Network Working Group                                     P. Hoffman
Request for Comments: 2487                  Internet Mail Consortium
Category: Standards Track                               January 1999


           SMTP Service Extension for Secure SMTP over TLS

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. 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.

2. 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.






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2.1 Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [RFC-2119].

3. STARTTLS Extension

  The STARTTLS extension to SMTP is laid out as follows:

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

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

  (3) the STARTTLS keyword has no parameters;

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

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

4. The STARTTLS Keyword

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

5. 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:

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

  A publicly-referenced SMTP server MUST NOT require use of the
  STARTTLS extension in order to deliver mail locally. This rule
  prevents the STARTTLS extension from damaging the interoperability of
  the Internet's SMTP infrastructure. A publicly-referenced SMTP server
  is an SMTP server which runs on port 25 of an Internet host listed in
  the MX record (or A record if an MX record is not present) for the
  domain name on the right hand side of an Internet mail address.




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  Any SMTP server may refuse to accept messages for relay based on
  authentication supplied during the TLS negotiation. An SMTP server
  that is not publicly referenced may refuse to accept any messages for
  relay or local delivery based on authentication supplied during the
  TLS negotiation.

  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:

  530 Must issue a STARTTLS command first

  to every command other than NOOP, EHLO, 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.

  After receiving a 220 response to a STARTTLS command, the client
  SHOULD start the TLS negotiation before giving any other SMTP
  commands.

  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 Processing After 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:





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   - 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.

5.2 Result of the STARTTLS Command

  Upon completion of the TLS handshake, 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 argument to the EHLO command,
  which was not obtained from the TLS negotiation itself. The client
  MUST discard any knowledge obtained from the server, such as the list
  of SMTP service extensions, which was not obtained from the TLS
  negotiation itself. The client SHOULD send an EHLO command as the
  first command after a successful TLS negotiation.

  The list of SMTP service extensions returned in response to an EHLO
  command received after the TLS handshake MAY be different than the
  list returned before the TLS handshake. For example, an SMTP server
  might not want to advertise support for a particular SASL mechanism
  [SASL] unless a client has sent an appropriate client certificate
  during a TLS handshake.

  Both the client and the server MUST know if there is a TLS session
  active.  A client MUST NOT attempt to start a TLS session if a TLS
  session is already active. A server MUST NOT return the TLS extension
  in response to an EHLO command received after a TLS handshake has
  completed.

6. 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 STARTTLS
  C: STARTTLS
  S: 220 Go ahead
  C: <starts TLS negotiation>



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  C & S: <negotiate a TLS session>
  C & S: <check result of negotiation>
  C: <continues by sending an SMTP command>
  . . .

7. 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 lengths 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
  STARTTLS" response from the server. This would cause the client not
  to try to start a TLS session. An SMTP client can protect against
  this attack by recording the fact that a particular SMTP server
  offers TLS during one session and generating an alarm if it does not
  appear in the EHLO response for a later session. The lack of TLS
  during a session SHOULD NOT result in the bouncing of email, although
  it could result in delayed processing.







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  Before the TLS handshake 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 TLS handshake upon completion of the TLS
  handshake.

  The STARTTLS extension is not suitable for authenticating the author
  of an email message unless every hop in the delivery chain, including
  the submission to the first SMTP server, is authenticated. Another
  proposal [SMTP-AUTH] can be used to authenticate delivery and MIME
  security multiparts [MIME-SEC] can be used to authenticate the author
  of an email message. In addition, the [SMTP-AUTH] proposal offers
  simpler and more flexible options to authenticate an SMTP client and
  the SASL EXTERNAL mechanism [SASL] MAY be used in conjunction with
  the STARTTLS command to provide an authorization identity.




































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A. References

  [RFC-821]   Postel, J., "Simple Mail Transfer Protocol", RFC 821,
              August 1982.

  [RFC-1869]  Klensin, J., Freed, N, Rose, M, Stefferud, E. and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.

  [RFC-2034]  Freed, N., "SMTP Service Extension for Returning Enhanced
              Error Codes", RFC 2034, October 1996.

  [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

  [SASL]      Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

  [SMTP-AUTH] "SMTP Service Extension for Authentication", Work in
              Progress.

  [TLS]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

B. Author's Address

  Paul Hoffman
  Internet Mail Consortium
  127 Segre Place
  Santa Cruz, CA  95060

  Phone: (831) 426-9827
  EMail: [email protected]


















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C.  Full Copyright Statement

  Copyright (C) The Internet Society (1999).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR 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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