One document matched: draft-myers-smtp-auth-11.txt
Differences from draft-myers-smtp-auth-10.txt
Network Working Group J. Myers
Internet Draft: SMTP Authentication February 1998
Document: draft-myers-smtp-auth-11.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
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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 SMTP service extension [ESMTP] 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].
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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. 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.
(4) a new SMTP verb "AUTH" is defined
(5) an optional parameter using the keyword "AUTH" is added to the
MAIL FROM command.
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
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 security layer for subsequent protocol
interactions. If the requested authentication mechanism is not
supported, the server rejects the AUTH command with a 504
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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 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 command is used
with a mechanism that sends data in the initial challenge, the
server rejects the AUTH command with a 535 reply.
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 unless a more specific error code, such as one listed
in section 6, is appropriate. 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 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 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.
If an AUTH command fails, the server MUST behave the same as if
the client had not issued the AUTH command.
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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 PLAIN
C: AUTH FOOBAR
S: 504 Unrecognized authentication type
C: AUTH SKEY c21pdGg=
S: 334 OTUgUWE1ODMwOA==
C: BsAY3g4gBNo=
S: 235 S/Key authentication successful
5. 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. To comply with the restrictions imposed
on ESMTP parameters, the addr-spec is encoded inside an xtext.
The syntax of an xtext is described in section 5 of [ESMTP-DSN].
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.
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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.
It is conforming for an implementation to be hard-coded to treat
all clients as being insufficiently trusted. In that case, the
implementation does nothing more than parse and discard
syntactically valid AUTH parameters to the MAIL FROM command.
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6. Error Codes
The following error codes may be used to indicate various conditions
as described.
422 A password transition is needed
This response to the AUTH command indicates that the user needs to
transition to the selected authentication mechanism. This typically
done by authenticating once using the PLAIN authentication mechanism.
522 Authentication mechanism is too weak
This response to the AUTH command indicates that the selected
authentication mechanism is weaker than server policy permits for
that user.
523 Encryption required for requested authentication mechanism
This response to the AUTH command indicates that the selected
authentication mechanism may only be used when the underlying SMTP
connection is encrypted.
454 Temporary authentication failure
This response to the AUTH command indicates that the authentication
failed due to a temporary server failure.
505 Authentication required
This response may be returned by any command other than AUTH, EHLO,
NOOP, or QUIT. It indicates that server policy requires
authentication in order to perform the requested action.
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7. 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 [SPACE base64]
*(CRLF base64) CRLF
auth_param = "AUTH=" xtext
;; xtext is defined in section 5 of RFC 1891
;; The decoded form of the xtext MUST be an addr-spec
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
[ESMTP] Klensin et al, "SMTP Service Extensions", RFC 1869, November
1995.
[ESMTP-DSN] Moore, K, "SMTP Service Extension for Delivery Status
Notifications", RFC 1890.
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[SASL] Myers, J., "Simple Authentication and Security Layer",
draft-myers-auth-sasl-13.txt, November 1997.
[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-
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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.
(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
security layer.
Additional security considerations are mentioned in the SASL
specification [SASL].
10. Author's Address:
John Gardiner Myers
Netscape Communications
501 East Middlefield Road
Mail Stop MV-029
Mountain View, CA 94043
Email: jgmyers@netscape.com
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