Network Working Group Robert Siemborski
INTERNET-DRAFT Google, Inc.
Intended Category: Proposed Standard February, 2005
SMTP Service Extension for Authentication
<draft-siemborski-rfc2554bis-04.txt>
Status of this Memo
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with RFC 3668.
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Abstract
This document defines a Simple Mail Transport Protocol (SMTP)
extension 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 during this session. This extension includes
a profile of the Simple Authentication and Security Layer (SASL) for
SMTP.
This document obsoletes RFC 2554 and replaces it as a Proposed
Standard.
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Table of Contents
1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 4
2. The Authentication Service Extension . . . . . . . . . . . . . . 4
3. The AUTH Command . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. The AUTH Parameter to the MAIL FROM command . . . . . . . . . . . 9
4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 15
9. Protocol Actions . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 15
11. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
13. Changes Since RFC 2554 . . . . . . . . . . . . . . . . . . . . . 18
14. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. How to Read This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
NOT", "RECOMMENDED", and "MAY" in this document are to be
interpreted as defined in "Key words for use in RFCs to Indicate
Requirement Levels" [KEYWORDS]
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively.
2. The Authentication Service Extension
1. The name of this [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 available [SASL] mechanisms.
The list of available mechanisms MAY change after a successful
STARTTLS command [SMTP-TLS].
4. A new [SMTP] verb "AUTH" is defined.
5. An optional parameter using the keyword "AUTH" is added to the
MAIL FROM command, and extends the maximum line length of the
MAIL FROM command by 500 characters.
6. This extension is appropriate for the submission protocol
[SUBMIT].
3. 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 described in Section
3 of [BASE64].
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 a 503 reply.
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The AUTH command is not permitted during a mail transaction.
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 invalid (e.g. is not
supported or requires an encrpytion layer), the server rejects
the AUTH command with a 504 reply, and if it supports the
[ESMTP-CODES] extension it SHOULD return a 5.5.4 enhanced
response code.
The SASL authentication 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 334 reply with the text part
containing the [BASE64] encoded string supplied by the SASL
mechanism. This challenge MUST NOT contain any text other
than the BASE64 encoded challenge.
A client response consists of a line containing a [BASE64]
encoded string. If the client wishes to cancel the
authentication exchange, it issues a line with a single "*".
If the server receives such a response, 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 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 5.1 of [SASL]. In SMTP, a server challenge that
contains no data is defined as a 334 reply with no text part.
Note that there is still a space following the reply code, so
the complete response line is "334 ".
Note that the AUTH command is still subject to the line length
limitations defined in [SMTP]. If use of the initial response
argument 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 [SASL]).
If the client is transmitting an initial response of zero
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length, it MUST instead 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 a
501 reply. Servers using the enhanced status codes extension
[ESMTP-CODES] SHOULD return an enhanced status code of 5.7.0
in this case.
If the server cannot [BASE64] decode a client response, it
MUST reject the AUTH command with a 501 reply (and an enhanced
status code of 5.5.2). If the client cannot BASE64 decode a
server challenge, 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 can be much longer than
normal SMTP commands. 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.
The authorization identity generated by this [SASL] exchange
is a simple username, and both client and server MUST use the
[SASLprep] profile of the [StringPrep] algorithm to prepare
these names for transmission or comparison. 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 the server is unable to authenticate the client, it SHOULD
reject the AUTH command with a 535 reply unless a more
specific error code is appropriate. Should the client
successfully complete the exchange, the SMTP server issues a
235 reply. These status codes, along with others defined by
this extension, are discussed in Section 5 of this document.
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
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client. For the server, it takes effect immediately following
the CRLF of its success reply.
When a security layer takes effect, the SMTP protocol is reset
to the initial state (the state in SMTP after a server issues
a 220 service ready greeting). The server MUST discard any
knowledge obtained from the client, such as the EHLO argument,
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 SMTP service extensions, which
was not obtained from the SASL negotiation itself (Note that a
client MAY compare the advertised SASL mechanisms before and
after authentication in order to detect an active down-
negotiation attack).
The client SHOULD send an EHLO command as the first command
after a successful SASL negotiation which results in the
enabling of a security layer.
When both [TLS] and SASL security layers are in effect, the
TLS encoding MUST be applied after the SASL encoding,
regardless of the order in which the layers were negotiated.
The service name specified by this protocol's profile of SASL
is "smtp". This service name is also to be used for the
[SUBMIT] protocol.
If an AUTH command fails, the client MAY proceed without
authentication, Alternatively, the client MAY try another
authentication mechanism or present different credentials by
issuing another AUTH command.
To ensure interoperability, client and server implementations
of this extension MUST implement the [DIGEST-MD5] SASL
mechanism.
3.1. Examples
Here is an example of a client attempting AUTH using the [PLAIN]
SASL mechanism under a TLS layer, and making use of the initial
client response:
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S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH GSSAPI
S: 250-ENHANCEDSTATUSCODES
S: 250 STARTTLS
C: STARTTLS
S: 220 Ready to start TLS
... TLS negotiation proceeds, further commands protected by TLS layer ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250 AUTH KERBEROS_V4 GSSAPI PLAIN
C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ=
S: 235 2.7.0 Authentication successful
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: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250 AUTH KERBEROS_V4 GSSAPI PLAIN
C: AUTH PLAIN
(note: there is a single space following the 334 on the following line)
S: 334
C: dGVzdAB0ZXN0ADEyMzQ=
S: 235 2.7.0 Authentication successful
Here is an example using a mechanism which does not support an
initial client send, and includes a server challenge:
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH CRAM-MD5
S: 250-ENHANCEDSTATUSCODES
S: 250 STARTTLS
C: AUTH CRAM-MD5
S: 334 PDQxOTI5NDIzNDEuMTI4Mjg0NzJAc291cmNlZm91ci5hbmRyZXcuY211LmVkdT4=
C: cmpzMyBlYzNhNTlmZWQzOTVhYmExZWM2MzY3YzRmNGI0MWFjMA==
S: 235 2.7.0 Authentication successful
Here is an example of a client attempting AUTH EXTERNAL under TLS,
using the derived authorization ID (and thus a zero-length initial
client send).
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S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250-AUTH GSSAPI
S: 250-ENHANCEDSTATUSCODES
S: 250 STARTTLS
C: STARTTLS
S: 220 Ready to start TLS
... TLS negotiation proceeds, further commands protected by TLS layer ...
C: EHLO client.example.com
S: 250-smtp.example.com Hello client.example.com, pleased to meet you
S: 250 AUTH EXTERNAL GSSAPI PLAIN
C: AUTH EXTERNAL =
S: 235 2.7.0 Authentication successful
4. The AUTH Parameter to the MAIL FROM command
AUTH=addr-spec
Arguments:
An addr-spec (see section 3.4.1 or [RFC2822]) that names the
identity which submitted the message to the delivery system, or
the two character sequence "<>" indicating such an identity is
unknown or insufficiently authenticated. To comply with
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].
Note:
For the purposes of this discussion, "authenticated identity"
refers to the identity (if any) derived from the authorization
identity of previous AUTH command, while the terms "authorized
identity" and "supplied addr-spec" refer to the sender identity
that is being associated with a particular message. Note that
one authenticated identity may be able to identify messages as
being sent by any number of authorized identities within a
single session. For example, this may be the case when an SMTP
server (one authenticated identity) is processing its queue
(many messages with distinct authorized identities).
Discussion:
The optional AUTH parameter to the MAIL FROM command allows
cooperating agents in a trusted environment to communicate the
authorization identity associated with individual messages.
If the server trusts the authenticated identity of the client to
assert that the message was originally submitted by the supplied
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addr-spec, then the server SHOULD supply the same addr-spec in
an AUTH parameter when relaying the message to any other server
which supports the AUTH extension.
For this reason, servers that advertise support for this
extension MUST support the AUTH parameter to the MAIL FROM
command even when the client has not authenticated itself to the
server.
A MAIL FROM parameter of AUTH=<> indicates that the original
submitter of the message is not known. The server MUST NOT
treat the message as having been originally submitted by
authenticated identity which resulted from the AUTH command.
If the AUTH parameter to the MAIL FROM command is not supplied,
the client has authenticated, and the server believes the
message is an original submission, the server MAY generate an
addr-spec from the user's authenticated identity for use in an
AUTH parameter when relaying the message to any server which
supports the AUTH extension. The generated addr-spec is
implementation specific, but it MUST conform to the syntax of
[RFC2822]. If the implementation cannot generate a valid addr-
spec, it MUST transmit AUTH=<> when relaying this message.
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 the AUTH=<> parameter was
supplied. The server MAY, however, write the value of any
supplied AUTH parameter to a log file.
If an AUTH=<> parameter was supplied, either explicitly or due
to the requirement in the previous paragraph, then the server
MUST supply the AUTH=<> parameter when relaying the message to
any server which it has authenticated to using the AUTH
extension.
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.
Note that an implementation which is hard-coded to treat all
clients as being insufficiently trusted is compliant with this
specification. In that case, the implementation does nothing
more than parse and discard syntactically valid AUTH parameters
to the MAIL FROM command, and supply AUTH=<> parameters to any
servers which it authenticates to.
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4.1. Examples
An example where the original identity of the sender is trusted and
known:
C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com
S: 250 OK
One example where the identity of the sender is not trusted or is
otherwise being suppressed by the client:
C: MAIL FROM:<john+@example.org> AUTH=<>
S: 250 OK
5. Status Codes
The following error codes may be used to indicate various success or
failure conditions. Servers that return enhanced status codes
[ESMTP-CODES] SHOULD use the enhanced codes suggested here.
235 2.7.0 Authentication Succeeded
This response to the AUTH command indicates that the authentication
was successful.
432 4.7.12 A password transition is needed
This response to the AUTH command indicates that the user needs to
transition to the selected authentication mechanism. This is
typically done by authenticating once using the [PLAIN]
authentication mechanism. The selected mechanism SHOULD then work
for authentications in subsequent sessions.
454 4.7.0 Temporary authentication failure
This response to the AUTH command indicates that the authentication
failed due to a temporary server failure. The client SHOULD NOT
prompt the user for another password in this case, and instead
notify the user of server failure.
534 5.7.9 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. The client SHOULD retry with a new authentication
mechanism.
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535 5.7.8 Authentication credentials invalid
This response to the AUTH command indicates that the authentication
failed due to invalid or insufficient authentication credentials.
In this case, the client SHOULD ask the user to supply new
credentials (such as by presenting a password dialog box).
530 5.7.0 Authentication required
This response SHOULD be returned by any command other than AUTH,
EHLO, HELO, NOOP, RSET, or QUIT when server policy requires
authentication in order to perform the requested action and
authentication is not currently in force.
538 5.7.11 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. Note that this response code is documented
here for historical purposes only. Modern implementations SHOULD
NOT advertise mechanisms that are not permitted due to lack of
encryption, unless an encryption layer of sufficient strength is
currently being employed.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form notation as specified in [ABNF].
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.
UPALPHA = %x41-5A ;; Uppercase: A-Z
LOALPHA = %x61-7A ;; Lowercase: a-z
ALPHA = UPALPHA / LOALPHA ;; case insensitive
DIGIT = %x30-39 ;; Digits 0-9
HEXDIGIT = %x41-46 / DIGIT ;; hexidecimal digit (uppercase)
hexchar = "+" HEXDIGIT HEXDIGIT
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xchar = %x21-2A / %x2C-3C / %x3E-7E
;; US-ASCII except for "+", "=", SPACE and CTL
xtext = *(xchar / hexchar)
AUTH_CHAR = ALPHA / DIGIT / "-" / "_"
auth_type = 1*20AUTH_CHAR
auth_command = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
*(CRLF [base64]) CRLF
auth_param = "AUTH=" xtext
;; The decoded form of the xtext MUST be either
;; an addr-spec or the two characters "<>"
base64 = base64_terminal /
( 1*(4base64_CHAR) [base64_terminal] )
base64_char = UPALPHA / LOALPHA / DIGIT / "+" / "/"
;; Case-sensitive
base64_terminal = (2base64_char "==") / (3base64_char "=")
continue_req = "334" SPACE [base64] CRLF
CR = %x0C ;; ASCII CR, carriage return
CRLF = CR LF
CTL = %x00-1F / %x7F ;; any ASCII control character and DEL
LF = %x0A ;; ASCII LF, line feed
SPACE = %x20 ;; ASCII SP, space
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.
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Before the [SASL] negotiation has begun, any protocol interactions
are performed in the clear and may be modified by an active
attacker. For this reason, clients and servers MUST discard any
knowledge obtained prior to the start of the SASL negotiation upon
the establishment of a security layer.
This mechanism does not protect the TCP port, so an active attacker
may redirect a relay connection attempt (i.e. a connection between
two MTAs) to the submission port [SUBMIT]. The AUTH=<> parameter
prevents such an attack from causing a relayed message, in the
absence of other envelope authentication, from picking up the
authentication of the relay client.
A message submission client may require the user to authenticate
whenever a suitable [SASL] mechanism is advertised. Therefore, it
may not be desirable for a submission server [SUBMIT] to advertise a
SASL mechanism when use of that mechanism grants the clients no
benefits over anonymous submission.
Servers MAY implement a policy whereby the connection is dropped
after a number of failed authentication attempts. If they do so,
they SHOULD NOT drop the connection until atleast 3 attempts to
authenticate have failed.
Implementations MUST support a configuration where SASL mechanisms
that are vulnerable to passive eavesdropping attacks (such as
[PLAIN]) are not advertised or used without the presence of an
external security layer such as [TLS].
This extension is not intended to replace or be used instead of end-
to-end message signature and encryption systems such as [S/MIME] 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. When mutual authentication is used along with a security
layer, it can give the sender some assurance that the message
was successfully delivered to the next hop.
Additional security considerations are mentioned in the [SASL]
specification.
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8. IANA Considerations
This document requests that the IANA update the entry for the "smtp"
SASL protocol name to point at this document.
This document requests that the IANA register the Authentication
SMTP service extension as defined in Section 2 of this document.
9. Protocol Actions
[RFC Editor: Remove this section prior to publication]
This document obsoletes RFC 2554 and replaces it as a Proposed
Standard.
10. 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 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.
11. Copyright
Copyright (C) The Internet Society (2005). 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
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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.
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12. References
The following documents contain normative definitions or
specifications that are necessary for correct understanding of this
protocol:
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[DIGEST-MD5]
Leach, P., Melnikov, A., and Newman C., "Using Digest
Authentication as a SASL Mechanism", draft-ietf-sasl-
rfc2831bis-*.txt, a work in progress.
[ESMTP-CODES]
Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, October 1996.
[ESMTP-DSN] Moore, K., "SMTP Service Extension for Delivery Status
Notifications", RFC 1891, January 1996.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC2822] Resnick, P. "Internet Message Format", RFC 2822, April 2001.
[SASL] Melnikov, A., "Simple Authentication and Security Layer
(SASL)", draft-ietf-sasl-rfc2222bis-*.txt, a work in
progress.
[SASLprep] Zeilega, K., "SASLprep: Stringprep profile for user names
and passwords", draft-ietf-sasl-saslprep-*.txt, a work in
progress
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[SMTP-TLS] Hoffman, P. "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[StringPrep]
Hoffman, P., Blanchet, M., "Preparation of Internationalized
Strings ("stringprep")", draft-hoffman-rfc3454bis-*.txt, a
work in progress
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[SUBMIT] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.
The following references are for informational purposes only:
[PGP] Elkins, M., "MIME Security with Pretty Good Privacy (PGP)",
RFC 2015, October 1996.
[PLAIN] Newman, C. "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
June 1999.
[S/MIME] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
13. Changes Since RFC 2554
1. Clarify that servers MUST support the use of the
AUTH=addr-spec parameter to MAIL FROM, even when the
client is not authenticated.
2. Clarify the initial-client-send requirements, and give
additional examples.
3. Update references to newer versions of various
specifications.
4. Require the minimum implementation of DIGEST-MD5.
5. Clarify that the mechanism list can change.
6. Deprecate the use of the 538 response code.
7. Add the use of the SASLprep profile for preparing
authorization identities.
8. Substantial cleanup of response codes and indicate
suggested enhanced response codes. Also indicate what
response codes should result in a client prompting the
user for new credentials.
9. General other editorial clarifications.
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SMTP Service Extension for Authentication February, 2005
14. Author's Address:
Robert Siemborski
Google, Inc.
1600 Ampitheatre Parkway
Mountain View, CA 94043
+1 650 623 6925
robsiemb@google.com
15. Acknowledgments:
The author would like to acknowledge the contributions of John Myers
and other contributors to RFC 2554, on which this document draws
from heavily.
The author would also like to thank Ken Murchison, Mark Crispin, and
Chris Newman for the time they devoted to reviewing early drafts of
this document.
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