Kerberos Working Group S. Hartman Internet-Draft MIT Expires: August 9, 2004 February 9, 2004 A Generalized Framework for Kerberos Preauthentication draft-ietf-krb-wg-preauth-framework-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. This Internet-Draft will expire on August 9, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract Kerberos is a protocol for verifying the identity of principals (e.g., a workstation user or a network server) on an open network. The Kerberos protocol provides a mechanism called preauthentication for proving the identity of a principal and for better protecting the long-term secret of the principal. This document describes a model for Kerberos preauthentication mechanisms. The model describes what state in the Kerberos request a preauthentication mechanism is likely to change. It also describes how multiple preauthentication mechanisms used in the same request will interact. This document also provides common tools needed by multiple Hartman Expires August 9, 2004 [Page 1] Internet-Draft Kerberos Preauth Framework February 2004 preauthentication mechanisms. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Model for Preauthentication . . . . . . . . . . . . . . . . . 4 2.1 Information Managed by Model . . . . . . . . . . . . . . . . . 5 2.2 The Preauth_Required Error . . . . . . . . . . . . . . . . . . 6 2.3 Client to KDC . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 KDC to Client . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Preauthentication Facilities . . . . . . . . . . . . . . . . . 9 3.1 Client Authentication . . . . . . . . . . . . . . . . . . . . 10 3.2 Strengthen Reply Key . . . . . . . . . . . . . . . . . . . . . 10 3.3 Replace Reply Key . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Verify Response . . . . . . . . . . . . . . . . . . . . . . . 11 4. Requirements for Preauthentication Mechanisms . . . . . . . . 12 5. Tools for Use in Preauthentication Mechanisms . . . . . . . . 13 5.1 Combine Keys . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2 Signing Requests/Responses . . . . . . . . . . . . . . . . . . 13 5.3 Managing State for the KDC . . . . . . . . . . . . . . . . . . 13 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 18 Normative References . . . . . . . . . . . . . . . . . . . . . 17 Informative References . . . . . . . . . . . . . . . . . . . . 18 A. Todo List . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . 20 Hartman Expires August 9, 2004 [Page 2] Internet-Draft Kerberos Preauth Framework February 2004 1. Introduction The core Kerberos specification treats preauthentication data as an opaque typed hole in the messages to the KDC that may influence the reply key used to encrypt the KDC response. This generality has been useful: preauthentication data is used for a variety of extensions to the protocol, many outside the expectations of the initial designers. However, this generality makes designing the more common types of preauthentication mechanisms difficult. Each mechanism needs to specify how it interacts with other mechanisms. Also, problems like combining a key with the long-term secret or proving the identity of the user are common to multiple mechanisms. Where there are generally well-accepted solutions to these problems, it is desirable to standardize one of these solutions so mechanisms can avoid duplication of work. In other cases, a modular approach to these problems is appropriate. The modular approach will allow new and better solutions to common preauth problems to be used by existing mechanisms as they are developed. This document specifies a framework for Kerberos preauthentication mechanisms. IT defines the common set of functions preauthentication mechanisms perform as well as how these functions affect the state of the request and response. In addition several common tools needed by preauthentication mechanisms are provided. Unlike [3], this framework is not complete--it does not describe all the inputs and outputs for the preauthentication mechanisms. Mechanism designers should try to be consistent with this framework because doing so will make their mechanisms easier to implement. Kerberos implementations are likely to have plugin architectures for preauthentication; such architectures are likely to support mechanisms that follow this framework plus commonly used extensions. This document should be read only after reading the documents describing the Kerberos cryptography framework [3] and the core Kerberos protocol [2]. This document freely uses terminology and notation from these documents without reference or further explanation. Hartman Expires August 9, 2004 [Page 3] Internet-Draft Kerberos Preauth Framework February 2004 2. Model for Preauthentication when a Kerberos client wishes to obtain a ticket using the authentication server, it sends an initial AS request. If preauthentication is being used, then the KDC will respond with a KDC_ERR_PREAUTH_REQUIRED error. Alternatively, if the client knows what preauthentication to use, it MAY optimize a round-trip and send an initial request with padata included. If the client includes the wrong padata, the server MAY return KDC_ERR_PREAUTH_FAILED with no indication of what padata should have been included. For interoperability reasons, clients that include optimistic preauth MUST retry with no padata and examine the KDC_ERR_PREAUTH_REQUIRED if they receive a KDC_ERR_PREAUTH_FAILED in response to their initial optimistic request. The KDC maintains no state between two requests; subsequent requests may even be processed by a different KDC. On the other hand, the client treats a series of exchanges with KDCs as a single authentication session. Each exchange accumulates state and hopefully brings the client closer to a successful authentication. These models for state management are in apparent conflict. For many of the simpler preauthentication scenarios, the client uses one round trip to find out what mechanisms the KDC supports. Then the next request contains sufficient preauthentication for the KDC to be able to return a successful response. For these simple scenarios, the client only sends one request with preauthentication data and so the authentication session is trivial. For more complex authentication sessions, the KDC needs to provide the client with a cookie to include in future requests to capture the current state of the authentication session. Handling of multiple round-trip mechanisms is discussed in Section 5.3. This framework specifies the behavior of Kerberos preauthentication mechanisms used to identify users or to modify the reply key used to encrypt the KDC response. The padata typed hole may be used to carry extensions to Kerberos that have nothing to do with proving the identity of the user or establishing a reply key. These extensions are outside the scope of this framework. However mechanisms that do accomplish these goals should follow this framework. This framework specifies the minimum state that a Kerberos implementation needs to maintain while handling a request in order to process preauthentication. It also specifies how Kerberos implementations process the preauthentication data at each step of the AS request process. Hartman Expires August 9, 2004 [Page 4] Internet-Draft Kerberos Preauth Framework February 2004 2.1 Information Managed by Model The following information is maintained by the client and KDC as each request is being processed: o The reply key used to encrypt the KDC response o How strongly the identity of the client has been authenticated o Whether the reply key has been used in this authentication session o Whether the contents of the KDC response can be verified by the client principal o Whether the contents of the KDC response can be verified by the client machine Conceptually, the reply key is initially the long-term key of the principal. However, principals can have multiple long-term keys because of support for multiple encryption types, salts and string2key parameters. As described in section 5.2.7.5 of the Kerberos protocol [2], the KDC sends PA-ETYPe-INFO2 to notify the client what types of keys are available. Thus in full generality, the reply key in the preauth model is actually a set of keys. At the beginning of a request, it is initialized to the set of long-term keys advertised in the PA-ETYPE-INFO2 element on the KDC. If multiple reply keys are available, the client chooses which one to use. Thus the client does not need to treat the reply key as a set. At the beginning of a handling a request, the client picks a reply key to use. KDC implementations MAY choose to offer only one key in the PA-ETYPE-INFO2 element. Since the KDC already knows the client's list of supported enctypes from the request, no interoperability problems are created by choosing a single possible reply key. This way, the KDC implementation avoids the complexity of treating the reply key as a set. At the beginning of handling a message on both the client and KDC, the client's identity is not authenticated. A mechanism may indicate that it has successfully authenticated the client's identity. This information is useful to keep track of on the client in order to know what preauthentication mechanisms should be used. The KDC needs to keep track of whether the client is authenticated because the primary purpose of preauthentication is to authenticate the client identity before issuing a ticket. Implementations that have preauthentication mechanisms offering significantly different strengths of client authentication MAY choose to keep track of the Hartman Expires August 9, 2004 [Page 5] Internet-Draft Kerberos Preauth Framework February 2004 strength of the authentication used as an input into policy decisions. For example, some principals might require strong preauthentication, while less sensitive principals can use relatively weak forms of preauthentication like encrypted timestamp. Initially the reply key has not been used. A preauthentication mechanism that uses the reply key either directly to encrypt or cheksum some data or indirectly in the generation of new keys MUST indicate that the reply key is used. This state is maintained by the client and KDC to enforce the security requirement stated in Section 3.3 that the reply key cannot be replaced after it is used. Without preauthentication, the client knows that the KDC request is authentic and has not been modified because it is encrypted in the long-term key of the client. Only the KDC and client know that key. So at the start of handling any message the KDC request is presumed to be verified to the client principal. Any preauthentication mechanism that sets a new reply key not based on the principal's long-term secret MUST either verify the KDC response some other way or indicate that the response is not verified. If a mechanism indicates that the response is not verified then the client implementation MUST return an error unless a subsequent mechanism verifies the response. The KDC needs to track this state so it can avoid generating a response that is not verified. The typical Kerberos request does not provide a way for the client machine to know that it is talking to the correct KDC. Someone who can inject packets into the network between the client machine and the KDC and who knows the password that the user will give to the client machine can generate a KDC response that will decrypt properly. So, if the client machine needs to authenticate that the user is in fact the named principal, then the client machine needs to do a TGS request for itself as a service. Some preauthentication mechanisms may provide a way for the client to authenticate the KDC. Examples of this include signing the response with a well-known public key or providing a ticket for the client machine as a service in addition to the requested ticket. 2.2 The Preauth_Required Error Typically a client starts an authentication session by sending an initial request with no preauthentication. If the KDC requires preauthentication, then it returns a KDC_ERR_PREAUTH_REQUIRED message. This message MAY also be returned for preauthentication configurations that use multi-round-trip mechanisms. This error contains a sequence of padata. Typically the padata contains the preauth type IDs of all the available preauthentication mechanisms. IN the initial error response, most mechanisms do not contain data. Hartman Expires August 9, 2004 [Page 6] Internet-Draft Kerberos Preauth Framework February 2004 If a mechanism requires multiple round trips or starts with a challenge from the KDC to the client, then it will likely contain data in the initial error response. The KDC SHOULD NOT send data that is encrypted in the long-term password-based key of the principal. Doing so has the same security exposures as the Kerberos protocol without preauthentication. There are few situations where preauthentication is desirable and where the KDC needs to expose ciphertext encrypted in a weak key before the client has proven knowledge of that key. In order to generate the error response, the KDC first starts by initializing the preauthentication state. Then it processes any padata in the client's request in the order provided by the client. Mechanisms that are not understood by the KDC are ignored. Mechanisms that are inappropriate for the client principal or request SHOULD also be ignored. Next, it generates padata for the error response, modifying the preauthentication state appropriately as each mechanism is processed. The KDC chooses the order in which it will generated padata (and thus the order of padata in the response), but it needs to modify the preauthentication state consistently with the choice of order. For example, if some mechanism establishes an authenticated client identity, then the mechanisms subsequent in the generated response receive this state as input. After the padata is generated, the error response is sent. 2.3 Client to KDC This description assumes a client has already received a KDC_ERR_PREAUTH_REQUIRED from the KDC. If the client performs optimistic preauthentication then the client needs to optimisticly choose the information it would normally receive from that error response. The client starts by initializing the preauthentication state as specified. It then processes the pdata in the KDC_ERR_PREAUTH_REQUIRED. After processing the pdata in the KDC error, the client generates a new request. It processes the preauthentication mechanisms in the order in which they will appear in the next request, updating the state as appropriate. When the request is complete it is sent. 2.4 KDC to Client When a KDC receives an AS request from a client, it needs to determine whether it will respond with an error or a AS reply. There are many causes for an error to be generated that have nothing Hartman Expires August 9, 2004 [Page 7] Internet-Draft Kerberos Preauth Framework February 2004 to do with preauthentication; they are discussed in the Kerberos specification. From the standpoint of evaluating the preauthentication, the KDC first starts by initializing the preauthentication state. IT then processes the padata in the request. AS mentioned in Section 2.2, the KDC MAY ignore padata that is inappropriate for the configuration and MUST ignore padata of an unknown type. At this point the KDC decides whether it will issue a preauthentication required error or a reply. Typically a KDC will issue a reply if the client's identity has been authenticated to a sufficient degree. The processing of the preauthentication required error is described in Section 2.2. The KDC generates the pdata modifying the preauthentication state as necessary. Then it generates the final response, encrypting it in the current preauthentication reply key. Hartman Expires August 9, 2004 [Page 8] Internet-Draft Kerberos Preauth Framework February 2004 3. Preauthentication Facilities Preauthentication mechanisms can be thought of as conceptually providing various facilities. This serves two useful purposes. First, mechanism authors can choose only to solve one specific small problem. It is often useful for a mechanism designed to offer key management not to directly provide client authentication but instead to allow one or more other mechanisms to handle this need. Secondly, thinking about the abstract services that a mechanism provides yields a minimum set of security requirements that all mechanisms providing that facility must meet. These security requirements are not complete; mechanisms will have additional security requirements based on the specific protocol they employ. A mechanism is not constrained to only offering one of these facilities. While such mechanisms can be designed and are sometimes useful, many preauthentication mechanisms implement several facilities. By combining multiple facilities in a single mechanism, it is often easier to construct a secure, simple solution than by solving the problem in full generality. Even when mechanisms provide multiple facilities, they need to meet the security requirements for all the facilities they provide. According to Kerberos extensibility rules (section 1.4.2 of the Kerberos specification [2]), an extension MUST NOT change the semantics of a message unless a recipient is known to understand that extension. Because a client does not know that the KDC supports a particular preauth mechanism when it sends an initial request, a preauth mechanism MUST NOT change the semantics of the request in a way that will break a KDC that does not understand that mechanism. Similarly, KDCs MUST not send messages to clients that affect the core semantics unless the clients have indicated support for the message. The only state in this model that would break the interpretation of a message is changing the expected reply key. If one mechanism changed the reply key and a later mechanism used that reply key, then a KDC that interpreted the second mechanism but not the first would fail to interpret the request correctly. In order to avoid this problem, extensions that change core semantics are typically divided into two parts. The first part proposes a change to the core semantic--for example proposes a new reply key. The second part acknowledges that the extension is understood and that the change takes effect. Section 3.2 discusses how to design mechanisms that modify the reply key to be split into a proposal and acceptance without requiring additional round trips to use the new reply key in subsiquent preauthentication. Other changes in the state described in Section 2.1 can safely be ignored by a KDC that does not understand a mechanism. Mechanisms Hartman Expires August 9, 2004 [Page 9] Internet-Draft Kerberos Preauth Framework February 2004 that modify the behavior of the request outside the scope of this framework need to carefully consider the Kerberos extensibility rules to avoid similar problems. 3.1 Client Authentication Binding to reply key Consider Secure ID case where you don't have anything to bind to 3.2 Strengthen Reply Key Particularly, when dealing with keys based on passwords it is desirable to increase the strength of the key by adding additional secrets to it. Examples of sources of additional secrets include the results of a Diffie-Hellman key exchange or key bits from the output of a smart card [5]. Typically these additional secrets are converted into a Kerberos protocol key. Then they are combined with the existing reply key as discussed in Section 5.1. If a mechanism implementing this facility wishes to modify the reply key before knowing that the other party in the exchange supports the mechanism, it proposes modifying the reply key. The other party then includes a message indicating that the proposal is accepted if it is understood and meets policy. In many cases it is desirable to use the new reply key for client authentication and for other facilities. Waiting for the other party to accept the proposal and actually modify the reply key state would add an additional round trip to the exchange. Instead, mechanism designers are encouraged to include a typed hole for additional padata in the message that proposes the reply key change. The padata included in the typed hole are generated assuming the new reply key. If the other party accepts the proposal, then these padata are interpreted as if they were included immediately following the proposal. The party generating the proposal can determine whether the padata were processed based on whether the proposal for the reply key is accepted. The specific formats of the proposal message, including where padata are are included is a matter for the mechanism specification. Similarly, the format of the message accepting the proposal is mechanism-specific. Mechanisms implementing this facility and including a typed hole for additional padata MUST checksum that padata using a keyed checksum or encrypt the padata. Typically the reply key is used to protect the padata. XXX If you are only minimally increasing the strength of the reply key, this may give the attacker access to something too close Hartman Expires August 9, 2004 [Page 10] Internet-Draft Kerberos Preauth Framework February 2004 to the original reply key. However, binding the padata to the new reply key seems potentially important from a security standpoint. There may also be objections to this from a double encryption standpoint because we also recommend client authentication facilities be tied to the reply key. 3.3 Replace Reply Key Containers to handle reply key when not sure whether other side supports mech Make sure reply key is not used previously Interactions with client authentication Reference to container argument 3.4 Verify Response Hartman Expires August 9, 2004 [Page 11] Internet-Draft Kerberos Preauth Framework February 2004 4. Requirements for Preauthentication Mechanisms State management for multi-round-trip mechs Security interactions with other mechs Hartman Expires August 9, 2004 [Page 12] Internet-Draft Kerberos Preauth Framework February 2004 5. Tools for Use in Preauthentication Mechanisms 5.1 Combine Keys 5.2 Signing Requests/Responses 5.3 Managing State for the KDC Hartman Expires August 9, 2004 [Page 13] Internet-Draft Kerberos Preauth Framework February 2004 6. IANA Considerations Hartman Expires August 9, 2004 [Page 14] Internet-Draft Kerberos Preauth Framework February 2004 7. Security Considerations Very little of the AS request is authenticated. Same for padata in the reply or error. Discuss implications Table of security requirements stated elsewhere in the document Hartman Expires August 9, 2004 [Page 15] Internet-Draft Kerberos Preauth Framework February 2004 8. Acknowledgements Hartman Expires August 9, 2004 [Page 16] Internet-Draft Kerberos Preauth Framework February 2004 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [2] Neuman, C., Yu, T., Hartman, S. and K. Raeburn, "The Kerberos Network Authentication Service (V5)", draft-ietf-krb-wg-kerberos-clarifications-04.txt (work in progress), June 2003. [3] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", draft-ietf-krb-wg-crypto-03.txt (work in progress). [4] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. Hartman Expires August 9, 2004 [Page 17] Internet-Draft Kerberos Preauth Framework February 2004 Informative References [5] Hornstein, K., Renard, K., Neuman, C. and G. Zorn, "Integrating Single-use Authentication Mechanisms with Kerberos", draft-ietf-krb-wg-kerberos-sam-02.txt (work in progress), October 2003. Author's Address Sam hartman MIT EMail: hartmans@mit.edu Hartman Expires August 9, 2004 [Page 18] Internet-Draft Kerberos Preauth Framework February 2004 Appendix A. Todo List Flesh out sections that are still outlines Discuss cookies and multiple-round-trip mechanisms. Talk about checksum contributions from each mechanism Hartman Expires August 9, 2004 [Page 19] Internet-Draft Kerberos Preauth Framework February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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