[14887] in cryptography@c2.net mail archive
Re: yahoo to use public key technology for anti-spam
daemon@ATHENA.MIT.EDU (Sidney Markowitz)
Sun Dec 7 18:39:29 2003
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Mon, 08 Dec 2003 09:33:18 +1300
From: Sidney Markowitz <sidney@sidney.com>
To: cryptography@metzdowd.com
In-Reply-To: <Pine.GSO.4.58.200312071146200.2164@sasas1.ms.com>
Victor.Duchovni@morganstanley.com wrote:
> To avoid replay attacks one needs to
> sign a string that is tied to a
> specific message or time period
I agree. Even time period and message content aren't good enough: Let's
say that the outgoing SMTP mailer at example.com is trusted. Spammer
gets an account at example.com, sends themselves one message, then
immediately copies the signature into forged headers for their spam that
is sent out through whatever open relays or compromised machines they
are using. The only way that the mail can be trusted is if it is being
received directly from the example.com SMTP server. If there is any
relaying, there is nothing that remains true and constant to sign.
But that is the situation we have today: My ISP's server can choose to
refuse to accept connections from servers that are on a blacklist of
open relays and spammers, and can, in theory, have a list of known good
servers who authenticate their clients. If all the new header does is
verify the sending mail server, that is done just as well by verifying
the ip address at the time of connection.
-- sidney
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