Internet Draft                              Paul Hoffman
draft-hoffman-smtp-ssl-04.txt               Internet Mail Consortium
November 1, 1997
Expires in six months

         SMTP Service Extension for Secure SMTP over TLS

Status of this memo

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. Abstract

This document describes an extension to the SMTP service that allows
an SMTP server and client to use transport-layer security to provide
private, authenticated communication over the Internet. This gives
SMTP agents the ability to protect some or all of their communications
from eavesdroppers and attackers.


. Introduction

SMTP [RFC-821] servers and clients normally communicate in the clear
over the Internet. In many cases, this communication goes through one
or more router that is not controlled or trusted by either entity.
Such an untrusted router might allow a third party to monitor or alter
the communications between the server and client.

Further, there is often a desire for two SMTP agents to be able to
authenticate each others' identities. For example, a secure SMTP
server might only allow communications from other SMTP agents it
knows, or it might act differently for messages received from an agent
it knows than from one it doesn't know.

TLS [TLS], more commonly known as SSL, is a popular mechanism for
enhancing TCP communications with privacy and authentication. TLS is
in wide use with the HTTP protocol, and is also being used for adding
security to many other common protocols that run over TCP.

2.1 Discussion of this Draft

This draft is being discussed on the "ietf-apps-tls" mailing list.
To subscribe, send a message to:
     ietf-apps-tls-request@imc.org
witht the single word
     subscribe
in the body of the message. There is a Web site for the mailing list
at <http://www.imc.org/ietf-apps-tls/>.


. TLS Extension

The TLS extension to SMTP is laid out as follows:

(1) the name of the SMTP service defined here is TLS;

(2) the EHLO keyword value associated with the extension is TLS;

(3) the TLS keyword has no parameters;

(4) a new SMTP verb, "STARTTLS", is defined;

(5) no additional parameters are added to any SMTP command.


. The TLS Keyword

The TLS keyword is used to tell the SMTP client that the SMTP server
allows use of TLS. It no parameters.


. The STARTTLS Command

The format for the STARTTLS command is:

STARTTLS

with no parameters.

After the client gives the STARTTLS command, the server responds with
one of the following reply codes:

 Ready to start TLS
 Syntax error (no parameters allowed)
 TLS not available due to temporary reason

A publicly-referenced server SHOULD  be able to accept other SMTP
commands before receiving a STARTTLS command.  After receiving a 220
response to a STARTTLS command, the client SHOULD issue a STARTTLS
command before giving any other SMTP commands.

A SMTP server that is not publicly referenced may choose to require
that the client perform a TLS negotiation before accepting any commands.
In this case, the server SHOULD return the reply code:

 Must issue a STARTTLS command first

to every command other than STARTTLS or QUIT. If the client and
server are using the ENHANCEDSTATUSCODES ESMTP extension [RFC-2034],
the status code to be returned SHOULD be 5.7.0.

If the SMTP client is using pipelining as defined in RFC 1854,
the STARTTLS command must be the last command in a group.

5.1 Result of the STARTTLS Command

After the TLS handshake has been completed, both parties MUST
immediately decide whether or not to continue based on the
authentication and privacy achieved. The SMTP client and server may
decide to move ahead even if the TLS negotiation ended with no
authentication and/or no privacy because most SMTP services are
performed with no authentication and no privacy, but some SMTP clients
or servers may want to continue only if a particular level of
authentication and/or privacy was achieved.

If the SMTP client decides that the level of authentication or privacy
is not high enough for it to continue, it SHOULD issue an SMTP QUIT
command immediately after the TLS negotiation is complete. If the SMTP
server decides that the level of authentication or privacy is not high
enough for it to continue, it SHOULD reply to every SMTP command from
the client (other than a QUIT command) with the 554 reply code (with a
possible text string such as "Command refused due to lack of
security").

The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:
 - A SMTP client would probably only want to authenticate an SMTP
   server whose server certificate has a domain name that is the
   domain name that the client thought it was connecting to.
 - A publicly-referenced  SMTP server would probably want to accept
   any certificate from an SMTP client, and would possibly want to
   put distinguishing information about the certificate in the
   Received header of messages that were relayed or submitted from
   the client.


. Usage Example

The following dialog illustrates how a client and server can start
a TLS session:

S: <waits for connection on TCP port 25>
C: <opens connection>
S: 220 mail.imc.org SMTP service ready
C: EHLO mail.ietf.org
S: 250-mail.imc.org offers a warm hug of welcome
S: 250 TLS
C: STARTTLS
S: 220 Go ahead
C: <starts TLS negotiation>
C & S: <negotiate a TLS session>
C & S: <check result of negotiation>
C: <continues by sending an SMTP command>
. . .


. Security Considerations

It should be noted that SMTP is not an end-to-end mechanism. Thus, if
an SMTP client/server pair decide to add TLS privacy, they are not
securing the transport from the originating mail user agent to the
recipient. Further, because delivery of a single piece of mail may go
between more than two SMTP servers, adding TLS privacy to one pair of
servers does not mean that the entire SMTP chain has been made
private. Further, just because an SMTP server can authenticate an SMTP
client, it does not mean that the mail from the SMTP client was
authenticated by the SMTP client when the client received it.

Both the STMP client and server must check the result of the TLS
negotiation to see whether acceptable authentication or privacy was
achieved. Ignoring this step completely invalidates using TLS for
security. The decision about whether acceptable authentication or
privacy was achieved is made locally, is implementation-dependant, and
is beyond the scope of this document.

The SMTP client and server should note carefully the result of the TLS
negotiation. If the negotiation results in no privacy, or if it
results in privacy using algorithms or key lengthths that are deemed
not strong enough, or if the authentication is not good enough for
either party, the client may choose to end the SMTP session with an
immediate QUIT command, or the server may choose to not accept any
more SMTP commands.

A server announcing in an EHLO response that it uses a particular TLS
protocol should not pose any security issues, since any use of TLS will
be at least as secure as no use of TLS.

A man-in-the-middle attack can be launched by deleting the "250 TLS"
response from the server. This would cause the client not to try
to start a TLS session.

Another draft, [SMTP-AUTH], proposes a different mechanism that can
also add privacy and security to SMTP. [SMTP-AUTH] does not allow for
using TLS, but instead describes how to enable other security protocols
when using SMTP.


A. References

[RFC-821] "Simple Mail Transfer Protocol", RFC 821

[RFC-1869] "SMTP Service Extensions", RFC 1869

[RFC-2034] "SMTP Service Extension for Returning Enhanced Error
Codes", RFC 2034

[SMTP-AUTH] "SMTP Service Extension for Authentication",
draft-myers-smtp-auth

[TLS] "The TLS Protocol Version 1.0", draft-ietf-tls-protocol


B. Revision History

***Changes from -03 to -04:

Minor grammar changes.

Expanded the wording in the third paragraph of section 7.

Added appendix C.


C. Revocation of smtps Port

An IANA port registration was made for an "smtps" port for use as a
TLS-negotiated SMTP port. The email community has reached rough concensus
that widespread use of such a port will be harmful to the performance,
interoperability and security of SMTP. This document hereby revokes the
IANA registration of the "smtps" port and forbids future registration of a
port for any "secure SMTP" service. IANA is directed to replace the port
registration with an indication that the port registration was revoked,
including the effective date. Two years after the effective date of
revocation, the port may be re-registered for a different purpose.


D. Author's Address

Paul Hoffman
Internet Mail Consortium
 Segre Place
Santa Cruz, CA  95060
(408) 426-9827
phoffman@imc.org