Key Signing

[David Turner]

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On Mon, 30 Apr 2001, Werner Koch wrote:

> > For example, if we were to organise a password at the meeting, then he
> > goes home and sends me an encrypted email containing the password and his
> 
> I am not sure whether I got it right.  If your problem is on how to
> sign someones key if he does not know his fingerprint when you meet
> him (or he has no key yet), you can use this protocol:
> 
> 1. Create a shared secret.  For example by using
>     gpg --gen-random 2 10 | gpg --enarmor
> 2. Both parties write it down and keep it secret.
> 3. Alice gives her fingerprint to Bob (she always carries it with her)
> 4. At home, Bob send the fingerprint along with the shared secret in
>    an _encrypted_ mail to Alice.  He can do so because he knows Alice
>    keys
> 5. Alice decrypt the mail from Bob, verifies the secret against her
>    copy and can now be sure that it is Bob's fingerprint
> 6. Alice signs Bobs key.
> 

The situation was less hypothetical than I had made out.  We met at a jazz
club, where there is a distinct lack of linux boxes (something I'm trying
to change by using one as a spectrum analyser, but that's another
story...)  I had my fingerprint on my palmtop, and can remember enough of
it to know that it hasn't been tampered with, so gave it to him.  Since he
managed to tell me what his first round was, what software he gave me,
what CDs I had with me, how far through Linux From Scratch I was *and*
what his fingerprint was, all correctly, in an encrypted message, I
deduced that the message has not been tampered with.  Similar protocol,
but the random secret was generated in a slightly non-standard way.

It's good to get confirmation from the experts, though :-)

- -- 
Dave Turner
dct25@cam.ac.uk
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