Network Working Group Robert Siemborski INTERNET-DRAFT Google, Inc. Intended Category: Proposed Standard Abhijit Menon-Sen Obsoletes: RFC 1734 Oryx Mail Systems GmbH Updates: RFC 2449 November 2006 POP3 SASL Authentication Mechanism draft-siemborski-rfc1734bis-07.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 in May 2007. Abstract This document defines a profile of the Simple Authentication and Security Layer (SASL) for the Post Office Protocol (POP3). This extension allows a POP3 client to indicate an authentication mechanism to the server, perform an authentication protocol exchange, and optionally negotiate a security layer for subsequent protocol interactions during this session. This document seeks to consolidate the information related to POP3 AUTH into a single document. To this end, this document obsoletes RFC 1734, replacing it as a Proposed Standard and updates Siemborski and Menon-Sen Expires May 2007 [Page 1]
POP3 SASL Authentication Mechanism November 2006 information contained in Section 6.3 of RFC 2449. 1. Conventions Used in This Document 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 [RFC2119]. In examples, "C:" and "S:" indicate lines sent by the client and server respectively. Formal syntax is defined by [RFC4234]. 2. Introduction The POP3 (see [RFC1939]) AUTH command (see [RFC1734]) has suffered several problems in its specification. The first is that it was very similar to a SASL framework defined by [RFC4422], but pre-dated the initial SASL specification. It was therefore missing some key components, such as a way to list the available authentication mechanisms. Later, [RFC2449] attempted to remedy this situation by adding the CAPA command and allowing an initial client response to the AUTH command, however problems in the clarity of the specification of how the initial client response was to be handled remained. Additionally, there is yet another document, [RFC3206], that provides additional response codes that are useful during authentication. Together, this means creating a full POP3 AUTH implementaiton requires an understanding of material in at least five (and probably six) different documents. This document attempts to combine the information in [RFC1734] and [RFC2449] to simplify this situation. Additionally, it aims to clarify and update the older specifications where appropriate. 3. The SASL Capability This section supersedes the definition of the SASL Capability in section 6.3 of [RFC2449]. CAPA tag: SASL Siemborski and Menon-Sen Expires May 2007 [Page 2]
POP3 SASL Authentication Mechanism November 2006 Arguments: Supported SASL Mechanisms Added commands: AUTH Standard Commands Affected None Announced states / possible differences: both / no Commands valid in states: AUTHORIZATION Specification Reference: This Document, [RFC4422] Discussion The SASL capability permits the use of the AUTH command (as defined in section 4 of this document) to begin a SASL negotiation (as defined in [RFC4422]). The argument to the SASL capability is a space-separated list of SASL mechanisms which are supported. If a server either does not support the CAPA command or does not advertise the SASL capability, clients SHOULD NOT attempt the AUTH command. If a client does attempt the AUTH command in such a situation, it MUST NOT supply the client initial response parameter (for backwards compatibility with [RFC1734]). Note that the list of available mechanisms MAY change after a successful STLS command (see [RFC2595]). However, as required by [RFC2449] implementations MUST continue to include the SASL capability even after a successful AUTH command has been completed (even though no further AUTH commands may be issued). Example S: +OK pop.example.com BlurdyBlurp POP3 server ready C: CAPA S: +OK List of capabilities follows S: SASL DIGEST-MD5 GSSAPI ANONYMOUS S: STLS S: IMPLEMENTATION BlurdyBlurp POP3 server S: . Siemborski and Menon-Sen Expires May 2007 [Page 3]
POP3 SASL Authentication Mechanism November 2006 4. The AUTH Command AUTH mechanism [initial-response] Arguments: mechanism: A string identifying a SASL authentication mechanism. initial-response: An optional initial client response. If present, this response MUST be encoded as Base64 (specified in Section 4 of [RFC4648]) or consist only of the single character "=", which represents an empty initial response. Restrictions: After an AUTH command has been successfully completed, no more AUTH commands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with an -ERR reply. The AUTH command may only be given during the AUTHORIZATION state. Discussion: The AUTH command initiates a SASL authentication exchange between the client and the server. The client identifies the SASL mechanism to use with the first parameter of the AUTH command. If the server supports the requested authentication mechanism, it performs the SASL exchange to authenticate the user. Optionally, it also negotiates a security layer for subsequent protocol interactions during this session. If the requested authentication mechanism is not supported, the server rejects the AUTH command with an -ERR reply. The authentication protocol exchange consists of a series of server challenges and client responses that are specific to the chosen SASL mechanism. A server challenge is sent as a line consisting of a "+" character followed by a single space and a string encoded using Base64 as specified in Section 4 of [RFC4648]. This line MUST NOT contain any text other than the BASE64 encoded challenge. A client response consists of a line containing a string encoded as Base64. If the client wishes to cancel the Siemborski and Menon-Sen Expires May 2007 [Page 4]
POP3 SASL Authentication Mechanism November 2006 authentication exchange, it issues a line with a single "*". If the server receives such a response, it MUST reject the AUTH command by sending an -ERR reply. The optional initial response argument to the AUTH command is used to save a round trip when using authentication mechanisms that support an initial client response. If the initial response argument is omitted and the chosen mechanism requires an initial client response, the server MUST proceed as defined in section 3 of [RFC4422]. In POP3, a server challenge with no data is defined as line with only a "+" followed by a single space. It MUST NOT contain any other data. For the purposes of the initial client response, the line length limitation defined in [RFC2449] still applies. If a client initial send would cause the AUTH command to exceed this length, the client MUST NOT use the initial response parameter (and instead proceed as defined in section 5.1 of [RFC4422]). If the client needs to send a zero-length initial response, the client MUST transmit the response as a single equals sign ("="). This indicates that the response is present, but contains no data. If the client uses an initial-response argument to the AUTH command with a SASL mechanism that does not support an initial client send, the server MUST reject the AUTH command with an -ERR reply. If the server cannot Base64 decode a client response, it MUST reject the AUTH command with an -ERR reply. If the client cannot Base64 decode any of the server's challenges, it MUST cancel the authentication using the "*" response. In particular, servers and clients MUST reject (and not ignore) any character not explicitly allowed by the Base64 alphabet, and MUST reject any sequence of Base64 characters that contains the pad character ('=') anywhere other than the end of the string (e.g. "=AAA" and "AAA=BBB" are not allowed). Note that these Base64 strings (excepting the initial client response) may be of arbitrarily length. Clients and servers MUST be able to handle the maximum encoded size of challenges and responses generated by their supported authentication mechanisms. This requirement is independent of any line length limitations the client or server may have in other parts of its protocol implementation. Siemborski and Menon-Sen Expires May 2007 [Page 5]
POP3 SASL Authentication Mechanism November 2006 If the server is unable to authenticate the client, it MUST reject the AUTH command with an -ERR reply. Should the client successfully complete the exchange, the server issues a +OK reply. Additionally, upon success, the POP3 session enters the TRANSACTION state. The authorization identity generated by the SASL exchange is a simple username, and SHOULD use the SASLprep profile (see [RFC4013]) of the StringPrep algorithm (see [RFC3454]) to prepare these names for matching. If preparation of the authorization identity fails or results in an empty string (unless it was transmitted as the empty string), the server MUST fail the authentication. If a security layer is negotiated during the SASL exchange, it takes effect for the client on the octet immediately following the CRLF that concludes the last response generated by the client. For the server, it takes effect immediately following the CRLF of its success reply. When a security layer takes effect, the server MUST discard any knowledge previously obtained from the client, which was not obtained from the SASL negotiation itself. Likewise, the client MUST discard any knowledge obtained from the server, such as the list of available POP3 service extensions. When both TLS (see [RFC4346]) and SASL security layers are in effect, the TLS encoding MUST be applied after the SASL encoding when sending data. (According to [RFC2595], STLS can only be issued before AUTH in any case.) Note that POP3 does not allow for additional data to be sent with a message indicating a successful outcome (see section 3.6 of [RFC4422]). The service name specified by this protocol's profile of SASL is "pop". If an AUTH command fails, the client may try another authentication mechanism or present different credentials by issuing another AUTH command (or by using one of the other POP3 authentication mechanisms). Likewise, the server MUST behave as if the client had not issued the AUTH command. To ensure interoperability, client and server implementations of this extension MUST implement the [DIGEST-MD5] SASL mechanism. Siemborski and Menon-Sen Expires May 2007 [Page 6]
POP3 SASL Authentication Mechanism November 2006 5. Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form notation as specified in [RFC4234]. The rules CRLF, ALPHA and DIGIT are imported from [RFC4234]. The sasl-mech rule is from [RFC4422]. Except as noted otherwise, all alphabetic characters are case- insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion. auth-command = "AUTH" SP sasl-mech [SP (base64 / "=")] *(CRLF [base64]) CRLF auth-resp = ("*" / base64) CRLF base64 = base64-terminal / ( 1*(4base64-CHAR) [base64-terminal] ) base64-char = ALPHA / DIGIT / "+" / "/" ;; Case-sensitive base64-terminal = (2base64-char "==") / (3base64-char "=") continue-req = "+" SP [base64] CRLF Additionally, the ABNF specified in [RFC2449] is updated as follows: challenge /= continue-req 6. Examples Here is an example of a client attempting AUTH PLAIN (see [RFC4616]) under TLS and making use of the initial client response: S: +OK pop.example.com BlurdyBlurp POP3 server ready C: CAPA S: +OK List of capabilities follows S: SASL DIGEST-MD5 GSSAPI ANONYMOUS S: STLS S: IMPLEMENTATION BlurdyBlurp POP3 server S: . C: STLS S: +OK Begin TLS negotiation now (TLS negotiation proceeds, further commands protected by TLS layer) Siemborski and Menon-Sen Expires May 2007 [Page 7]
POP3 SASL Authentication Mechanism November 2006 C: CAPA S: +OK List of capabilities follows S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS S: IMPLEMENTATION BlurdyBlurp POP3 server S: . C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q= S: +OK Maildrop locked and ready Here is another client that is attempting AUTH PLAIN under a TLS layer, this time without the initial response. Parts of the negotiation before the TLS layer was established have been omitted: (TLS negotiation proceeds, further commands protected by TLS layer) C: CAPA S: +OK List of capabilities follows S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS S: IMPLEMENTATION BlurdyBlurp POP3 server S: . C: AUTH PLAIN (note that there is a space following the '+' on the following line) S: + C: dGVzdAB0ZXN0AHRlc3Q= S: +OK Maildrop locked and ready Here is an example using a mechanism in which the exchange begins with a server challenge (the long lines are broken for editorial clarity only): S: +OK pop.example.com BlurdyBlurp POP3 server ready C: CAPA S: +OK List of capabilities follows S: SASL DIGEST-MD5 GSSAPI ANONYMOUS S: STLS S: IMPLEMENTATION BlurdyBlurp POP3 server S: . C: AUTH DIGEST-MD5 S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0 RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh cnNldD11dGYtOA== C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2 QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg= S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA== C: Siemborski and Menon-Sen Expires May 2007 [Page 8]
POP3 SASL Authentication Mechanism November 2006 S: +OK Maildrop locked and ready 7. Security Considerations Security issues are discussed throughout this document. 8. IANA Considerations The IANA is requested to refer to this RFC instead of [RFC1734] in http://www.iana.org/assignments/pop3-extension-mechanism (the POP3 extension registry). 9. Acknowledgments The authors would like to acknowledge the contributions of John Myers, Randall Gellens, Chris Newman, Laurence Lundblade, and other contributors to RFC 1734 and RFC 2554, on which this document draws heavily. The authors would also like to thank Ken Murchison, Randall Gellens, Alexey Melnikov, Mark Crispin, and Arnt Gulbrandsen for the time they devoted to reviewing early drafts of this document. 10. Changes From RFC 1734, RFC 2449. 1. The SASL-based semantics defined in RFC 2449 are now normative for the AUTH extension. 2. Clarifications and examples of the proper behavior of initial client response handling. 3. Minimum requirement of support for DIGEST-MD5. 4. Clarify ordering of TLS and SASL security layers. 5. Update references to newer versions of various specifications. 6. Clarify that the mechanism list can change. 7. Add the use of the SASLprep profile for preparing authorization identities. 8. General other editorial clarifications. Siemborski and Menon-Sen Expires May 2007 [Page 9]
POP3 SASL Authentication Mechanism November 2006 9. Consolidation of much applicable information into a single document. 10. CR is no longer (incorrectly) defined here. 11. Include M-T-I DIGEST-MD5 in the SASL capability response. 12. Explicitly mention that "=" means a zero-length initial response. 13. Change MUST to SHOULD use SASLprep, because nobody does. 14. Clarify that the TLS encoding should be applied after any SASL one. 15. Note that POP3 doesn't allow additional data to be sent with +OK. 16. Change "_" to "-" in the ABNF, and use the sasl-mech rule instead of AUTH_CHAR. 17. Change the KERBEROS_V4 example to DIGEST-MD5 for now; remove KERBEROS_V4. 11. Normative References [RFC1939] Myers, Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [RFC2119] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2449] Gellens, Newman, Lundblade, "POP3 Extension Mechanism", RFC 2449, November 1998. [RFC2595] Newman, "Using TLS with IMAP, POP3, and ACAP", RFC 2595, June 1999. [RFC3454] Hoffman, Blanchet, "Preparation of Internationalized Strings ( RFC 3454, December 2002. [RFC4013] Zeilenga, "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, OpenLDAP Foundation, February 2005. [RFC4234] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, Brandenburg Siemborski and Menon-Sen Expires May 2007 [Page 10]
POP3 SASL Authentication Mechanism November 2006 Internetworking, Demon Internet Ltd, October 2005. [RFC4422] Melnikov, Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006. [RFC4648] Josefsson, "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2003. [DIGEST-MD5] Melnikov, "Using Digest Authentication as a SASL Mechanism", draft-ietf-sasl-rfc2831bis-11.txt, Isode Ltd., November 2006 12. Informative References [RFC1734] Myers, "POP3 AUTHentication Command", RFC 1734, January 1994. [RFC3206] Gellens, "The SYS and AUTH POP Response Codes", RFC 3206, February 2002. [RFC4346] Dierks, Rescorla, "The Transport Layer Security (TLS) Protocol, Version 1.1", RFC 4346, April 2006. [RFC4616] Zeilenga, "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, OpenLDAP Foundation, August 2006. 13. Authors' Addresses Robert Siemborski Google, Inc. 1600 Ampitheatre Parkway Mountain View, CA 94043 Phone: +1 650 623 6925 Email: robsiemb@google.com Abhijit Menon-Sen Oryx Mail Systems GmbH Email: ams@oryx.com Siemborski and Menon-Sen Expires May 2007 [Page 11]
POP3 SASL Authentication Mechanism November 2006 Protocol Actions [RFC Editor: Remove this section before publication] This document obsoletes RFC 1734 and replaces it as a Proposed Standard. By moving RFC 1734 to Historic, RFC 1731 can also be moved to Historic (as RFC 1734 was the last document to have a normative reference). It also updates information contained in Section 6.3 of RFC 2449. Siemborski and Menon-Sen Expires May 2007 [Page 12]
POP3 SASL Authentication Mechanism November 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Full Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Siemborski and Menon-Sen Expires May 2007 [Page 13]
