draft-ietf-sasl-rfc2222bis-02.txt [plain text]
Network Working Group A. Melnikov
Internet Draft Editor
Document: draft-ietf-sasl-rfc2222bis-02.txt August 2003
Expires in six months
Simple Authentication and Security Layer (SASL)
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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. Internet Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is not appropriate 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.
A revised version of this draft document will be submitted to the RFC
editor as a Draft Standard for the Internet Community. Discussion
and suggestions for improvement are requested. Distribution of this
draft is unlimited.
A. Melnikov FORMFEED[Page i]
Internet DRAFT SASL 19 August 2003
1. Abstract
SASL provides a method for adding authentication support with an
optional security layer to connection-based protocols. It also
describes a structure for authentication mechanisms. The result is
an abstraction layer between protocols and authentication mechanisms
such that any SASL-compatible authentication mechanism can be used
with any SASL-compatible protocol.
This document describes how a SASL authentication mechanism is
structured, how a protocol adds support for SASL, defines the
protocol for carrying a security layer over a connection, and defines
the EXTERNAL SASL authentication mechanism.
2. Organization of this document
2.1. How to read this document
This document is written to serve two different audiences, protocol
designers using this specification to support authentication in their
protocol, and implementors of clients or servers for those protocols
using this specification.
The sections "Overview", "Authentication Mechanisms", "Protocol
Profile Requirements", "Specific Issues", and "Security
Considerations" cover issues that protocol designers need to
understand and address in profiling this specification for use in a
specific protocol.
Implementors of a protocol using this specification need the
protocol-specific profiling information in addition to the
information in this document.
2.2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [KEYWORDS].
3. Overview
The Simple Authentication and Security Layer (SASL) is a method for
adding authentication support to connection-based protocols.
The SASL specification has three layers, as indicated in the diagram
A. Melnikov FORMFEED[Page 2]
Internet DRAFT SASL 19 August 2003
below. At the top, a protocol definition using SASL specifies a
profile, including a command for identifying and authenticating a
user to a server and for optionally negotiating a security layer for
subsequent protocol interactions. At the bottom, a SASL mechanism
definition specifies an authentication mechanism. The SASL
framework, specified by this document, constrains the behavior of
protocol profiles and mechanisms, separating protocol from mechanism
and defining how they interact.
SMTP Protocol LDAP Protocol Etc
Profile Profile . . .
----- | -----//
| //
SASL framework
/ | \
/----- | -----\
EXTERNAL DIGEST-MD5 Etc
SASL mechanism SASL mechanism . . .
This separation between the definition of protocols and the
definition of authentication mechanisms is crucial. It permits an
authentication mechanism to be defined once, making it usable by any
SASL protocol profiles. In many implementations, the same SASL
mechanism code is used for multiple protocols.
4. Authentication mechanisms
SASL mechanisms are named by strings, from 1 to 20 characters in
length, consisting of upper-case letters, digits, hyphens, and/or
underscores. SASL mechanism names must be registered with the IANA.
IETF Standards Track documents may override this registration
requirement by reserving a portion of the SASL mechanism namespace
for their own use; the GSSAPI mechanism specification [SASL-GSSAPI]
does this. Procedures for registering new SASL mechanisms are given
in the section "Registration procedures".
The "sasl-mech" rule below defines the syntax of a SASL mechanism
name. This uses the augmented Backus-Naur Form (BNF) notation as
specified in [ABNF] and the ABNF core rules as specified in Appendix
A of the ABNF specification [ABNF].
sasl-mech = 1*20mech-char
mech-char = %x41-5A / DIGIT / "-" / "_"
; mech names restricted to uppercase letters,
; digits, "-" and "_"
A. Melnikov FORMFEED[Page 3]
Internet DRAFT SASL 19 August 2003
4.1. Authentication protocol exchange
A SASL mechanism is responsible for conducting an authentication
protocol exchange. This consists of a series of server challenges
and client responses, the contents of which are specific to and
defined by the mechanism. To the protocol, the challenges and
responses are opaque binary tokens of arbitrary length. The
protocol's profile then specifies how these binary tokens are then
encoded for transfer over the connection.
After receiving an authentication command or any client response, a
server mechanism may issue a challenge, indicate failure, or indicate
completion. The server mechanism MAY return additional data with a
completion indication. The protocol's profile specifies how each of
these is then represented over the connection.
After receiving a challenge, a client mechanism may issue a response
or abort the exchange. The protocol's profile specifies how each of
these is then represented over the connection.
During the authentication protocol exchange, the mechanism performs
authentication, transmits an authorization identity (frequently known
as a userid) from the client to server, and negotiates the use of a
mechanism-specific security layer. If the use of a security layer is
agreed upon, then the mechanism must also define or negotiate the
maximum security layer buffer size that each side is able to receive.
4.2. Authorization identities and proxy authentication
An authorization identity is a string of zero or more ISO 10646
[ISO-10646] coded characters. The NUL (U+0000) character is not
permitted in authorization identities. The meaning of an
authorization identity of the empty string (zero length) is defined
below in this section. The authorization identity is used by the
server as the primary identity for making access policy decisions.
The character encoding scheme used for transmitting an authorization
identity over protocol is specified in each authentication mechanism
(with the authentication mechanism's blob being further
restricted/encoded by the protocol profile). Per IETF character set
policy [CHARSET-POLICY], authentication mechanisms SHOULD encode
these and other strings in UTF-8 [UTF-8]. While some legacy
mechanisms are incapable of transmitting an authorization identity
other than the empty string, newly defined mechanisms are expected to
be capable of carrying the entire Unicode repertoire (with the
exception of the NUL character). An authorization identity of the
empty string and and an absent authorization identity MUST be treated
as equivalent. However, mechanisms SHOULD NOT allow both (i.e. if an
A. Melnikov FORMFEED[Page 4]
Internet DRAFT SASL 19 August 2003
authorization identity is allowed to be absent, but one is
transferred, it SHOULD NOT be an empty string).
The identity derived from the client's authentication credentials is
known as the "authentication identity". With any mechanism,
transmitting an authorization identity of the empty string directs
the server to derive an authorization identity from the client's
authentication identity.
If the authorization identity transmitted during the authentication
protocol exchange is not the empty string, this is typically referred
to as "proxy authentication". This feature permits agents such as
proxy servers to authenticate using their own credentials, yet
request the access privileges of the identity for which they are
proxying.
The server makes an implementation defined policy decision as to
whether the authentication identity is permitted to have the access
privileges of the authorization identity and whether the
authorization identity is permitted to receive service. If it is
not, the server indicates failure of the authentication protocol
exchange.
As a client might not have the same information as the server,
clients SHOULD NOT themselves try to derive authorization identities
from authentication identities when clients could instead transmit an
authorization identity of the empty string.
The server SHOULD verify the correctness of a received authorization
identity. Profiles whose authorization identities are simple user
names (e.g. IMAP [RFC 3501]) are encouraged to employ [SASLPrep]
profile [SASLPrep] of the "stringprep" algorithm [StringPrep] to
prepare these names for matching. If the preparation of the
authorization identity fails or results in an empty string, the
server MUST fail the authentication exchange. The only exception to
this rule is when the received authorization identity is already the
empty string.
4.3. Security layers
If use of a security layer is negotiated by the authentication
protocol exchange, the security layer is applied to all subsequent
data sent over the connection. The security layer takes effect
immediately following the last response of the authentication
exchange for data sent by the client and the completion indication
for data sent by the server.
A. Melnikov FORMFEED[Page 5]
Internet DRAFT SASL 19 August 2003
Once the security layer is in effect, the protocol stream is
processed by the security layer into buffers of security encoded
data. Each buffer of security encoded data is transferred over the
connection as a stream of octets prepended with a four octet field in
network byte order that represents the length of the following
buffer. The length of the security encoded data buffer MUST be no
larger than the maximum size that was either defined in the mechanism
specification or negotiated by the other side during the
authentication protocol exchange. Upon the receipt of a data buffer
which is larger than the defined/negotiated maximal buffer size, the
receiver SHOULD close the connection. This might be a sign of an
attack or a buggy implementation.
4.4. Character string issues
Per IETF character set policy [CHARSET-POLICY], authentication
mechanisms SHOULD encode character strings in UTF-8 [UTF-8]. In
order to avoid noninteroperability due to differing normalizations,
when a mechanism specifies that a string authentication identity or
password used as input to a cryptographic function (or used for
comparison) it SHOULD specify that the string first be prepared using
the "SASLPrep" profile [SASLPrep], of the "stringprep" algorithm
[StringPrep]. This should be done by both the client (upon getting
user input or retrieving a value from configuration) and by the
server (upon receiving the value from the client). If preparation
fails or results in an empty string, the client/server SHALL fail the
authentication exchange.
5. Protocol profile requirements
In order to use this specification, a protocol definition MUST supply
the following information:
A service name, to be selected from the IANA registry of "service"
elements for the GSSAPI host-based service name form. [GSSAPI] This
service name is made available to the authentication mechanism.
The registry is available at the URL
"http://www.iana.org/assignments/gssapi-service-names" A definition
of the command to initiate the authentication protocol exchange.
This command must have as a parameter the name of the mechanism being
selected by the client.
The command SHOULD have an optional parameter giving an initial
response. This optional parameter allows the client to avoid a round
trip when using a mechanism which is defined to have the client send
data first. When this initial response is sent by the client and the
A. Melnikov FORMFEED[Page 6]
Internet DRAFT SASL 19 August 2003
selected mechanism is defined to have the server start with an
initial challenge, the command fails. See section 6.1 of this
document for further information.
A definition of the method by which the authentication protocol
exchange is carried out, including how the challenges and responses
are encoded, how the server indicates completion or failure of the
exchange, how the client aborts an exchange, and how the exchange
method interacts with any line length limits in the protocol.
The command SHOULD have a method for the server to include an
optional challenge with a success notification. This allows the
server to avoid a round trip when using a mechanism which is defined
to have the server send additional data along with the indication of
successful completion. See section 6.2 of this document for further
information.
In addition, a protocol profile SHOULD specify a mechanism through
which a client may obtain the names of the SASL mechanisms available
to it. This is typically done through the protocol's extensions or
capabilities mechanism.
Identification of the octet where any negotiated security layer
starts to take effect, in both directions.
If both TLS and SASL security layer are allowed to be negotiated by
the protocol, the protocol profile MUST define in which order they
are applied to a cleartext data sent over the connection.
A protocol profile MAY further refine the definition of an
authorization identity by adding additional syntactic restrictions
and protocol-specific semantics. A protocol profile MUST specify the
form of the authorization identity (as it is protocol specific, as
opposed to the authentication identity which is mechanism specific)
and how authorization identities are to be compared. Profiles whose
authorization identities are simple user names (e.g. IMAP [RFC 3501])
are encouraged to employ [SASLPrep] profile [SASLPrep] of the
"stringprep" algorithm [StringPrep] to prepare these names for
matching.
Certain SASL mechanisms, e.g. DIGEST-MD5 [SASL-DIGEST], use a concept
of realm. Conceptually, realm is the name of a collection of user's
accounts. Realms allow the protected resources on a server to be
partitioned into a set of protection spaces, each with its own
authentication mechanisms and/or authorization database. For example,
a proxy/frontend can use different realms for different
servers/backends it represents. The realm value is a case-
insensitive string, generally assigned by the origin server, which
A. Melnikov FORMFEED[Page 7]
Internet DRAFT SASL 19 August 2003
may have additional semantics specific to the authentication
mechanism.
A protocol profile SHOULD provide a guidance how realms are to be
constructed and used in the protocol and MAY further restrict its
syntax and protocol-specific semantics.
A protocol profile SHOULD NOT attempt to amend the definition of
mechanisms or make mechanism-specific encodings. This breaks the
separation between protocol and mechanism that is fundamental to the
design of SASL.
6. Specific issues
6.1. Client sends data first
Some mechanisms specify that the first data sent in the
authentication protocol exchange is from the client to the server.
If a protocol's profile permits the command which initiates an
authentication protocol exchange to contain an initial client
response, this parameter SHOULD be used with such mechanisms.
If the initial client response parameter is not given, or if a
protocol's profile does not permit the command which initiates an
authentication protocol exchange to contain an initial client
response, then the server issues a challenge with no data. The
client's response to this challenge is then used as the initial
client response. (The server then proceeds to send the next
challenge, indicates completion, or indicates failure.)
6.2. Server returns success with additional data
Some mechanisms may specify that additional data be sent to the
client along with an indication of successful completion of the
exchange. This data would, for example, authenticate the server to
the client.
If a protocol's profile does not permit this additional data to be
returned with a success indication, then the server issues the data
as a server challenge, without an indication of successful
completion. The client then responds with no data. After receiving
this empty response, the server then indicates successful completion
(with no additional data).
Client implementors should be aware of an additional failure case
that might occur when the profile supports sending the additional
data with success. Imagine that an active attacker is trying to
A. Melnikov FORMFEED[Page 8]
Internet DRAFT SASL 19 August 2003
impersonate the server and sends a faked data, that should be used to
authenticate the server to the client, with success. (A similar
situation can happen when either the server and/or the client has a
bug and they calculate different responses). After checking the data
the client will think that the authentication exchange has failed,
however the server will think that the authentication exchange has
completed successfully. At this point the client can't abort the
authentication exchange, it SHOULD close the connection instead.
However, if the profile didn't support sending of additional data
with success, the client could have aborted the exchange at the very
last step of the authentication exchange.
6.3. Multiple authentications
Unless otherwise stated by the protocol's profile, only one
successful SASL negotiation may occur in a protocol session. In this
case, once an authentication protocol exchange has successfully
completed, further attempts to initiate an authentication protocol
exchange fail.
In the case that a profile explicitly permits multiple successful
SASL negotiations to occur, then in no case may multiple security
layers be simultaneously in effect. If a security layer is in effect
and a subsequent SASL negotiation selects a second security layer,
then the second security layer replaces the first. If a security
layer is in effect and a subsequent SASL negotiation selects no
security layer, the original security layer must be removed. The next
paragraph explains why this is important.
Below the term "realm" has the meaning as defined in the section 5.
A security layer that remains in effect when a client, which already
has authenticated and established the security layer with "Realm A",
authenticates to "Realm B", without negotiating a new security layer,
enables "Realm B" to make guesses about previously observed
ciphertext using the server's SASL engine as an oracle. "Realm B" may
observe how known cleartext is encrypted.
A. Melnikov FORMFEED[Page 9]
Internet DRAFT SASL 19 August 2003
+---------+ +---------+
| | | |
| Realm B | | Realm A |
| | | |
+---------+ +---------+
| ^ |
| : +-----------+ |
Traffic from | : | Encryption| | Traffic from A
B to client +-------->| end point |<-------+ to client
: | (SSL/SASL)|
: +-----------+
: |
: |
: +---+
: | |
: | |
: | | Encryption tunnel, e.g. SASL or SSL,
: | | between the server
(1) Recording +---------:| | and a single client only.
encrypted | | Separate tunnels to different
traffic between | | clients.
Realm A and client +---+
|
|
+-----------> Traffic to clients
7. The EXTERNAL mechanism
The mechanism name associated with external authentication is
"EXTERNAL".
The client sends an initial response with the UTF-8 encoding of the
authorization identity. The form of the authorization identity is
further restricted by the application-level protocol's SASL profile.
The server uses information, external to SASL, to determine whether
the client is authorized to authenticate as the authorization
identity. If the client is so authorized, the server indicates
successful completion of the authentication exchange; otherwise the
server indicates failure.
The system providing this external information may be, for example,
IPsec or TLS.
If the client sends the empty string as the authorization identity
(thus requesting the authorization identity be derived from the
client's authentication credentials), the authorization identity is
to be derived from authentication credentials which exist in the
A. Melnikov FORMFEED[Page 10]
Internet DRAFT SASL 19 August 2003
system which is providing the external authentication.
7.1. Formal syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in [ABNF]. This uses the ABNF core
rules as specified in Appendix A of the ABNF specification [ABNF].
Non-terminals referenced but not defined below are as defined by
[UTF-8].
The "initial-response" rule below defines the initial response sent
from client to server.
initial-response = *( UTF8-char-no-null )
UTF8-char-no-null = UTF8-1-no-null / UTF8-2 / UTF8-3 / UTF8-4
UTF8-1-no-null = %x01-7F
7.2. Example
The following is an example of an EXTERNAL authentication in the SMTP
protocol [SMTP-AUTH]. In this example, the client is proxy
authenticating, sending the authorization id "fred". The server has
determined the client's identity through IPsec and has a security
policy that permits that identity to proxy authenticate as any other
identity.
To the protocol profile, the four octet sequence "fred" is an opaque
binary blob. The SASL protocol profile for SMTP specifies that
server challenges and client responses are encoded in BASE64; the
BASE64 encoding of "fred" is "ZnJlZA==".
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
S: 250-smtp.example.com
S: 250 AUTH DIGEST-MD5 EXTERNAL
C: AUTH EXTERNAL ZnJlZA==
S: 235 Authentication successful.
8. IANA Considerations
Registration of a SASL mechanism is done by filling in the template
in section 8.4 and sending it in to iana@iana.org. IANA has the
right to reject obviously bogus registrations, but will perform no
review of claims made in the registration form.
A. Melnikov FORMFEED[Page 11]
Internet DRAFT SASL 19 August 2003
There is no naming convention for SASL mechanisms; any name that
conforms to the syntax of a SASL mechanism name can be registered.
An IETF Standards Track document may reserve a portion of the SASL
mechanism namespace for its own use, amending the registration rules
for that portion of the namespace.
While the registration procedures do not require it, authors of SASL
mechanisms are encouraged to seek community review and comment
whenever that is feasible. Authors may seek community review by
posting a specification of their proposed mechanism as an internet-
draft. SASL mechanisms intended for widespread use should be
standardized through the normal IETF process, when appropriate.
8.1. Comments on SASL mechanism registrations
Comments on registered SASL mechanisms should first be sent to the
"owner" of the mechanism. Submitters of comments may, after a
reasonable attempt to contact the owner, request IANA to attach their
comment to the SASL mechanism registration itself. If IANA approves
of this the comment will be made accessible in conjunction with the
SASL mechanism registration itself.
8.2. Location of registered SASL mechanism list
SASL mechanism registrations are available at the URL
"http://www.iana.org/assignments/sasl-mechanisms" The SASL mechanism
description and other supporting material may also be published as an
Informational RFC by sending it to "rfc-editor@rfc-editor.org"
(please follow the instructions to RFC authors [RFC-INSTRUCTIONS]).
8.3. Change control
Once a SASL mechanism registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a SASL mechanism may pass responsibility for the SASL
mechanism to another person or agency by informing IANA; this can be
done without discussion or review.
The IESG may reassign responsibility for a SASL mechanism. The most
common case of this will be to enable changes to be made to
mechanisms where the author of the registration has died, moved out
of contact or is otherwise unable to make changes that are important
to the community.
SASL mechanism registrations may not be deleted; mechanisms which are
no longer believed appropriate for use can be declared OBSOLETE by a
A. Melnikov FORMFEED[Page 12]
Internet DRAFT SASL 19 August 2003
change to their "intended use" field; such SASL mechanisms will be
clearly marked in the lists published by IANA.
The IESG is considered to be the owner of all SASL mechanisms which
are on the IETF standards track.
8.4. Registration template
To: iana@isi.edu
Subject: Registration of SASL mechanism X
SASL mechanism name:
Security considerations:
Published specification (optional, recommended):
Person & email address to contact for further information:
Intended usage:
(One of COMMON, LIMITED USE or OBSOLETE)
Owner/Change controller:
(Any other information that the author deems interesting may be
added below this line.)
8.5. The EXTERNAL mechanism registration
It is requested that the SASL Mechanism registry [IANA-SASL] entry
for the EXTERNAL mechanism be updated to reflect that this document
now provides its technical specification.
To: iana@iana.org
Subject: Updated Registration of SASL mechanism EXTERNAL
SASL mechanism name: EXTERNAL
Security considerations: See RFC XXXX, section 10.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
Alexey Melnikov <mel@isode.com>
Intended usage: COMMON
A. Melnikov FORMFEED[Page 13]
Internet DRAFT SASL 19 August 2003
Owner/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for EXTERNAL
9. References
9.1. Normative References
[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
ABNF", RFC 2234, November 1997
[CHARSET-POLICY] Alvestrand, "IETF Policy on Character Sets and
Languages", RFC 2277, January 1998
[GSSAPI] Linn, "Generic Security Service Application Program
Interface, Version 2, Update 1", RFC 2743, January 2000
[ISO-10646] "Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane", ISO/IEC 10646-1 : 1993.
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
[Stringprep] P. Hoffman, M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, December 2002.
[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords", Work in progress, draft-ietf-sasl-saslprep-XX.txt.
[UTF-8] Yergeau, "UTF-8, a transformation format of ISO 10646", work
in progress (draft-yergeau-rfc2279bis-XX) that replaces RFC 2279,
Janyary 1998
9.2. Informative References
<<Update the reference below>> [SASL-GSSAPI] Myers, "SASL GSSAPI
mechanisms", work in progress, draft-ietf-cat-sasl-gssapi-XX.txt,
September 2000
[SASL-DIGEST] Leach, P., Newman, C., Melnikov, A., "Using Digest
Authentication as a SASL Mechanism", work in progress, draft-ietf-
sasl-rfc2831bis-XX.txt, replaces RFC 2831
[SASL-OTP] Newman, "The One-Time-Password SASL Mechanism", RFC 2444,
October 1998
[SMTP-AUTH] Myers, "SMTP Service Extension for Authentication", RFC
2554, March 1999
A. Melnikov FORMFEED[Page 14]
Internet DRAFT SASL 19 August 2003
[RFC-INSTRUCTIONS] Postel, Reynolds, "Instructions to RFC Authors",
RFC 2223, October 1997
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", http://www.iana.org/assignments/sasl-mechanisms.
10. Security considerations
Security issues are discussed throughout this memo.
The mechanisms that support integrity protection are designed such
that the negotiation of the security layer and authorization identity
is integrity protected. When the client selects a security layer
with at least integrity protection, this protects against an active
attacker hijacking the connection and modifying the authentication
exchange to negotiate a plaintext connection.
When a server or client supports multiple authentication mechanisms,
each of which has a different security strength, it is possible for
an active attacker to cause a party to use the least secure mechanism
supported. To protect against this sort of attack, a client or
server which supports mechanisms of different strengths should have a
configurable minimum strength that it will use. It is not sufficient
for this minimum strength check to only be on the server, since an
active attacker can change which mechanisms the client sees as being
supported, causing the client to send authentication credentials for
its weakest supported mechanism.
The client's selection of a SASL mechanism is done in the clear and
may be modified by an active attacker. It is important for any new
SASL mechanisms to be designed such that an active attacker cannot
obtain an authentication with weaker security properties by modifying
the SASL mechanism name and/or the challenges and responses.
Any protocol interactions prior to authentication are performed in
the clear and may be modified by an active attacker. In the case
where a client selects integrity protection, it is important that any
security-sensitive protocol negotiations be performed after
authentication is complete. Protocols should be designed such that
negotiations performed prior to authentication should be either
ignored or revalidated once authentication is complete.
When use of a security layer is negotiated by the authentication
protocol exchange, the receiver should handle gracefully any security
encoded data buffer larger than the defined/negotiated maximal size.
In particular, it must not blindly allocate the amount of memory
specified in the buffer size field, as this might cause the "out of
memory" condition. If the receiver detects a large block, it SHOULD
A. Melnikov FORMFEED[Page 15]
Internet DRAFT SASL 19 August 2003
close the connection.
"stringprep" and Unicode security considerations apply to
authentication identities, authorization identities and passwords.
The EXTERNAL mechanism provides no security protection; it is
vulnerable to spoofing by either client or server, active attack, and
eavesdropping. It should only be used when external security
mechanisms are present and have sufficient strength.
11. Editor's Address
Alexey Melnikov
Isode
Email: mel@isode.com
12. Acknowledgments
This document is a revision of RFC 2222 written by John G. Myers
<jgmyers@netscape.com>. He also wrote the major part of this
document.
Thank you to Magnus Nystrom for the ASCII picture used in section
6.3.
Definition of realm was extracted from RFC 2617 ("HTTP
Authentication: Basic and Digest Access Authentication").
Contributions of many members of the SASL mailing list are gratefully
acknowledged.
13. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
A. Melnikov FORMFEED[Page 16]
Internet DRAFT SASL 19 August 2003
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Appendix A. Relation of SASL to transport security
Questions have been raised about the relationship between SASL and
various services (such as IPsec and TLS) which provide a secured
connection.
Two of the key features of SASL are:
The separation of the authorization identity from the identity in
the client's credentials. This permits agents such as proxy
servers to authenticate using their own credentials, yet request
the access privileges of the identity for which they are proxying.
Upon successful completion of an authentication exchange, the
server knows the authorization identity the client wishes to use.
This allows servers to move to a "user is authenticated" state in
the protocol.
These features are extremely important to some application protocols,
yet Transport Security services do not always provide them. To
define SASL mechanisms based on these services would be a very messy
task, as the framing of these services would be redundant with the
framing of SASL and some method of providing these important SASL
features would have to be devised.
Sometimes it is desired to enable within an existing connection the
use of a security service which does not fit the SASL model. (TLS is
an example of such a service.) This can be done by adding a command,
for example "STARTTLS", to the protocol. Such a command is outside
the scope of SASL, and should be different from the command which
A. Melnikov FORMFEED[Page 17]
Internet DRAFT SASL 19 August 2003
starts a SASL authentication protocol exchange.
In certain situations, it is reasonable to use SASL underneath one of
these Transport Security services. The transport service would
secure the connection, either service would authenticate the client,
and SASL would negotiate the authorization identity. The SASL
negotiation would be what moves the protocol from "unauthenticated"
to "authenticated" state. The "EXTERNAL" SASL mechanism is
explicitly intended to handle the case where the transport service
secures the connection and authenticates the client and SASL
negotiates the authorization identity.
When using SASL underneath a sufficiently strong Transport Security
service, a SASL security layer would most likely be redundant. The
client and server would thus probably want to negotiate off the use
of a SASL security layer.
Appendix B. IANA considerations
The IANA is directed to modify the SASL mechanisms registry as
follows:
Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
registrations to OBSOLETE. Change the "Published specification"
of the EXTERNAL mechanism to this document.
Appendix C. Changes since RFC 2222
The GSSAPI mechanism was removed. It is now specified in a separate
document [SASL-GSSAPI].
The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has
been removed.
The "SKEY" mechanism described in RFC 2222 is obsolete and has been
removed. It has been replaced by the OTP mechanism [SASL-OTP].
The overview has been substantially reorganized and clarified.
Clarified the definition and semantics of the authorization identity.
Prohibited the NULL character in authorization identities.
Added a section on character string issues.
The word "must" in the first paragraph of the "Protocol profile
requirements" section was changed to "MUST".
A. Melnikov FORMFEED[Page 18]
Internet DRAFT SASL 19 August 2003
Specified that protocol profiles SHOULD provide a way for clients to
discover available SASL mechanisms.
Made the requirement that protocol profiles specify the semantics of
the authorization identity optional to the protocol profile.
Clarified that such a specification is a refinement of the definition
in the base SASL spec.
Added a requirement discouraging protocol profiles from breaking the
separation between protocol and mechanism.
Mentioned that standards track documents may carve out their own
portions of the SASL mechanism namespace.
Specified that the authorization identity in the EXTERNAL mechanism
is encoded in UTF-8.
Added a statement that a protocol profile SHOULD allow challenge data
to be sent with a success indication.
Added a security consideration for the EXTERNAL mechansim.
Clarified sections concerning success with additional data.
Updated IANA related URLs.
Updated references and split them into Informative and Normative.
Added text to the Security Considerations section regarding handling
of extremely large SASL blocks.
Replaced UTF-8 ABNF with the reference to the UTF-8 document.
Added text about SASLPrep for authentication identities and
passwords.
Added paragraph about verifying authorization identities.
This document requires to drop a security layer on reauthentication
when no security layer is negotiated. This differs from RFC 2222,
which required to keep the last security layer in this case.
Added a protocol profile requirement to specify interaction between
SASL and TLS security layers.
A. Melnikov FORMFEED[Page 19]
Internet DRAFT SASL 19 August 2003
Status of this Memo ......................................... i
1. Abstract .............................................. 2
2. Organization of this document ......................... 2
2.1. How to read this document ............................. 2
2.2. Conventions used in this document ..................... 2
3. Overview .............................................. 2
4. Authentication mechanisms ............................. 3
4.1. Authentication protocol exchange ...................... 4
4.2. Authorization identities and proxy authentication ..... 4
4.3. Security layers ....................................... 5
4.4. Character string issues ............................... 6
5. Protocol profile requirements ......................... 6
6. Specific issues ....................................... 8
6.1. Client sends data first ............................... 8
6.2. Server returns success with additional data ........... 8
6.3. Multiple authentications .............................. 9
7. The EXTERNAL mechanism ............................... 10
7.1. Formal syntax ........................................ 11
7.2. Example .............................................. 11
8. IANA Considerations .................................. 11
8.1. Comments on SASL mechanism registrations ............. 12
8.2. Location of registered SASL mechanism list ........... 12
8.3. Change control ....................................... 12
8.4. Registration template ................................ 13
8.5. The EXTERNAL mechanism registration .................. 13
9. References ........................................... 14
9.1. Normative References ................................. 14
9.2. Informative References ............................... 14
10. Security considerations .............................. 15
11. Editor's Address ..................................... 16
12. Acknowledgments ...................................... 16
13. Full Copyright Statement ............................. 16
Appendix A. Relation of SASL to transport security ......... 17
Appendix B. IANA considerations ............................ 18
Appendix C. Changes since RFC 2222 ......................... 18
A. Melnikov FORMFEED[Page ii]