From: Jiri Baum <jirib@sweeney.cs.monash.edu.au>
Date: Wed, 17 Jan 1996 08:52:07 +0800
To: ckey2@eng.ua.edu (Christopher R. Key)
Subject: Re: A weakness in PGP signatures, and a suggested solution (long)
In-Reply-To: <1996Jan11.152134.127675@ua1ix.ua.edu>
Message-ID: <199601162319.KAA09830@sweeney.cs.monash.edu.au>
MIME-Version: 1.0
Content-Type: text/plain


-----BEGIN PGP SIGNED MESSAGE-----

Hello ckey2@eng.ua.edu (Christopher R. Key)
  and cypherpunks@toad.com
 
> In article <Pine.ULT.3.91.960110182255.18692H-100000@xdm011>, Jeffrey Goldberg <cc047@Cranfield.ac.uk> says:
...
> First of all, if the recipient is a newsgroup, why would that particular
> information need to be part of the signed information?  If you post to a
...

Somebody already pointed out an adult message being re-posted to a kidgroup.

...
> Secondly, if you are sending email to some one and sign it using pgp, wouldn't
> that person need pgp to prove that in fact you did sign it?  Then it can be
...
> So if all that needs be done to a message to insure that the appropriate 
> person reads it is encrypt it using their public key, why does pgp (or one
> of the pgp interfaces) need to be changed to include header information?  
...

But then the recipient has a PGP-signed message from you which
isn't encrypted (using pgp -d). That person could then impersonate
you. Eg Alice the jilted lover could resend the goodbye message
with forged headers to Bob's new girlfriend to get back at him.

What a sentence. Here it is again, hopefully understandable:

Bob->Alice
  From:Bob; Encrypted(Signed("We're through",Bob),Alice)

Alice does pgp -d, leaving her with Signed("We're through",Bob)

Alice->Carol
  From:Bob; Encrypted(Signed("We're through",Bob),Carol)

Later, when Bob gets another girlfriend,

Alice->Danielle
  From:Bob; Encrypted(Signed("We're through",Bob),Danielle)

Later still,

Alice->Eve
  From:Bob; Encrypted(Signed("We're through",Bob),Eve)