I just got this little draft specification which relates to security for email.
I expect that our solution to the IPP security requirements may look
something along the same lines.
Regards,
Carl-Uno
--Internet Draft Paul Hoffman draft-hoffman-smtp-ssl-00.txt Internet Mail Consortium March 24, 1997 Expires September 24, 1997
SMTP Service Extension for Secure SMTP over TLS
Status of this memo
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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.
3. 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 an optional parameter that contains a space-separated list of tokens describing the type and version of TLS that can be used;
(4) a new SMTP verb, "STARTTLS", and its parameter, are defined;
(5) no additional parameters are added to any SMTP command.
4. The TLS Keyword
The TLS keyword is used to tell the SMTP client that the SMTP server allows use of TLS. It has one optional parameter: a space-separated list of tokens describing the type and version of TLS that can be used. Each token describes a TLS protocol that has a description that has been published by the IESG.
The only tokens currently defined are: TLS1.0 [TLS] SSL3.0 [SSL30]
5. The STARTTLS Command
The format for the STARTTLS command is:
STARTTLS token
where the token parameter is one of the tokens described in Section 4.
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 parameter, more than one parameters) 504 - TLS not available due to the server not being able to use the specified protocol 554 - TLS not available due to some other reason
In the case of a 220 response code, the client SHOULD start the TLS procedure immediately. However, a server SHOULD be able to accept other SMTP command between sending a 220 response code and receiving a STARTTLS command.
6. Usage Example
The following dialog illustrates how a client and server can start a TLS session on the first attempt:
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 greets you with a warm hug S: 250 TLS TLS1.0 SSL3.0 C: STARTTLS TLS1.0 S: 220 Go ahead with TLS1.0 C: <starts TLS session>
The following dialog illustrates how a client and server can start a TLS session after trying a TLS protocol the server doesn't support:
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 smiles broadly and invites you in S: 250 TLS C: STARTTLS SSL3.0 S: 504 Sorry, I don't know how to SSL3.0 C: STARTTLS TLS1.0 S: 220 Go ahead with TLS1.0 C: <starts TLS session>
7. Security Considerations
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.
The SMTP client should note carefully the result of the TLS negotiation. If the negotiation results in no privacy, the client may choose to abort the SMTP 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
[SMTP-AUTH] "SMTP Service Extension for Authentication", draft-myers-smtp-auth
[SSL30] "The SSL Protocol Version 3.0", draft-ietf-tls-ssl-version3
[TLS] "The TLS Protocol Version 1.0", draft-ietf-tls-protocol
B. Outstanding Issues
Need to create an IANA registry and vetting process for adding new tokens.
C. Author's Address
Paul Hoffman Internet Mail Consortium 127 Segre Place Santa Cruz, CA 95060 (408) 426-9827 phoffman@imc.org