One document matched: draft-myers-smtp-auth-02.txt
Differences from draft-myers-smtp-auth-01.txt
Network Working Group J. Myers
Internet Draft: SMTP Authentication Carnegie Mellon
Document: draft-myers-smtp-auth-02.txt January 1996
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
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``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 protection mechanism for subsequent protocol interactions. This
extension is a profile of the Simple Authentication and Session Layer
[SASL]. A mechanism is also provided for a client to transfer
envelope authentication of individual messages.
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2. 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) no parameter is used with the AUTH EHLO keyword
(4) a new SMTP verb "AUTH" is defined
(5) an optional parameter using the keyword "AUTH" is added to the
MAIL FROM command.
3. The AUTH command
AUTH mechanism
Arguments:
a string identifying a SASL authentication mechanism.
Restrictions:
after an AUTH command has 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 a 503 reply.
Discussion:
The AUTH command indicates an authentication mechanism to the
server. If the server supports the requested authentication
mechanism, it performs an authentication protocol exchange to
authenticate and identify the user. Optionally, it also
negotiates a protection mechanism 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
authentication 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 authentication exchange, it should issue a
line with a single "*". If the server receives such an answer,
it must reject the AUTH command by sending a 501 reply.
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If the server cannot BASE64 decode the argument, it should
reject 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 session layer is negotiated through the SASL
authentication exchange, it takes effect immediately following
the CRLF that concludes 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 protection mechanisms. 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
C: AUTH FOOBAR
S: 504 Unrecognized authentication type
C: AUTH SKEY
S: 334
C: c21pdGg=
S: 334 OTUgUWE1ODMwOA==
C: BsAY3g4gBNo=
S: 235 S/Key authentication successful
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4. The AUTH parameter to the MAIL FROM command
AUTH=addr-spec
Arguments:
an addr-spec containing the identity which submitted the
message to the delivery system. The length of the addr-
spec is limited to 129 characters, per [RFC821]. To comply
with the restrictions imposed on ESMTP parameters, the
addr-spec may not conatain space, `=', or control
characters.
Discussion:
The optional AUTH parameter to the MAIL FROM command allows
cooperating agents in a trusted environment to communicate
the authentication of individual messages.
If the server trusts the authenticated identity of the
client to assert that the message was originally submitted
by the supplied addr-spec, then the server SHOULD supply
the same addr-spec in an AUTH parameter when relaying the
message to any server which supports the AUTH extension.
If the server does not sufficiently trust the authenticated
identity of the client, or if the client is not
authenticated, then the server MUST behave as if no AUTH
parameter was supplied. The server MAY, however, place the
value of the AUTH parameter in a comment in the inserted
Received: header and/or write it to a log file.
A server MAY treat expansion of a mailing list as a new
submission, setting the AUTH parameter to the mailing list
address or mailing list administration address when
relaying the message to list subscribers.
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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-
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.
ATOM_CHAR ::= <any CHAR except atom_specials>
atom_specials ::= "(" / ")" / "{" / SPACE / CTLs / "%" / "*" /
<"> / "\"
auth_command ::= "AUTH" SPACE auth_type *(CRLF base64) CRLF
auth_param ::= "AUTH=" addr-spec
;; addr-spec may not contain SPACE, "="
;; or CTL characters.
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
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CTL ::= <any ASCII control character and DEL,
0x00 - 0x1f, 0x7f>
LF ::= <ASCII LF, line feed, 0x0A>
SPACE ::= <ASCII SP, space, 0x20>
8. References
[SASL] Myers, J., "Simple Authentication and Session 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.
9. 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
authentication 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
message's body.
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(3) it authenticates the message submission, not authorship of the
message content
(4) it can give the sender some assurance the message was delivered
to the next hop in the case where the sender mutually
authenticates with the next hop and negotiates an appropriate
protection mechanism.
8. Author's Address:
John G. Myers
Carnegie-Mellon University
5000 Forbes Ave
Pittsburgh, PA 15213
EMail: jgm+@cmu.edu
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