Network Working Group J. Myers Internet Draft: SMTP Authentication February 1997 Document: draft-myers-smtp-auth-05.txt SMTP Service Extension for Authentication Status of this Memo This document is an Internet Draft. 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 a ``working draft'' or ``work in progress``. To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or munnari.oz.au. A revised version of this draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. This document will expire before July 1996. Distribution of this draft is unlimited. 1. Introduction This document defines an extension to the SMTP service whereby an SMTP client may indicate an authentication mechanism to the server, perform an authentication protocol exchange, and optionally negotiate a security layer for subsequent protocol interactions. This extension is a profile of the Simple Authentication and Security Layer [SASL]. Myers [Page 1]
Internet Draft SMTP Authentication 26 February 1997 2. Conventions Used in this Document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. The following terms are used in this document to signify the requirements of this specification. 1) MUST, or the adjective REQUIRED, means that the definition is an absolute requirement of the specification. 2) MUST NOT that the definition is an absolute prohibition of the specification. 3) SHOULD means that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications MUST be understood and carefully weighed before choosing a different course. 4) SHOULD NOT means that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications SHOULD be understood and the case carefully weighed before implementing any behavior described with this label. 5) MAY, or the adjective OPTIONAL, means that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option. "Can" is used instead of "may" when referring to a possible circumstance or situation, as opposed to an optional facility of the protocol. 3. The Authentication service extension (1) the name of the SMTP service extension is "Authentication" (2) the EHLO keyword value associated with this extension is "AUTH" (3) The AUTH EHLO keyword contains as a parameter a space separated list of the names of supported SASL mechanisms. Myers [Page 2]
Internet Draft SMTP Authentication 26 February 1997 (4) a new SMTP verb "AUTH" is defined (5) No additional parameters are added to either the MAIL FROM or RCPT TO commands. 4. The AUTH command AUTH mechanism [initial-response] Arguments: a string identifying a SASL authentication mechanism. an optional base64-encoded response Restrictions: after an AUTH command has successfully completed, no more AUTH com- mands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with a 503 reply. Discussion: The AUTH command indicates an authentication mechanism to the server. If the server supports the requested authentication mecha- nism, it performs an authentication protocol exchange to authenti- cate and identify the user. Optionally, it also negotiates a secu- rity layer for subsequent protocol interactions. If the requested authentication mechanism is not supported, the server rejects the AUTH command with a 504 reply. The authentication protocol exchange consists of a series of server challenges and client answers that are specific to the authentica- tion mechanism. A server challenge, otherwise known as a ready response, is a 334 reply with the text part containing a BASE64 encoded string. The client answer consists of a line containing a BASE64 encoded string. If the client wishes to cancel an authenti- cation exchange, it issues a line with a single "*". If the server receives such an answer, it MUST reject the AUTH command by sending a 501 reply. The optional initial-response argument to the AUTH command is used to save a round trip when using authentication mechanisms that are defined to send no data in the initial challenge. When the initial- response argument is used with such a mechanism, the initial empty challenge is not sent to the client and the server uses the data in the initial-response argument as if it were sent in response to the empty challenge. If the initial-response argument to the AUTH com- mand is used with a mechanism that sends data in the initial chal- lenge, the server rejects the AUTH command with a 535 reply. Myers [Page 3]
Internet Draft SMTP Authentication 26 February 1997 If the server cannot BASE64 decode the argument, it rejects the AUTH command with a 501 reply. If the server rejects the authentication data, it SHOULD reject the AUTH command with a 535 reply. Should the client successfully complete the authentication exchange, the SMTP server issues a 235 reply. The service name specified by this protocol's profile of SASL is "smtp". If a security layer is negotiated through the SASL authentication exchange, it takes effect immediately following the CRLF that con- cludes the authentication exchange for the client, and the CRLF of the success reply for the server. The server is not required to support any particular authentication mechanism, nor are authentication mechanisms required to support any security layers. If an AUTH command fails, the client may try another authentication mechanism by issuing another AUTH command. In other words, the client may request authentication types in decreasing order of preference. The BASE64 string may in general be arbitrarily long. Clients and servers MUST be able to support challenges and responses that are as long as are generated by the authentication mechanisms they support, independent of any line length limitations the client or server may have in other parts of its protocol implementation. Examples: S: 220 smtp.andrew.cmu.edu ESMTP server ready C: EHLO jgm.pc.cc.cmu.edu S: 250-smtp.andrew.cmu.edu S: 250 AUTH=SKEY C: AUTH FOOBAR S: 504 Unrecognized authentication type C: AUTH SKEY c21pdGg= S: 334 OTUgUWE1ODMwOA== C: BsAY3g4gBNo= S: 235 S/Key authentication successful Myers [Page 4]
Internet Draft SMTP Authentication 26 February 1997 5. Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in [RFC822]. Except as noted otherwise, all alphabetic characters are case-insen- sitive. 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. ATOM_CHAR ::= <any CHAR except atom_specials> atom_specials ::= "(" / ")" / "{" / SPACE / CTLs / "%" / "*" / <"> / "\" auth_command ::= "AUTH" SPACE auth_type [SPACE base64] *(CRLF base64) CRLF auth_type ::= 1*ATOM_CHAR base64 ::= *(4base64_CHAR) [base64_terminal] base64_char ::= "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H" / "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P" / "Q" / "R" / "S" / "T" / "U" / "V" / "W" / "X" / "Y" / "Z" / "a" / "b" / "c" / "d" / "e" / "f" / "g" / "h" / "i" / "j" / "k" / "l" / "m" / "n" / "o" / "p" / "q" / "r" / "s" / "t" / "u" / "v" / "w" / "x" / "y" / "z" / "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" / "+" / "/" ;; Case-sensitive base64_terminal ::= (2base64_char "==") / (3base64_char "=") CHAR ::= <any 7-bit US-ASCII character except NUL, 0x01 - 0x7f> continue_req ::= "334" SPACE base64 CRLF CR ::= <ASCII CR, carriage return, 0x0C> CRLF ::= CR LF CTL ::= <any ASCII control character and DEL, 0x00 - 0x1f, 0x7f> Myers [Page 5]
Internet Draft SMTP Authentication 26 February 1997 LF ::= <ASCII LF, line feed, 0x0A> SPACE ::= <ASCII SP, space, 0x20> 6. References [SASL] Myers, J., "Simple Authentication and Security Layer", draft-myers-auth-sasl-XX.txt, Carnegie Mellon. [RFC821] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August 1982. [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", RFC 822, August 1982. 7. Security Considerations Security issues are discussed throughout this memo. If a client uses this extension to get an encrypted tunnel through an insecure network to a cooperating server, it needs to be configured to never send mail to that server when the connection is not mutually authenticated and encrypted. Otherwise, an attacker could steal the client's mail by hijacking the SMTP connection and either pretending the server does not support the Authentication extension or causing all AUTH commands to fail. This extension does not provide a defined mechanism for authentica- tion using a plaintext password. This omission is intentional. This extension is not intended to replace or be used instead of end- to-end message signature and encryption systems such as PEM or PGP. This extension addresses a different problem than end-to-end systems; it has the following key differences: (1) it is generally useful only within a trusted enclave (2) it protects the entire envelope of a message, not just the mes- sage's body. (3) it authenticates the message submission, not authorship of the message content Myers [Page 6]
Internet Draft SMTP Authentication 26 February 1997 (4) it can give the sender some assurance the message was delivered to the next hop in the case where the sender mutually authenti- cates with the next hop and negotiates an appropriate security layer. Additional security considerations are mentioned in the SASL specification [SASL]. 8. Author's Address: John G. Myers 220 Palo Alto Ave, Apt 102 Palo Alto, CA 94301 Email: jgm@cmu.edu Myers [Page 7]