padata fields, each corresponding to an acceptable pre-authentication method and optionally containing data for the method: METHOD-DATA ::= SEQUENCE OF PA-DATA For error codes defined in this document other than "kdc-err- preauth-required", the format and contents of the e-data field are implementation-defined. Similarly, for future error codes, the format and contents of the e-data field are implementation- defined unless specified. lang-tag Indicates the language of the message in the "e-text" field. It is only present in the extensible KRB-ERROR. nonce is the nonce from a KDC-REQ. It is only present in the extensible KRB-ERROR. typed-data TYPED-DATA is a typed hole allowing for additional data to be returned in error conditions, since "e-data" is insufficiently flexible for some purposes. TYPED-DATA is only present in the extensible KRB-ERROR. Yu Expires: Jul 2005 [Page 60] Internet-Draft rfc1510ter-00 21 Jan 2005 TDType ::= TH-id TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { data-type [0] TDType, data-value [1] OCTET STRING OPTIONAL } 10. Session Key Exchange Session key exchange consists of the AP-REQ and AP-REP messages. The client sends the AP-REQ message, and the service responds with an AP-REP message if mutual authentication is desired. Following session key exchange, the client and service share a secret session key, or possibly a subsesion key, which can be used to protect further communications. Additionally, the session key exchange process can establish initial sequence numbers which the client and service can use to detect replayed messages. 10.1. AP-REQ An AP-REQ message contains a ticket and a authenticator. The authenticator is ciphertext encrypted with the session key contained in the ticket. The plaintext contents of the authenticator are: -- plaintext of authenticator Authenticator ::= [APPLICATION 2] SEQUENCE { authenticator-vno [0] INTEGER (5), crealm [1] Realm, cname [2] PrincipalName, cksum [3] Checksum {{ key-session }, { ku-Authenticator-cksum | ku-pa-TGSReq-cksum }} OPTIONAL, cusec [4] Microseconds, ctime [5] KerberosTime, subkey [6] EncryptionKey OPTIONAL, seq-number [7] SeqNum OPTIONAL, authorization-data [8] AuthorizationData OPTIONAL } The common parts between the RFC 1510 and the extensible versions of the AP-REQ are: Yu Expires: Jul 2005 [Page 61] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REQ-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (14 | 18), ap-options [2] APOptions, ticket [3] Ticket, authenticator [4] EncryptedData { Authenticator, { key-session }, { ku-APReq-authenticator | ku-pa-TGSReq-authenticator } }, ..., extensions [5] ApReqExtensions OPTIONAL, lang-tag [6] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } The complete definition of AP-REQ is: AP-REQ ::= CHOICE { rfc1510 [APPLICATION 14] AP-REQ-1510, ext [APPLICATION 18] Signed { AP-REQ-EXT, { key-session }, { ku-APReq-cksum } } } AP-REQ-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (14 | 18), ap-options [2] APOptions, ticket [3] Ticket, authenticator [4] EncryptedData { Authenticator, { key-session }, { ku-APReq-authenticator | ku-pa-TGSReq-authenticator } }, ..., extensions [5] ApReqExtensions OPTIONAL, lang-tag [6] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } Yu Expires: Jul 2005 [Page 62] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REQ-1510 ::= SEQUENCE { COMPONENTS OF AP-REQ-COMMON } (WITH COMPONENTS { ..., msg-type (14), authenticator (EncryptedData { Authenticator (WITH COMPONENTS { ..., crealm (RealmIA5), cname (PrincipalNameIA5), seqnum (UInt32) }), { key-session }, { ku-APReq-authenticator }}) }) AP-REQ-EXT ::= AP-REQ-COMMON (WITH COMPONENTS { ..., msg-type (18), -- The following constraints on Authenticator assume that -- we want to restrict the use of AP-REQ-EXT with TicketExt -- only, since that is the only way we can enforce UTF-8. authenticator (EncryptedData { Authenticator (WITH COMPONENTS { ..., crealm (RealmExt), cname (PrincipalNameExt), authorization-data (SIZE (1..MAX)) }), { key-session }, { ku-APReq-authenticator }}) }) APOptions ::= KerberosFlags { APOptionsBits } APOptionsBits ::= BIT STRING { reserved (0), use-session-key (1), mutual-required (2) } 10.2. AP-REP An AP-REP message provides mutual authentication of the service to the client. EncAPRepPart ::= CHOICE { rfc1510 [APPLICATION 27] EncAPRepPart1510, ext [APPLICATION 31] EncAPRepPartExt } Yu Expires: Jul 2005 [Page 63] Internet-Draft rfc1510ter-00 21 Jan 2005 EncAPRepPartCom ::= SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] SeqNum OPTIONAL, ..., authorization-data [4] AuthorizationData OPTIONAL, ... } EncAPRepPart1510 ::= SEQUENCE { COMPONENTS OF ENCAPRepPartCom } (WITH COMPONENTS { ..., seq-number (UInt32), authorization-data ABSENT }) EncAPRepPartExt ::= EncAPRepPartCom AP-REP ::= CHOICE { rfc1510 [APPLICATION 15] AP-REP-1510, ext [APPLICATION 19] Signed { AP-REP-EXT, { key-session | key-subsession }, { ku-APRep-cksum }} } AP-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (15 | 19), enc-part [2] EncryptedData { EncPart, { key-session | key-subsession }, { ku-EncAPRepPart }}, ..., extensions [3] ApRepExtensions OPTIONAL, ... } AP-REP-1510 ::= SEQUENCE { COMPONENTS OF AP-REP-COMMON { EncAPRepPart1510 } } (WITH COMPONENTS { ..., msg-type (15) }) Yu Expires: Jul 2005 [Page 64] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REP-EXT ::= [APPLICATION 19] AP-REP-COMMON { EncAPRepPartExt } (WITH COMPONENTS { ..., msg-type (19) }) 11. Session Key Use Once a session key has been established, the client and server can use several kinds of messages to securely transmit data. KRB-SAFE provides integrity protection for application data, while KRB-PRIV provides confidentiality along with integrity protection. The KRB- CRED message provides a means of securely forwarding credentials from the client host to the server host. 11.1. KRB-SAFE The KRB-SAFE message provides integrity protection for an included cleartext message. -- Do we chew up another tag for KRB-SAFE-EXT? That would -- allow us to make safe-body optional, allowing for a GSS-MIC -- sort of message. KRB-SAFE ::= [APPLICATION 20] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (20), safe-body [2] KRB-SAFE-BODY, cksum [3] ChecksumOf { KRB-SAFE-BODY, { key-session | key-subsession }, { ku-KrbSafe-cksum }}, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } KRB-SAFE-BODY ::= SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress, r-address [5] HostAddress OPTIONAL, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } 11.2. KRB-PRIV The KRB-PRIV message provides confidentiality and integrity protection. Yu Expires: Jul 2005 [Page 65] Internet-Draft rfc1510ter-00 21 Jan 2005 KRB-PRIV ::= [APPLICATION 21] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (21), enc-part [3] EncryptedData { EncKrbPrivPart, { key-session | key-subsession }, { ku-EncKrbPrivPart }}, ... } EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress -- sender's addr --, r-address [5] HostAddress OPTIONAL -- recip's addr --, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } 11.3. KRB-CRED The KRB-CRED message provides a means of securely transferring credentials from the client to the service. KRB-CRED ::= CHOICE { rfc1510 [APPLICATION 22] KRB-CRED-1510, ext [APPLICATION 24] Signed { KRB-CRED-EXT, { key-session | key-subsession }, { ku-KrbCred-cksum }} } KRB-CRED-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (22 | 24), tickets [2] SEQUENCE OF Ticket, enc-part [3] EncryptedData { EncKrbCredPart, { key-session | key-subsession }, { ku-EncKrbCredPart }}, ... } KRB-CRED-1510 ::= SEQUENCE { COMPONENTS OF KRB-CRED-COMMON } (WITH COMPONENTS { ..., msg-type (22) }) Yu Expires: Jul 2005 [Page 66] Internet-Draft rfc1510ter-00 21 Jan 2005 KRB-CRED-EXT ::= [APPLICATION 24] KRB-CRED-COMMON (WITH COMPONENTS { ..., msg-type (24) }) EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { ticket-info [0] SEQUENCE OF KrbCredInfo, nonce [1] Nonce OPTIONAL, timestamp [2] KerberosTime OPTIONAL, usec [3] Microseconds OPTIONAL, s-address [4] HostAddress OPTIONAL, r-address [5] HostAddress OPTIONAL } KrbCredInfo ::= SEQUENCE { key [0] EncryptionKey, prealm [1] Realm OPTIONAL, pname [2] PrincipalName OPTIONAL, flags [3] TicketFlags OPTIONAL, authtime [4] KerberosTime OPTIONAL, starttime [5] KerberosTime OPTIONAL, endtime [6] KerberosTime OPTIONAL, renew-till [7] KerberosTime OPTIONAL, srealm [8] Realm OPTIONAL, sname [9] PrincipalName OPTIONAL, caddr [10] HostAddresses OPTIONAL } 12. Security Considerations 12.1. Time Synchronization Time synchronization between the KDC and application servers is necessary to prevent acceptance of expired tickets. Time synchronization is needed between application servers and clients to prevent replay attacks if a replay cache is being used. If negotiated subsession keys are used to encrypt application data, replay caches may not be necessary. 12.2. Replays 12.3. Principal Name Reuse See Section 5.3. 12.4. Password Guessing Yu Expires: Jul 2005 [Page 67] Internet-Draft rfc1510ter-00 21 Jan 2005 12.5. Forward Secrecy [from KCLAR 10.; needs some rewriting] The Kerberos protocol in its basic form does not provide perfect forward secrecy for communications. If traffic has been recorded by an eavesdropper, then messages encrypted using the KRB-PRIV message, or messages encrypted using application-specific encryption under keys exchanged using Kerberos can be decrypted if any of the user's, application server's, or KDC's key is subsequently discovered. This is because the session key used to encrypt such messages is transmitted over the network encrypted in the key of the application server, and also encrypted under the session key from the user's ticket-granting ticket when returned to the user in the TGS-REP message. The session key from the ticket-granting ticket was sent to the user in the AS-REP message encrypted in the user's secret key, and embedded in the ticket-granting ticket, which was encrypted in the key of the KDC. Application requiring perfect forward secrecy must exchange keys through mechanisms that provide such assurance, but may use Kerberos for authentication of the encrypted channel established through such other means. 12.6. Authorization As an authentication service, Kerberos provides a means of verifying the identity of principals on a network. Kerberos does not, by itself, provide authorization. Applications SHOULD NOT accept the mere issuance of a service ticket by the Kerberos server as granting authority to use the service. 12.7. Login Authentication Some applications, particularly those which provide login access when provided with a password, SHOULD NOT treat successful acquisition of credentials as sufficient proof of the user's identity. An attacker posing as a user could generate an illegitimate KDC-REP message which decrypts properly. To authenticate a user logging on to a local system, the credentials obtained SHOULD be used in a TGS exchange to obtain credentials for a local service. Successful use of those credentials to authenticate to the local service assures that the initially obtained credentials are from a valid KDC. 13. IANA Considerations [ needs more work ] Each use of Int32 in this document defines a number space. [ XXX enumerate these ] Negative numbers are reserved for private use; local and experimental extensions should use these values. Zero is reserved and may not be assigned. Yu Expires: Jul 2005 [Page 68] Internet-Draft rfc1510ter-00 21 Jan 2005 Typed hole contents may be identified by either Int32 values or by RELATIVE-OID values. Since RELATIVE-OIDs define a hierarchical namespace, assignments to the top level of the RELATIVE-OID space may be made on a first-come, first-served basis. 14. Acknowledgments Much of the text here is adapted from draft-ietf-krb-wg-kerberos- clarifications-07. The description of the general form of the extensibility framework was derived from text by Sam Hartman. Appendices A. ASN.1 Module (Normative) KerberosV5Spec3 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec3(4) } DEFINITIONS EXPLICIT TAGS ::= BEGIN -- OID arc for KerberosV5 -- -- This OID may be used to identify Kerberos protocol messages -- encapsulated in other protocols. -- -- This OID also designates the OID arc for KerberosV5-related -- OIDs. -- -- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its -- OID. id-krb5 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) } Yu Expires: Jul 2005 [Page 69] Internet-Draft rfc1510ter-00 21 Jan 2005 -- top-level type -- -- Applications should not directly reference any types -- other than KRB-PDU and its component types. -- KRB-PDU ::= CHOICE { ticket Ticket, as-req AS-REQ, as-rep AS-REP, tgs-req TGS-REQ, tgs-rep TGS-REP, ap-req AP-REQ, ap-rep AP-REP, krb-safe KRB-SAFE, krb-priv KRB-PRIV, krb-cred KRB-CRED, tgt-req TGT-REQ, krb-error KRB-ERROR, ... } -- -- *** basic types -- -- signed values representable in 32 bits -- -- These are often used as assigned numbers for various things. Int32 ::= INTEGER (-2147483648..2147483647) -- Typed hole identifiers TH-id ::= CHOICE { int32 Int32, rel-oid RELATIVE-OID } -- unsigned 32 bit values UInt32 ::= INTEGER (0..4294967295) -- unsigned 64 bit values UInt64 ::= INTEGER (0..18446744073709551615) -- sequence numbers SeqNum ::= UInt64 Yu Expires: Jul 2005 [Page 70] Internet-Draft rfc1510ter-00 21 Jan 2005 -- nonces Nonce ::= UInt64 -- microseconds Microseconds ::= INTEGER (0..999999) KerberosTime ::= GeneralizedTime (CONSTRAINED BY { -- MUST NOT include fractional seconds }) -- used for names and for error messages KerberosString ::= CHOICE { ia5 GeneralString (IA5String), utf8 UTF8String, ... -- no extension may be sent -- to an rfc1510 implementation -- } -- IA5 choice only; useful for constraints KerberosStringIA5 ::= KerberosString (WITH COMPONENTS { ia5 PRESENT }) -- IA5 excluded; useful for constraints KerberosStringExt ::= KerberosString (WITH COMPONENTS { ia5 ABSENT }) -- used for language tags LangTag ::= PrintableString (FROM ("A".."Z" | "a".."z" | "0".."9" | "-")) Yu Expires: Jul 2005 [Page 71] Internet-Draft rfc1510ter-00 21 Jan 2005 -- assigned numbers for name types (used in principal names) NameType ::= Int32 -- Name type not known nt-unknown NameType ::= 0 -- Just the name of the principal as in DCE, or for users nt-principal NameType ::= 1 -- Service and other unique instance (krbtgt) nt-srv-inst NameType ::= 2 -- Service with host name as instance (telnet, rcommands) nt-srv-hst NameType ::= 3 -- Service with host as remaining components nt-srv-xhst NameType ::= 4 -- Unique ID nt-uid NameType ::= 5 -- Encoded X.509 Distingished name [RFC 2253] nt-x500-principal NameType ::= 6 -- Name in form of SMTP email name (e.g. user@foo.com) nt-smtp-name NameType ::= 7 -- Enterprise name - may be mapped to principal name nt-enterprise NameType ::= 10 PrincipalName ::= SEQUENCE { name-type [0] NameType, -- May have zero elements, or individual elements may be -- zero-length, but this is NOT RECOMMENDED. name-string [1] SEQUENCE OF KerberosString } -- IA5 only PrincipalNameIA5 ::= PrincipalName (WITH COMPONENTS { ..., name-string (WITH COMPONENT (KerberosStringIA5)) }) -- IA5 excluded PrincipalNameExt ::= PrincpalName (WITH COMPONENTS { ..., name-string (WITH COMPONENT (KerberosStringExt)) }) Yu Expires: Jul 2005 [Page 72] Internet-Draft rfc1510ter-00 21 Jan 2005 Realm ::= KerberosString -- IA5 only RealmIA5 ::= Realm (KerberosStringIA5) -- IA5 excluded RealmExt ::= Realm (KerberosStringExt) KerberosFlags { NamedBits } ::= BIT STRING (SIZE (32..MAX)) (CONSTRAINED BY { -- MUST be a valid value of -- NamedBits -- but if the value to be sent would be truncated to shorter -- than 32 bits according to DER, the value MUST be padded -- with trailing zero bits to 32 bits. Otherwise, no -- trailing zero bits may be present. }) AddrType ::= Int32 HostAddress ::= SEQUENCE { addr-type [0] AddrType, address [1] OCTET STRING } -- NOTE: HostAddresses is always used as an OPTIONAL field and -- should not be a zero-length SEQUENCE OF. -- -- The extensible messages explicitly constrain this to be -- non-empty. HostAddresses ::= SEQUENCE OF HostAddress -- -- *** crypto-related types and assignments -- Yu Expires: Jul 2005 [Page 73] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Assigned numbers denoting encryption mechanisms. EType ::= Int32 -- assigned numbers for encryption schemes et-des-cbc-crc EType ::= 1 et-des-cbc-md4 EType ::= 2 et-des-cbc-md5 EType ::= 3 -- [reserved] 4 et-des3-cbc-md5 EType ::= 5 -- [reserved] 6 et-des3-cbc-sha1 EType ::= 7 et-dsaWithSHA1-CmsOID EType ::= 9 et-md5WithRSAEncryption-CmsOID EType ::= 10 et-sha1WithRSAEncryption-CmsOID EType ::= 11 et-rc2CBC-EnvOID EType ::= 12 et-rsaEncryption-EnvOID EType ::= 13 et-rsaES-OAEP-ENV-OID EType ::= 14 et-des-ede3-cbc-Env-OID EType ::= 15 et-des3-cbc-sha1-kd EType ::= 16 -- AES et-aes128-cts-hmac-sha1-96 EType ::= 17 -- AES et-aes256-cts-hmac-sha1-96 EType ::= 18 -- Microsoft et-rc4-hmac EType ::= 23 -- Microsoft et-rc4-hmac-exp EType ::= 24 -- opaque; PacketCable et-subkey-keymaterial EType ::= 65 Yu Expires: Jul 2005 [Page 74] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Assigned numbers denoting key usages. KeyUsage ::= UInt32 -- -- Actual identifier names are provisional and subject to -- change. -- ku-pa-enc-ts KeyUsage ::= 1 ku-Ticket KeyUsage ::= 2 ku-EncASRepPart KeyUsage ::= 3 ku-TGSReqAuthData-sesskey KeyUsage ::= 4 ku-TGSReqAuthData-subkey KeyUsage ::= 5 ku-pa-TGSReq-cksum KeyUsage ::= 6 ku-pa-TGSReq-authenticator KeyUsage ::= 7 ku-EncTGSRepPart-sesskey KeyUsage ::= 8 ku-EncTGSRepPart-subkey KeyUsage ::= 9 ku-Authenticator-cksum KeyUsage ::= 10 ku-APReq-authenticator KeyUsage ::= 11 ku-EncAPRepPart KeyUsage ::= 12 ku-EncKrbPrivPart KeyUsage ::= 13 ku-EncKrbCredPart KeyUsage ::= 14 ku-KrbSafe-cksum KeyUsage ::= 15 ku-ad-KDCIssued-cksum KeyUsage ::= 19 -- The following numbers are provisional... -- conflicts may exist elsewhere. ku-Ticket-cksum KeyUsage ::= 25 ku-ASReq-cksum KeyUsage ::= 26 ku-TGSReq-cksum KeyUsage ::= 27 ku-ASRep-cksum KeyUsage ::= 28 ku-TGSRep-cksum KeyUsage ::= 29 ku-APReq-cksum KeyUsage ::= 30 ku-APRep-cksum KeyUsage ::= 31 ku-KrbCred-cksum KeyUsage ::= 32 ku-KrbError-cksum KeyUsage ::= 33 ku-KDCRep-cksum KeyUsage ::= 34 Yu Expires: Jul 2005 [Page 75] Internet-Draft rfc1510ter-00 21 Jan 2005 -- KeyToUse identifies which key is to be used to encrypt or -- sign a given value. -- -- Values of KeyToUse are never actually transmitted over the -- wire, or even used directly by the implementation in any -- way, as key usages are; it exists primarily to identify -- which key gets used for what purpose. Thus, the specific -- numeric values associated with this type are irrelevant. KeyToUse ::= ENUMERATED { -- unspecified key-unspecified, -- server long term key key-server, -- client long term key key-client, -- key selected by KDC for encryption of a KDC-REP key-kdc-rep, -- session key from ticket key-session, -- subsession key negotiated via AP-REQ/AP-REP key-subsession, ... } EncryptionKey ::= SEQUENCE { keytype [0] EType, keyvalue [1] OCTET STRING } Yu Expires: Jul 2005 [Page 76] Internet-Draft rfc1510ter-00 21 Jan 2005 -- "Type" specifies which ASN.1 type is encrypted to the -- ciphertext in the EncryptedData. "Keys" specifies a set of -- keys of which one key may be used to encrypt the data. -- "KeyUsages" specifies a set of key usages, one of which may -- be used to encrypt. -- -- None of the parameters is transmitted over the wire. EncryptedData { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { etype [0] EType, kvno [1] UInt32 OPTIONAL, cipher [2] OCTET STRING (CONSTRAINED BY { -- must be encryption of -- OCTET STRING (CONTAINING Type), -- with one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }), ... } CksumType ::= Int32 -- The parameters specify which key to use to produce the -- signature, as well as which key usage to use. The -- parameters are not actually sent over the wire. Checksum { KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksumtype [0] CksumType, checksum [1] OCTET STRING (CONSTRAINED BY { -- signed using one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }) } Yu Expires: Jul 2005 [Page 77] Internet-Draft rfc1510ter-00 21 Jan 2005 -- a Checksum that must contain the checksum -- of a particular type ChecksumOf { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS { ..., checksum (CONSTRAINED BY { -- must be checksum of -- OCTET STRING (CONTAINING Type) }) }) -- parameterized type for wrapping authenticated plaintext Signed { InnerType, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksum [0] ChecksumOf { InnerType, Keys, KeyUsages } OPTIONAL, inner [1] InnerType, ... } -- -- *** Tickets -- Ticket ::= CHOICE { rfc1510 [APPLICATION 1] Ticket1510, ext [APPLICATION 4] Signed { TicketExt, { key-server }, { ku-Ticket-cksum } } } -- takes a parameter specifying which type gets encrypted TicketCommon { EncPart } ::= SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData { EncPart, { key-server }, { ku-Ticket } }, ..., extensions [4] TicketExtensions OPTIONAL, ... } Yu Expires: Jul 2005 [Page 78] Internet-Draft rfc1510ter-00 21 Jan 2005 Ticket1510 ::= SEQUENCE { COMPONENTS OF TicketCommon { EncTicketPart1510 } } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings realm (RealmIA5), sname (PrincipalNameIA5) }) -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. TicketExt ::= [APPLICATION 4] TicketCommon { EncTicketPartExt } (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings realm (RealmExt), sname (PrincipalNameExt) }) -- Encrypted part of ticket EncTicketPart ::= CHOICE { rfc1510 [APPLICATION 3] EncTicketPart1510, ext [APPLICATION 5] EncTicketPartExt } EncTicketPartCommon ::= SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL, ... } Yu Expires: Jul 2005 [Page 79] Internet-Draft rfc1510ter-00 21 Jan 2005 EncTicketPart1510 ::= SEQUENCE { COMPONENTS OF EncTicketPartCommon } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings crealm (RealmIA5), cname (PrincipalNameIA5) }) EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings crealm (RealmExt), cname (PrincipalNameExt), -- explicitly constrain caddr to be non-empty if present caddr (SIZE (1..MAX)), -- forbid empty authorization-data encodings authorization-data (SIZE (1..MAX)) }) -- -- *** Authorization Data -- ADType ::= TH-id AuthorizationData ::= SEQUENCE OF SEQUENCE { ad-type [0] ADType, ad-data [1] OCTET STRING } ad-if-relevant ADType ::= int32 : 1 -- Encapsulates another AuthorizationData. -- Intended for application servers; receiving application servers -- MAY ignore. AD-IF-RELEVANT ::= AuthorizationData Yu Expires: Jul 2005 [Page 80] Internet-Draft rfc1510ter-00 21 Jan 2005 -- KDC-issued privilege attributes ad-kdcissued ADType ::= int32 : 4 AD-KDCIssued ::= SEQUENCE { ad-checksum [0] ChecksumOf { AuthorizationData, { key-session }, { ku-ad-KDCIssued-cksum }}, i-realm [1] Realm OPTIONAL, i-sname [2] PrincipalName OPTIONAL, elements [3] AuthorizationData } ad-and-or ADType ::= int32 : 5 AD-AND-OR ::= SEQUENCE { condition-count [0] INTEGER, elements [1] AuthorizationData } -- KDCs MUST interpret any AuthorizationData wrapped in this. ad-mandatory-for-kdc ADType ::= int32 : 8 AD-MANDATORY-FOR-KDC ::= AuthorizationData TrType ::= TH-id -- must be registered -- encoded Transited field TransitedEncoding ::= SEQUENCE { tr-type [0] TrType, contents [1] OCTET STRING } TEType ::= TH-id -- ticket extensions: for TicketExt only TicketExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { te-type [0] TEType, te-data [1] OCTET STRING } Yu Expires: Jul 2005 [Page 81] Internet-Draft rfc1510ter-00 21 Jan 2005 TicketFlags ::= KerberosFlags { TicketFlagsBits } TicketFlagsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), may-postdate (5), postdated (6), invalid (7), renewable (8), initial (9), pre-authent (10), hw-authent (11), transited-policy-checked (12), ok-as-delegate (13), anonymous (14), cksummed-ticket (15) } -- -- *** KDC protocol -- Yu Expires: Jul 2005 [Page 82] Internet-Draft rfc1510ter-00 21 Jan 2005 AS-REQ ::= CHOICE { rfc1510 [APPLICATION 10] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (10), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }), ext [APPLICATION 6] Signed { -- APPLICATION tag goes inside Signed{} as well as -- outside, to prevent possible substitution attacks. [APPLICATION 6] KDC-REQ-EXT, -- not sure this is correct key to use; do we even want -- to sign AS-REQ? { key-client }, { ku-ASReq-cksum } } (WITH COMPONENTS { ..., msg-type (6), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }) } TGS-REQ ::= CHOICE { rfc1510 [APPLICATION 12] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (12), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }), ext [APPLICATION 8] Signed { -- APPLICATION tag goes inside Signed{} as well as -- outside, to prevent possible substitution attacks. [APPLICATION 8] KDC-REQ-EXT, { key-session }, { ku-TGSReq-cksum } } (WITH COMPONENTS { ..., msg-type (8), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }) } Yu Expires: Jul 2005 [Page 83] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REQ-COMMON ::= SEQUENCE { -- NOTE: first tag is [1], not [0] pvno [1] INTEGER (5), msg-type [2] INTEGER (10 -- AS-REQ.rfc1510 -- | 12 -- TGS-REQ.rfc1510 -- | 6 -- AS-REQ.ext -- | 8 -- TGS-REQ.ext -- ), padata [3] SEQUENCE OF PA-DATA OPTIONAL -- NOTE: not empty } KDC-REQ-1510 ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-1510 } (WITH COMPONENTS { ..., msg-type (10 | 12) }) -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. KDC-REQ-EXT ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-EXT, ... } (WITH COMPONENTS { ..., msg-type (6 | 8), padata (SIZE (1..MAX)) }) Yu Expires: Jul 2005 [Page 84] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REQ-BODY-COMMON ::= SEQUENCE { kdc-options [0] KDCOptions, cname [1] PrincipalName OPTIONAL -- Used only in AS-REQ --, realm [2] Realm -- Server's realm; also client's in AS-REQ --, sname [3] PrincipalName OPTIONAL, from [4] KerberosTime OPTIONAL, till [5] KerberosTime OPTIONAL -- was required in rfc1510; -- still required for compat versions -- of messages --, rtime [6] KerberosTime OPTIONAL, nonce [7] Nonce, etype [8] SEQUENCE OF EType -- in preference order --, addresses [9] HostAddresses OPTIONAL, enc-authorization-data [10] EncryptedData { AuthorizationData, { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } } OPTIONAL, additional-tickets [11] SEQUENCE OF Ticket OPTIONAL -- NOTE: not empty --, ... lang-tags [5] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } KDC-REQ-BODY-1510 ::= SEQUENCE { COMPONENTS OF KDC-REQ-BODY-COMMON } (WITH COMPONENTS { ..., cname (PrincipalNameIA5), realm (RealmIA5), sname (PrincipalNameIA5), till PRESENT, nonce (UInt32) }) Yu Expires: Jul 2005 [Page 85] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REQ-BODY-EXT ::= KDC-REQ-BODY-COMMON (WITH COMPONENTS { ..., cname (PrincipalNameExt), realm (RealmExt), sname (PrincipalNameExt), addresses (SIZE (1..MAX)), enc-authorization-data (EncryptedData { AuthorizationData (SIZE (1..MAX)), { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } }), additional-tickets (SIZE (1..MAX)) }) KDCOptions ::= KerberosFlags { KDCOptionsBits } KDCOptionsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), allow-postdate (5), postdated (6), unused7 (7), renewable (8), unused9 (9), unused10 (10), unused11 (11), unused12 (12), unused13 (13), requestanonymous (14), canonicalize (15), disable-transited-check (26), renewable-ok (27), enc-tkt-in-skey (28), renew (30), validate (31) -- XXX need "need ticket1" flag? } Yu Expires: Jul 2005 [Page 86] Internet-Draft rfc1510ter-00 21 Jan 2005 AS-REP ::= CHOICE { rfc1510 [APPLICATION 11] KDC-REP-1510 { EncASRepPart1510 } (WITH COMPONENTS { ..., msg-type (11) }), ext [APPLICATION 7] Signed { [APPLICATION 7] KDC-REP-EXT { EncASRepPartExt }, { key-reply }, { ku-ASRep-cksum } } (WITH COMPONENTS { ..., msg-type (7) }) } TGS-REP ::= CHOICE { rfc1510 [APPLICATION 13] KDC-REP-1510 { EncTGSRepPart1510 } (WITH COMPONENTS { ..., msg-type (13) }), ext [APPLICATION 9] Signed { [APPLICATION 9] KDC-REP-EXT { EncTGSRepPartExt }, { key-reply }, { ku-TGSRep-cksum } } (WITH COMPONENTS { ..., msg-type (9) }) } Yu Expires: Jul 2005 [Page 87] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (11 -- AS-REP.rfc1510 -- | 13 -- TGS.rfc1510 -- | 7 -- AS-REP.ext -- | 9 -- TGS-REP.ext -- ), padata [2] SEQUENCE OF PA-DATA OPTIONAL, crealm [3] Realm, cname [4] PrincipalName, ticket [5] Ticket, enc-part [6] EncryptedData { EncPart, { key-reply }, -- maybe reach into EncryptedData in AS-REP/TGS-REP -- definitions to apply constraints on key usages? { ku-EncASRepPart -- if AS-REP -- | ku-EncTGSRepPart-subkey -- if TGS-REP and -- using Authenticator -- session key -- | ku-EncTGSRepPart-sesskey -- if TGS-REP and using -- subsession key -- } }, ..., -- In extensible version, KDC signs original request -- to avoid replay attacks against client. req-cksum [7] ChecksumOf { CHOICE { as-req AS-REQ, tgs-req TGS-REQ }, { key-reply }, { ku-KDCRep-cksum }} OPTIONAL, lang-tag [8] LangTag OPTIONAL, ... } KDC-REP-1510 { EncPart } ::= SEQUENCE { COMPONENTS OF KDC-REP-COMMON { EncPart } } (WITH COMPONENTS { ..., msg-type (11 | 13), crealm (RealmIA5), cname (PrincipalNameIA5) }) Yu Expires: Jul 2005 [Page 88] Internet-Draft rfc1510ter-00 21 Jan 2005 KDC-REP-EXT { EncPart } ::= KDC-REP-COMMON { EncPart } (WITH COMPONENTS { ..., msg-type (7 | 9), crealm (RealmExt), cname (PrincipalNameExt) }) EncASRepPart1510 ::= [APPLICATION 25] EncKDCRepPart1510 EncTGSRepPart1510 ::= [APPLICATION 26] EncKDCRepPart1510 EncASRepPartExt ::= [APPLICATION 32] EncKDCRepPartExt EncTGSRepPartExt ::= [APPLICATION 33] EncKDCRepPartExt EncKDCRepPartCom ::= SEQUENCE { key [0] EncryptionKey, last-req [1] LastReq, nonce [2] Nonce, key-expiration [3] KerberosTime OPTIONAL, flags [4] TicketFlags, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, srealm [9] Realm, sname [10] PrincipalName, caddr [11] HostAddresses OPTIONAL, ... } EncKDCRepPart1510 ::= SEQUENCE { COMPONENTS OF EncKDCRepPartCom } (WITH COMPONENTS { ..., srealm (RealmIA5), sname (PrincipalNameIA5), nonce UInt32 }) EncKdcRepPartExt ::= EncKDCRepPartCom (WITH COMPONENTS { ..., srealm (RealmExt), sname (PrincipalNameExt) }) Yu Expires: Jul 2005 [Page 89] Internet-Draft rfc1510ter-00 21 Jan 2005 LRType ::= TH-id LastReq ::= SEQUENCE OF SEQUENCE { lr-type [0] LRType, lr-value [1] KerberosTime } -- -- *** preauth -- PaDataType ::= TH-id PaDataOID ::= RELATIVE-OID PA-DATA ::= SEQUENCE { -- NOTE: first tag is [1], not [0] padata-type [1] PaDataType, padata-value [2] OCTET STRING } -- AP-REQ authenticating a TGS-REQ pa-tgs-req PaDataType ::= int32 : 1 PA-TGS-REQ ::= AP-REQ -- Encrypted timestamp preauth -- Encryption key used is client's long-term key. pa-enc-timestamp PaDataType ::= int32 : 2 PA-ENC-TIMESTAMP ::= EncryptedData { PA-ENC-TS-ENC, { key-client }, { ku-pa-enc-ts } } PA-ENC-TS-ENC ::= SEQUENCE { patimestamp [0] KerberosTime -- client's time --, pausec [1] Microseconds OPTIONAL } Yu Expires: Jul 2005 [Page 90] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Hints returned in AS-REP or KRB-ERROR to help client -- choose a password-derived key, either for preauthentication -- or for decryption of the reply. pa-etype-info PaDataType ::= int32 : 11 ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY ETYPE-INFO-ENTRY ::= SEQUENCE { etype [0] EType, salt [1] OCTET STRING OPTIONAL } -- Similar to etype-info, but with parameters provided for -- the string-to-key function. pa-etype-info2 PaDataType ::= int32 : 19 ETYPE-INFO2 ::= SEQUENCE (SIZE (1..MAX)) OF ETYPE-INFO-ENTRY ETYPE-INFO2-ENTRY ::= SEQUENCE { etype [0] EType, salt [1] KerberosString OPTIONAL, s2kparams [2] OCTET STRING OPTIONAL } -- Obsolescent. Salt for client's long-term key. -- Its character encoding is unspecified. pa-pw-salt PaDataType ::= int32 : 3 -- The "padata-value" does not encode an ASN.1 type. -- Instead, "padata-value" must consist of the salt string to -- be used by the client, in an unspecified character -- encoding. -- An extensible AS-REQ may be sent as a padata in a -- non-extensible AS-REQ to allow for backwards compatibility. pa-as-req PaDataType ::= int32 : 42 -- provisional PA-AS-REQ ::= AS-REQ (WITH COMPONENTS ext) -- -- *** Session key exchange -- Yu Expires: Jul 2005 [Page 91] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REQ ::= CHOICE { rfc1510 [APPLICATION 14] AP-REQ-1510, ext [APPLICATION 18] Signed { AP-REQ-EXT, { key-session }, { ku-APReq-cksum } } } AP-REQ-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (14 | 18), ap-options [2] APOptions, ticket [3] Ticket, authenticator [4] EncryptedData { Authenticator, { key-session }, { ku-APReq-authenticator | ku-pa-TGSReq-authenticator } }, ..., extensions [5] ApReqExtensions OPTIONAL, lang-tag [6] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } AP-REQ-1510 ::= SEQUENCE { COMPONENTS OF AP-REQ-COMMON } (WITH COMPONENTS { ..., msg-type (14), authenticator (EncryptedData { Authenticator (WITH COMPONENTS { ..., crealm (RealmIA5), cname (PrincipalNameIA5), seqnum (UInt32) }), { key-session }, { ku-APReq-authenticator }}) }) Yu Expires: Jul 2005 [Page 92] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REQ-EXT ::= AP-REQ-COMMON (WITH COMPONENTS { ..., msg-type (18), -- The following constraints on Authenticator assume that -- we want to restrict the use of AP-REQ-EXT with TicketExt -- only, since that is the only way we can enforce UTF-8. authenticator (EncryptedData { Authenticator (WITH COMPONENTS { ..., crealm (RealmExt), cname (PrincipalNameExt), authorization-data (SIZE (1..MAX)) }), { key-session }, { ku-APReq-authenticator }}) }) ApReqExtType ::= TH-id ApReqExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { apReqExt-Type [0] ApReqExtType, apReqExt-Data [1] OCTET STRING } APOptions ::= KerberosFlags { APOptionsBits } APOptionsBits ::= BIT STRING { reserved (0), use-session-key (1), mutual-required (2) } -- plaintext of authenticator Authenticator ::= [APPLICATION 2] SEQUENCE { authenticator-vno [0] INTEGER (5), crealm [1] Realm, cname [2] PrincipalName, cksum [3] Checksum {{ key-session }, { ku-Authenticator-cksum | ku-pa-TGSReq-cksum }} OPTIONAL, cusec [4] Microseconds, ctime [5] KerberosTime, subkey [6] EncryptionKey OPTIONAL, seq-number [7] SeqNum OPTIONAL, authorization-data [8] AuthorizationData OPTIONAL } Yu Expires: Jul 2005 [Page 93] Internet-Draft rfc1510ter-00 21 Jan 2005 AP-REP ::= CHOICE { rfc1510 [APPLICATION 15] AP-REP-1510, ext [APPLICATION 19] Signed { AP-REP-EXT, { key-session | key-subsession }, { ku-APRep-cksum }} } AP-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (15 | 19), enc-part [2] EncryptedData { EncPart, { key-session | key-subsession }, { ku-EncAPRepPart }}, ..., extensions [3] ApRepExtensions OPTIONAL, ... } AP-REP-1510 ::= SEQUENCE { COMPONENTS OF AP-REP-COMMON { EncAPRepPart1510 } } (WITH COMPONENTS { ..., msg-type (15) }) AP-REP-EXT ::= [APPLICATION 19] AP-REP-COMMON { EncAPRepPartExt } (WITH COMPONENTS { ..., msg-type (19) }) ApRepExtType ::= TH-id ApRepExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { apRepExt-Type [0] ApRepExtType, apRepExt-Data [1] OCTET STRING } EncAPRepPart ::= CHOICE { rfc1510 [APPLICATION 27] EncAPRepPart1510, ext [APPLICATION 31] EncAPRepPartExt } Yu Expires: Jul 2005 [Page 94] Internet-Draft rfc1510ter-00 21 Jan 2005 EncAPRepPart1510 ::= SEQUENCE { COMPONENTS OF ENCAPRepPartCom } (WITH COMPONENTS { ..., seq-number (UInt32), authorization-data ABSENT }) EncAPRepPartExt ::= EncAPRepPartCom EncAPRepPartCom ::= SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] SeqNum OPTIONAL, ..., authorization-data [4] AuthorizationData OPTIONAL, ... } -- -- *** Application messages -- -- Do we chew up another tag for KRB-SAFE-EXT? That would -- allow us to make safe-body optional, allowing for a GSS-MIC -- sort of message. KRB-SAFE ::= [APPLICATION 20] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (20), safe-body [2] KRB-SAFE-BODY, cksum [3] ChecksumOf { KRB-SAFE-BODY, { key-session | key-subsession }, { ku-KrbSafe-cksum }}, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } Yu Expires: Jul 2005 [Page 95] Internet-Draft rfc1510ter-00 21 Jan 2005 KRB-SAFE-BODY ::= SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress, r-address [5] HostAddress OPTIONAL, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } KRB-PRIV ::= [APPLICATION 21] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (21), enc-part [3] EncryptedData { EncKrbPrivPart, { key-session | key-subsession }, { ku-EncKrbPrivPart }}, ... } EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress -- sender's addr --, r-address [5] HostAddress OPTIONAL -- recip's addr --, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } KRB-CRED ::= CHOICE { rfc1510 [APPLICATION 22] KRB-CRED-1510, ext [APPLICATION 24] Signed { KRB-CRED-EXT, { key-session | key-subsession }, { ku-KrbCred-cksum }} } Yu Expires: Jul 2005 [Page 96] Internet-Draft rfc1510ter-00 21 Jan 2005 KRB-CRED-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (22 | 24), tickets [2] SEQUENCE OF Ticket, enc-part [3] EncryptedData { EncKrbCredPart, { key-session | key-subsession }, { ku-EncKrbCredPart }}, ... } KRB-CRED-1510 ::= SEQUENCE { COMPONENTS OF KRB-CRED-COMMON } (WITH COMPONENTS { ..., msg-type (22) }) KRB-CRED-EXT ::= [APPLICATION 24] KRB-CRED-COMMON (WITH COMPONENTS { ..., msg-type (24) }) EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { ticket-info [0] SEQUENCE OF KrbCredInfo, nonce [1] Nonce OPTIONAL, timestamp [2] KerberosTime OPTIONAL, usec [3] Microseconds OPTIONAL, s-address [4] HostAddress OPTIONAL, r-address [5] HostAddress OPTIONAL } KrbCredInfo ::= SEQUENCE { key [0] EncryptionKey, prealm [1] Realm OPTIONAL, pname [2] PrincipalName OPTIONAL, flags [3] TicketFlags OPTIONAL, authtime [4] KerberosTime OPTIONAL, starttime [5] KerberosTime OPTIONAL, endtime [6] KerberosTime OPTIONAL, renew-till [7] KerberosTime OPTIONAL, srealm [8] Realm OPTIONAL, sname [9] PrincipalName OPTIONAL, caddr [10] HostAddresses OPTIONAL } Yu Expires: Jul 2005 [Page 97] Internet-Draft rfc1510ter-00 21 Jan 2005 TGT-REQ ::= [APPLICATION 16] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (16), sname [2] PrincipalName OPTIONAL, srealm [3] Realm OPTIONAL, ... } -- -- *** Error messages -- ErrCode ::= Int32 KRB-ERROR ::= CHOICE { rfc1510 [APPLICATION 30] KRB-ERROR-1510, ext [APPLICATION 23] Signed { KRB-ERROR-EXT, { ku-KrbError-cksum } } } KRB-ERROR-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (30 | 23), ctime [2] KerberosTime OPTIONAL, cusec [3] Microseconds OPTIONAL, stime [4] KerberosTime, susec [5] Microseconds, error-code [6] ErrCode, crealm [7] Realm OPTIONAL, cname [8] PrincipalName OPTIONAL, realm [9] Realm -- Correct realm --, sname [10] PrincipalName -- Correct name --, e-text [11] KerberosString OPTIONAL, e-data [12] OCTET STRING OPTIONAL, ..., typed-data [13] TYPED-DATA OPTIONAL, nonce [14] Nonce OPTIONAL, lang-tag [15] LangTag OPTIONAL, ... } Yu Expires: Jul 2005 [Page 98] Internet-Draft rfc1510ter-00 21 Jan 2005 KRB-ERROR-1510 ::= SEQUENCE { COMPONENTS OF KRB-ERROR-COMMON } (WITH COMPONENTS { ..., msg-type (30) }) KRB-ERROR-EXT ::= [APPLICATION 23] KRB-ERROR-COMMON (WITH COMPONENTS { ..., msg-type (23) }) METHOD-DATA ::= SEQUENCE OF PA-DATA TDType ::= TH-id TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { data-type [0] TDType, data-value [1] OCTET STRING OPTIONAL } Yu Expires: Jul 2005 [Page 99] Internet-Draft rfc1510ter-00 21 Jan 2005 -- -- *** Error codes -- -- No error kdc-err-none ErrCode ::= 0 -- Client's entry in database has expired kdc-err-name-exp ErrCode ::= 1 -- Server's entry in database has expired kdc-err-service-exp ErrCode ::= 2 -- Requested protocol version number not supported kdc-err-bad-pvno ErrCode ::= 3 -- Client's key encrypted in old master key kdc-err-c-old-mast-kvno ErrCode ::= 4 -- Server's key encrypted in old master key kdc-err-s-old-mast-kvno ErrCode ::= 5 -- Client not found in Kerberos database kdc-err-c-principal-unknown ErrCode ::= 6 -- Server not found in Kerberos database kdc-err-s-principal-unknown ErrCode ::= 7 -- Multiple principal entries in database kdc-err-principal-not-unique ErrCode ::= 8 -- The client or server has a null key kdc-err-null-key ErrCode ::= 9 -- Ticket not eligible for postdating kdc-err-cannot-postdate ErrCode ::= 10 -- Requested start time is later than end time kdc-err-never-valid ErrCode ::= 11 -- KDC policy rejects request kdc-err-policy ErrCode ::= 12 -- KDC cannot accommodate requested option kdc-err-badoption ErrCode ::= 13 -- KDC has no support for encryption type kdc-err-etype-nosupp ErrCode ::= 14 -- KDC has no support for checksum type kdc-err-sumtype-nosupp ErrCode ::= 15 -- KDC has no support for padata type kdc-err-padata-type-nosupp ErrCode ::= 16 -- KDC has no support for transited type kdc-err-trtype-nosupp ErrCode ::= 17 -- Clients credentials have been revoked kdc-err-client-revoked ErrCode ::= 18 -- Credentials for server have been revoked kdc-err-service-revoked ErrCode ::= 19 -- TGT has been revoked kdc-err-tgt-revoked ErrCode ::= 20 Yu Expires: Jul 2005 [Page 100] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Client not yet valid - try again later kdc-err-client-notyet ErrCode ::= 21 -- Server not yet valid - try again later kdc-err-service-notyet ErrCode ::= 22 -- Password has expired - change password to reset kdc-err-key-expired ErrCode ::= 23 -- Pre-authentication information was invalid kdc-err-preauth-failed ErrCode ::= 24 -- Additional pre-authenticationrequired kdc-err-preauth-required ErrCode ::= 25 -- Requested server and ticket don't match kdc-err-server-nomatch ErrCode ::= 26 -- Server principal valid for user2user only kdc-err-must-use-user2user ErrCode ::= 27 -- KDC Policy rejects transited path kdc-err-path-not-accpeted ErrCode ::= 28 -- A service is not available kdc-err-svc-unavailable ErrCode ::= 29 -- Integrity check on decrypted field failed krb-ap-err-bad-integrity ErrCode ::= 31 -- Ticket expired krb-ap-err-tkt-expired ErrCode ::= 32 -- Ticket not yet valid krb-ap-err-tkt-nyv ErrCode ::= 33 -- Request is a replay krb-ap-err-repeat ErrCode ::= 34 -- The ticket isn't for us krb-ap-err-not-us ErrCode ::= 35 -- Ticket and authenticator don't match krb-ap-err-badmatch ErrCode ::= 36 -- Clock skew too great krb-ap-err-skew ErrCode ::= 37 -- Incorrect net address krb-ap-err-badaddr ErrCode ::= 38 -- Protocol version mismatch krb-ap-err-badversion ErrCode ::= 39 -- Invalid msg type krb-ap-err-msg-type ErrCode ::= 40 Yu Expires: Jul 2005 [Page 101] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Message stream modified krb-ap-err-modified ErrCode ::= 41 -- Message out of order krb-ap-err-badorder ErrCode ::= 42 -- Specified version of key is not available krb-ap-err-badkeyver ErrCode ::= 44 -- Service key not available krb-ap-err-nokey ErrCode ::= 45 -- Mutual authentication failed krb-ap-err-mut-fail ErrCode ::= 46 -- Incorrect message direction krb-ap-err-baddirection ErrCode ::= 47 -- Alternative authentication method required krb-ap-err-method ErrCode ::= 48 -- Incorrect sequence number in message krb-ap-err-badseq ErrCode ::= 49 -- Inappropriate type of checksum in message krb-ap-err-inapp-cksum ErrCode ::= 50 -- Policy rejects transited path krb-ap-path-not-accepted ErrCode ::= 51 -- Response too big for UDP, retry with TCP krb-err-response-too-big ErrCode ::= 52 -- Generic error (description in e-text) krb-err-generic ErrCode ::= 60 Yu Expires: Jul 2005 [Page 102] Internet-Draft rfc1510ter-00 21 Jan 2005 -- Field is too long for this implementation krb-err-field-toolong ErrCode ::= 61 -- Reserved for PKINIT kdc-error-client-not-trusted ErrCode ::= 62 -- Reserved for PKINIT kdc-error-kdc-not-trusted ErrCode ::= 63 -- Reserved for PKINIT kdc-error-invalid-sig ErrCode ::= 64 -- Reserved for PKINIT kdc-err-key-too-weak ErrCode ::= 65 -- Reserved for PKINIT kdc-err-certificate-mismatch ErrCode ::= 66 -- No TGT available to validate USER-TO-USER krb-ap-err-no-tgt ErrCode ::= 67 -- USER-TO-USER TGT issued different KDC kdc-err-wrong-realm ErrCode ::= 68 -- Ticket must be for USER-TO-USER krb-ap-err-user-to-user-required ErrCode ::= 69 -- Reserved for PKINIT kdc-err-cant-verify-certificate ErrCode ::= 70 -- Reserved for PKINIT kdc-err-invalid-certificate ErrCode ::= 71 -- Reserved for PKINIT kdc-err-revoked-certificate ErrCode ::= 72 -- Reserved for PKINIT kdc-err-revocation-status-unknown ErrCode ::= 73 -- Reserved for PKINIT kdc-err-revocation-status-unavailable ErrCode ::= 74 END B. Kerberos and Character Encodings (Informative) [adapted from KCLAR 5.2.1] The original specification of the Kerberos protocol in RFC 1510 uses GeneralString in numerous places for human-readable string data. Historical implementations of Kerberos cannot utilize the full power of GeneralString. This ASN.1 type requires the use of designation and invocation escape sequences as specified in ISO 2022 | ECMA-35 [ISO2022] to switch character sets, and the default character set that is designated as G0 is the ISO 646 | ECMA-6 [ISO646] International Reference Version (IRV) (aka U.S. ASCII), which mostly works. ISO 2022 | ECMA-35 defines four character-set code elements (G0..G3) and two Control-function code elements (C0..C1). DER previously [X690-1997] prohibited the designation of character sets as any but the G0 and C0 sets. This had the side effect of prohibiting the use Yu Expires: Jul 2005 [Page 103] Internet-Draft rfc1510ter-00 21 Jan 2005 of (ISO Latin) character-sets such as ISO 8859-1 [ISO8859-1] or any other character-sets that utilize a 96-character set, since it is prohibited by ISO 2022 | ECMA-35 to designate them as the G0 code element. Recent revisions to the ASN.1 standards resolve this contradiction. In practice, many implementations treat RFC 1510 GeneralStrings as if they were 8-bit strings of whichever character set the implementation defaults to, without regard for correct usage of character-set designation escape sequences. The default character set is often determined by the current user's operating system dependent locale. At least one major implementation places unescaped UTF-8 encoded Unicode characters in the GeneralString. This failure to conform to the GeneralString specifications results in interoperability issues when conflicting character encodings are utilized by the Kerberos clients, services, and KDC. This unfortunate situation is the result of improper documentation of the restrictions of the ASN.1 GeneralString type in prior Kerberos specifications. [the following should probably be rewritten and moved into the principal name section] For compatibility, implementations MAY choose to accept GeneralString values that contain characters other than those permitted by IA5String, but they should be aware that character set designation codes will likely be absent, and that the encoding should probably be treated as locale-specific in almost every way. Implementations MAY also choose to emit GeneralString values that are beyond those permitted by IA5String, but should be aware that doing so is extraordinarily risky from an interoperability perspective. Some existing implementations use GeneralString to encode unescaped locale-specific characters. This is a violation of the ASN.1 standard. Most of these implementations encode US-ASCII in the left- hand half, so as long the implementation transmits only US-ASCII, the ASN.1 standard is not violated in this regard. As soon as such an implementation encodes unescaped locale-specific characters with the high bit set, it violates the ASN.1 standard. Other implementations have been known to use GeneralString to contain a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 is a different encoding, not a 94 or 96 character "G" set as defined by ISO 2022. It is believed that these implementations do not even use the ISO 2022 escape sequence to change the character encoding. Even if implementations were to announce the change of encoding by using that escape sequence, the ASN.1 standard prohibits the use of any escape sequences other than those used to designate/invoke "G" or "C" sets allowed by GeneralString. Yu Expires: Jul 2005 [Page 104] Internet-Draft rfc1510ter-00 21 Jan 2005 C. Kerberos History (Informative) [Adapted from KCLAR "BACKGROUND"] The Kerberos model is based in part on Needham and Schroeder's trusted third-party authentication protocol [NS78] and on modifications suggested by Denning and Sacco [DS81]. The original design and implementation of Kerberos Versions 1 through 4 was the work of two former Project Athena staff members, Steve Miller of Digital Equipment Corporation and Clifford Neuman (now at the Information Sciences Institute of the University of Southern California), along with Jerome Saltzer, Technical Director of Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members of Project Athena have also contributed to the work on Kerberos. Version 5 of the Kerberos protocol (described in this document) has evolved from Version 4 based on new requirements and desires for features not available in Version 4. The design of Version 5 of the Kerberos protocol was led by Clifford Neuman and John Kohl with much input from the community. The development of the MIT reference implementation was led at MIT by John Kohl and Theodore Ts'o, with help and contributed code from many others. Since RFC1510 was issued, extensions and revisions to the protocol have been proposed by many individuals. Some of these proposals are reflected in this document. Where such changes involved significant effort, the document cites the contribution of the proposer. Reference implementations of both version 4 and version 5 of Kerberos are publicly available and commercial implementations have been developed and are widely used. Details on the differences between Kerberos Versions 4 and 5 can be found in [KNT94]. D. Notational Differences from [KCLAR] [ possible point for discussion ] [KCLAR] uses notational conventions slightly different from this document. As a derivative of RFC 1510, the text of [KCLAR] uses C- language style identifier names for defined values. In ASN.1 notation, identifiers referencing defined values must begin with a lowercase letter and contain hyphen (-) characters rather than underscore (_) characters, while identifiers referencing types begin with an uppercase letter. [KCLAR] and RFC 1510 use all-uppercase identifiers with underscores to identify defined values. This has the potential to create confusion, but neither document defines values using actual ASN.1 value-assignment notation. It is debatable whether it is advantageous to write all identifier names (regardless of their ASN.1 token type) in all-uppercase letters for the purpose of emphasis in running text. The alternative is to Yu Expires: Jul 2005 [Page 105] Internet-Draft rfc1510ter-00 21 Jan 2005 use double-quote characters (") when ambiguity is possible. Normative References [ISO646] "7-bit coded character set", ISO/IEC 646:1991 | ECMA-6:1991. [ISO2022] "Information technology -- Character code structure and extension techniques", ISO/IEC 2022:1994 | ECMA-35:1994. [KCRYPTO] K. Raeburn, "Encryption and Checksum Specifications for Kerberos 5", draft-ietf-krb-wg-crypto-07.txt, work in progress. [RFC2119] S. Bradner, RFC2119: "Key words for use in RFC's to Indicate Requirement Levels", March 1997. [RFC3660] H. Alvestrand, "Tags for the Identification of Languages", RFC 3660, January 2001. [SASLPREP] Kurt D. Zeilenga, "SASLprep: Stringprep profile for user names and passwords", draft-ietf-sasl-saslprep-10.txt, work in progress. [X680] "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002. [X682] "Information technology -- Abstract Syntax Notation One (ASN.1): Constraint specification", ITU-T Recommendation X.682 (2002) | ISO/IEC 8824-3:2002. [X683] "Information technology -- Abstract Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications", ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002. [X690] "Information technology -- ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002. Yu Expires: Jul 2005 [Page 106] Internet-Draft rfc1510ter-00 21 Jan 2005 Informative References [DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key Distribution Protocols," Communications of the ACM, Vol. 24(8), pp. 533-536 (August 1981). [Dub00] Olivier Dubuisson, "ASN.1 - Communication between Heterogeneous Systems", Elsevier-Morgan Kaufmann, 2000. [ISO8859-1] "Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1", ISO/IEC 8859- 1:1998. [KCLAR] Clifford Neuman, Tom Yu, Sam Hartman, Ken Raeburn, "The Kerberos Network Authentication Service (V5)", draft-ietf-krb-wg- kerberos-clarifications-07.txt, work in progress. [KNT94] John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The Evolution of the Kerberos Authentication System". In Distributed Open Systems, pages 78-94. IEEE Computer Society Press, 1994. [Lar96] John Larmouth, "Understanding OSI", International Thomson Computer Press, 1996. [Lar99] John Larmouth, "ASN.1 Complete", Elsevier-Morgan Kaufmann, 1999. [NS78] Roger M. Needham and Michael D. Schroeder, "Using Encryption for Authentication in Large Networks of Computers", Communications of the ACM, Vol. 21(12), pp. 993-999 (December, 1978). [RFC1510] J. Kohl and B. C. Neuman, "The Kerberos Network Authentication Service (v5)", RFC1510, September 1993, Status: Proposed Standard. [RFC1964] J. Linn, "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996, Status: Proposed Standard. Yu Expires: Jul 2005 [Page 107] Internet-Draft rfc1510ter-00 21 Jan 2005 [X690-2002] "Information technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002. Author's Address Tom Yu 77 Massachusetts Ave Cambridge, MA 02139 USA tlyu@mit.edu Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Yu Expires: Jul 2005 [Page 108]