Martin Lambers <[email protected]> writes:

> If a server uses user/password authentication, then PLAIN is really the
> only method that needs to be supported, since nowadays an SMTP session
> should be protected by TLS encryption anyway.
>
> If a server wants to offer an additional user/password authentication
> scheme that does not reveal the password even in the absence of TLS
> encryption, then it should offer the properly standardized
> and documented SCRAM-SHA1. Disadvantage to PLAIN: the server must know
> the clear text password; storing a hash is not enough. This means a
> larger risk for attacks.

That's true for CRAM-MD5, but not for SCRAM-SHA1.  SCRAM-SHA1 allows the
server to only store a hash, so it is nice.  However, that makes it
harder to in the future migrate to another mechanism which uses another
hash.  I would say SCRAM-SHA1 is still better than CRAM-MD5, DIGEST-MD5,
and PLAIN though.

> For the special case of authentication via TLS client certificates,
> method EXTERNAL may be used.

Yep.  Alas, it is a bit underspecified whether EXTERNAL means
TLS-and-client-certs or something else.  You need to out-of-band policy
agreement about this between client and server.

> These three methods are really the only useful ones now, as far as I
> can see.

GSSAPI and GS2-KRB5 are useful for Kerberos people.  There ought to be
some checks to make sure GSSAPI/GS2 isn't selected if it is likely that
it won't succeed (e.g., no Kerberos tickets available).  I thought GNU
SASL checked that, but I'm not confident.

/Simon

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