INTERNET-DRAFT Editor: Kurt D. Zeilenga Intended Category: Standards Track OpenLDAP Foundation Expires August 2004 15 February 2004 Updates: RFC 2595 The Plain SASL Mechanism <draft-ietf-sasl-plain-04.txt> Status of Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. This document is intended to be, after appropriate review and revision, submitted to the RFC Editor as a Standards Track document. Distribution of this memo is unlimited. Technical discussion of this document will take place on the IETF SASL mailing list <ietf-sasl@imc.org>. Please send editorial comments directly to the document editor <Kurt@OpenLDAP.org>. 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 (C) The Internet Society (2004). All Rights Reserved. Please see the Full Copyright section near the end of this document for more information. Abstract This document defines a simple clear-text user/password Simple Authentication and Security Layer (SASL) mechanism called the PLAIN mechanism. The PLAIN mechanism is intended to be used, in combination with data confidentiality services provided by a lower layer, in protocols which lack a simple password authentication command. Zeilenga Plain SASL Mechanism [Page 1]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 Conventions 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 [Keywords]. 1. Background and Intended Usage Clear-text passwords are simple, interoperate with almost all existing operating system authentication databases, and are useful for a smooth transition to a more secure password-based authentication mechanism. The drawback is that they are unacceptable for use over an unencrypted network connection. This document defines the PLAIN Simple Authentication and Security Layer ([SASL]) mechanism for use in protocols with no clear-text login command (e.g., [ACAP] or [SMTP-AUTH]). The name associated with this mechanism is "PLAIN". The PLAIN SASL mechanism does not provide a security layer. This mechanism MUST NOT be used without adequate security protection as the mechanism affords no integrity nor confidentiality protection itself. The PLAIN SASL mechanism MUST NOT be advertised unless a strong encryption layer, such as provided by Transport Layer Security ([TLS]), is active or backwards compatibility dictates otherwise. This document updates RFC 2595, replacing Section 6. Changes since RFC 2595 are detailed in Appendix A. 2. PLAIN SASL mechanism The mechanism consists of a single message from the client to the server. The client sends the authorization identity (identity to login as), followed by a NUL (U+0000) character, followed by the authentication identity (identity whose password will be used), followed by a NUL (U+0000) character, followed by the clear-text password. The client leaves the authorization identity empty if it wishes the server to derive the authorization identity from the authentication identity. The formal grammar for the client message using Augmented BNF [ABNF] follows. message = [authzid] NUL authcid NUL passwd authcid = 1*SAFE ; MUST accept up to 255 octets Zeilenga Plain SASL Mechanism [Page 2]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 authzid = 1*SAFE ; MUST accept up to 255 octets passwd = 1*SAFE ; MUST accept up to 255 octets NUL = %x00 SAFE = UTF1 / UTF2 / UTF3 / UTF4 ;; any UTF-8 encoded Unicode character except NUL UTF1 = %x01-7F ;; except NULL UTF2 = %xC2-DF UTF0 UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) / %xED %x80-9F UTF0 / %xEE-EF 2(UTF0) UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) / %xF4 %x80-8F 2(UTF0) UTF0 = %x80-BF The authorization identity (authzid), authentication identity (authcid) and password (passwd) SHALL be transferred as [UTF-8] encoded strings of [Unicode] characters. As NUL (U+0000) is used as a deliminator, the NUL (U+0000) MUST NOT appear in authzid, authcid, or passwd productions. The form of the authzid production is specific to the application-level protocol's SASL profile [SASL]. The authcid and passwd productions are form-free. Use of non-visible characters or characters which a user may be unable to enter on some keyboards is discouraged. Servers MUST be capable of accepting authzid, authcid, and passwd productions up to and including 255 octets. It is noted that the UTF-8 encoding of a Unicode character may be as long as 4 octets. Upon receipt of the message, the server will verify the presented authentication identity (authcid) and password (passwd) with the system authentication database and verify the authentication credentials permit the client to login as the (presented or derived) authorization identity. If both steps succeed, the user is authenticated. The presented authentication identity and password strings are not to be compared directly with stored strings. The server SHALL first prepare authentication identity and password strings using the [SASLPrep] profile of the [StringPrep] algorithm. Per Section 7 of [StringPrep], unassigned code points may appear in prepared presented (query) strings but are prohibited in stored strings. If preparation fails or results in an empty string, verification SHALL fail. If the server stores only the hash of expected string, the string MUST be prepared before generation of the hash. Zeilenga Plain SASL Mechanism [Page 3]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 When the no authorization identity is provided, the server SHALL derive the authorization identity from the prepared representation of the provided authentication identity string. This ensures that the derivation of different representations of the authentication identity produce the same authorization identity. The verification function (using hashed password) can be written (in pseudo-code): boolean Verify(string authzid, string authcid, string passwd) { string pAuthcid = SASLprep(authcid, true); # prepare authcid string pPasswd = SASLprep(passwd, true); # prepare passwd if (pAuthcid == NULL || pPasswd == NULL) { return false; # preparation failed } if (pAuthcid == "" || pPasswd == "") { return false; # empty prepared string } storedHash = FetchPasswordHash(pAuthcid); if (storedHash == NULL || storedHash == "") { return false; # error or unknown authcid } if (!Compare(storedHash, Hash(pPasswd))) { return false; # incorrect password } if (authzid == NULL) { authzid = DeriveAuthzid(pAuthcid); if (authzid == NULL || authzid == "") { return false; # could not derive authzid } } if (!Authorize(pAuthcid, authzid)) { return false; # not authorized } return true; } The second parameter of the SASLprep function, when true, indicates that the input string is a "query" string [StringPrep] and, hence, unassigned code points are allowed. When the SASLprep function is called to prepared the password prior to computing the stored hash, the second parameter would be false as unassigned code points are not allowed in "stored strings" [StringPrep]. Zeilenga Plain SASL Mechanism [Page 4]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 The second parameter provided to the Authorize function is not prepared by this code. The application-level SASL profile should be consulted to determine what, if any, preparation is necessary. The server MAY also use the credentials to initialize any new authentication database, such as one suitable for [CRAM-MD5] or [DIGEST-MD5]. 4. Examples This section provides examples of PLAIN authentication exchanges. The examples are intended to help the readers understand the above text. The examples are not definitive. "C:" and "S:" indicate lines sent by the client and server respectively. "<NUL>" represents a single NUL (U+0000) character. The Application Configuration Access Protocol ([ACAP]) is used in the examples. The first example shows how the PLAIN mechanism might be used for user authentication. S: * ACAP (SASL "CRAM-MD5") (STARTTLS) C: a001 STARTTLS S: a001 OK "Begin TLS negotiation now" <TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "CRAM-MD5" "PLAIN") C: a002 AUTHENTICATE "PLAIN" S: + "" C: {21} C: <NUL>tim<NUL>tanstaaftanstaaf S: a002 OK "Authenticated" The second example shows how the PLAIN mechanism might be used to assuming the identity of another user. In this example, the server rejects the request. Also, this example makes use of the protocol optional initial response capability to eliminate a round-trip. S: * ACAP (SASL "CRAM-MD5") (STARTTLS) C: a001 STARTTLS S: a001 OK "Begin TLS negotiation now" <TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "CRAM-MD5" "PLAIN") C: a002 AUTHENTICATE "PLAIN" {20+} C: Ursel<NUL>Kurt<NUL>xipj3plmq S: a002 NO "Not authorized to assume asserted identity" Zeilenga Plain SASL Mechanism [Page 5]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 5. Security Considerations The PLAIN mechanism relies on the TLS encryption layer for security. When used without TLS, it is vulnerable to a common network eavesdropping attack. Therefore PLAIN MUST NOT be advertised or used unless a suitable TLS encryption layer is active or backwards compatibility dictates otherwise. When the PLAIN mechanism is used, the server gains the ability to impersonate the user to all services with the same password regardless of any encryption provided by TLS or other network privacy mechanisms. While many other authentication mechanisms have similar weaknesses, stronger SASL mechanisms address this issue. Clients are encouraged to have an operational mode where all mechanisms which are likely to reveal the user's password to the server are disabled. General SASL security considerations apply to this mechanism. "stringprep" and Unicode [StringPrep] security considerations also apply, as do [UTF-8] security considerations. 6. IANA Considerations It is requested that the SASL Mechanism registry [IANA-SASL] entry for the PLAIN mechanism be updated to reflect that this document now provides its technical specification. To: iana@iana.org Subject: Updated Registration of SASL mechanism PLAIN SASL mechanism name: PLAIN Security considerations: See RFC XXXX. Published specification (optional, recommended): RFC XXXX Person & email address to contact for further information: Kurt Zeilenga <kurt@openldap.org> IETF SASL WG <ietf-sasl@imc.org> Intended usage: COMMON Author/Change controller: IESG <iesg@ietf.org> Note: Updates existing entry for PLAIN 7. Acknowledgment This document is a revision of RFC 2595 by Chris Newman. Portions of the grammar defined in Section 2 were borrowed from [UTF-8] by Francois Yergeau. This document is a product of the IETF SASL WG. Zeilenga Plain SASL Mechanism [Page 6]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 8. Normative References [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [Keywords] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [SASL] Melnikov, A. (Editor), "Simple Authentication and Security Layer (SASL)", draft-ietf-sasl-rfc2222bis-xx.txt, a work in progress. [SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names and passwords", draft-ietf-sasl-saslprep-xx.txt, a work in progress. [StringPrep] Hoffman P. and M. Blanchet, "Preparation of Internationalized Strings ('stringprep')", draft-hoffman-rfc3454bis-xx.txt, a work in progress. [Unicode] The Unicode Consortium, "The Unicode Standard, Version 3.2.0" is defined by "The Unicode Standard, Version 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the "Unicode Standard Annex #27: Unicode 3.1" (http://www.unicode.org/reports/tr27/) and by the "Unicode Standard Annex #28: Unicode 3.2" (http://www.unicode.org/reports/tr28/). [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3639 (also STD 63), November 2003. [TLS] Dierks, T. and, E. Rescorla, "The TLS Protocol Version 1.1", draft-ietf-tls-rfc2246-bis-xx.txt, a work in progress. 9. Informative References [ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", draft-ietf-sasl-crammd5-xx.txt, a work in progress. [DIGEST-MD5] Leach, P., C. Newman, and A. Melnikov, "Using Digest Authentication as a SASL Mechanism", draft-ietf-sasl-rfc2831bis-xx.txt, a work in progress. [IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL) Zeilenga Plain SASL Mechanism [Page 7]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 MECHANISMS", <http://www.iana.org/assignments/sasl- mechanisms>. [SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. 10. Editor's Address Kurt D. Zeilenga OpenLDAP Foundation Email: Kurt@OpenLDAP.org Appendix A. Changes since RFC 2595 This appendix is non-normative. This document replaces Section 6 of RFC 2595. The specification details how the server is to compare client-provided character strings with stored character strings. The ABNF grammar was updated. In particular, the grammar now allows LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the authzid, authcid, passwd productions. However, whether these control characters may be used depends on the string preparation rules applicable to the production. For passwd and authcid productions, control characters are prohibited. For authzid, one must consult the application-level SASL profile. Pseudo-code was added. The example section was expanded to illustrate more features of the PLAIN mechanism. Intellectual Property Rights 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 Zeilenga Plain SASL Mechanism [Page 8]
INTERNET-DRAFT draft-ietf-sasl-plain-04.txt 15 February 2004 procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Copyright (C) The Internet Society (2004). 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 followed, or as required to translate it into languages other than English. Zeilenga Plain SASL Mechanism [Page 9]
