From: mccoy@io.com (Jim McCoy)
Date: Wed, 28 Sep 94 13:53:15 PDT
To: dps@kafka.atinc.com (Doug Shapter)
Subject: Re: Mandatory email verification
In-Reply-To: <9409280832.ZM12030@kafka.atinc.com>
Message-ID: <199409282052.PAA18770@pentagon.io.com>
MIME-Version: 1.0
Content-Type: text/plain


> From: dps@kafka.atinc.com (Doug Shapter)
> 
> Its my understanding that to be truly useful on multi-user
> systems, digital signatures require some user input (eg, PGP
> requires entering a pass phrase).

Not really.  The system I was sketching out would not require the user
to enter any information at all, the sendmail daemon would handle
everything and have the key for that mail server held internally.  The
purpose would not be to say that "User X" did or did not sign a
message, but to say _with reasonable assurance_ that the message
either came from someone logged in as userx@foo.com (there are other
alternatives, like the mail server being hacked, etc.)  The purpose of
such a system would not be to link mail messages to any real person or
identity, but to link it to an account on the sending host (and mostly
to link it to the sending host.)  Thus someone who just did a telnet
to port 25 and forged off a mail message would not be able to generate
the necessary site signature to pull off the charade unless they
managed to actually hack _into_ the mail server.  Bouncing messages
off a smtp port would no longer be enough to work.

In actual practice the keys would not need to be monstrously huge and
one could probably get by with a public key small enough to fit into a
TXT record in the DNS system.  It would be easier to break in to the
system than crack a 512 bit key...

The mail system would not be signing the messages for you, it would
just do a hash of a few choice lines from the header and sign those
with the mail system key.  It would not try to say that any particular
person sent a mail message but would instead say "to the best of my
knowledge this message came from my system and was sent by someone
accessing account userx" and no more.  This would probably be enough
to cut mail forgery through smtp by 90% among sites using the system.

jim