Re: Man In The Middle Attacks and STARTTLS

At 01:39 AM 4/16/99 +0200, Bodo Moeller wrote:
>The discussion of authentication in RFC 2487 is rather vague, anyway.
>The RFC defines a protocol mechanism, but does not thoroughly discuss
>the goals that may be the reason to use this mechanism.  Depending on
>what one is trying to achieve, things can look quite different.

Exactly right. The reasons are briefly stated at the beginning of the RFC, 
but are left for local implementation. Just like they are in the main TLS RFC.

>The hints in RFC 2487 are rather fuzzy, e.g.:
>
>     - 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.
>
>When taken literally, this way of authenticating the server is all but
>useless because the name of the server to connect to will typically be
>obtained by a DNS query that does not use strong authentication

Not true. Even without authentication, if the client gets its DNS lookups 
over a trusted pathway (such as an internal network), authentication may 
not be needed, especially when asking for DNS records for which the server 
asked is authoritative.

>  -- and
>the domain name of the MX need not have anything to do with the domain
>name in the recipient's e-mail address.

True, but this is an intractable problem. There is no other way (that I can 
think of, at least) to determine whether or not the client should trust the 
server's authenticity for the purpose of transferring mail, even if they 
share a common root CA.

>   Only if the client knows from
>the start where to send the mail, authenticating the server makes
>sense.

Such as getting the information from a trusted DNS server.

>To summarize, it appears to me that one important question is usually
>neglected: What problem is it that we trying to solve by using TLS?

I didn't neglect the question, I just covered it very lightly. :-)

BTW, I'm going to turn in the first draft of the Internet Draft now, so why 
don't we wait a few days until it is published and then bash the specific 
wording in the draft.

--Paul Hoffman, Director
--Internet Mail Consortium