From: yanek@novavax.nova.edu (Yanek Martinson)
Date: Fri, 27 Nov 92 21:00:47 PST
To: gnu@toad.com  (John Gilmore)
Subject: Alternative to physically meeting
In-Reply-To: <9211280415.AA12997@toad.com>
Message-ID: <9211280500.AA08808@novavax.nova.edu>
MIME-Version: 1.0
Content-Type: text/plain


> person identified in the name field".  Don't sign someone's key unless
> you are sure you can make that statement (like, they're standing in the
> same room with you and they verify that they key ID matches their real key).
> Don't sign a key that you received by email or over a modem; it might
> be from someone impersonating your friend (when they left their keyboard

Here's an alternative method if you know the person (know them well
enough to recognize the voice on the phone):

You transfer the key over a non-trusted channel such as electronic
mail..  Then both of you run a secure hash function (for example MD5)
on the key.  The result (128bits in the case of MD5) is then converted
to alphanumerics using something like base64.  In the case of 128bit
hash, you end up with 22 character verification code.

Then you call each other up on the phone, and spell out the 22 letters 
and verify they match what you independently computed.  If they do,
that means the key transferred over e-mail is correct.

This is of course susceptible to the kind of attack where someone stands
with a gun pointed at you and makes you give the wrong key, but that
attack can also be done if meeting in person.  I.e. someone tells you they
are going to kill you as soon as you step out of the room if you don't give
the compromised key.

But at least with this attack one of the persons knows they key is 
no good, and you will avoid using it for sensitive material.

Can you think of any other attack that this method is susceptible to?


--
Yanek Martinson    mthvax.cs.miami.edu!safe0!yanek     uunet!medexam!yanek
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