Re: PGP Signed Messages

["Kurt Seifried" <bugtraq () seifried org>]

Yes and no. When you verify the message a box comes up, with the following
(for example):

==================
*** PGP Signature Status: good
*** Signer: Kurt M. Seifried  <seifried () seifried org>
*** Signed: 10/15/01 3:25:13 PM
*** Verified: 10/15/01 3:27:21 PM
*** BEGIN PGP VERIFIED MESSAGE ***

test testijng. 1 2 3.

Kurt

*** END PGP VERIFIED MESSAGE ***
==================

the rest is ignored (i.e. the stuff I stuck in after -----BEGIN PGP
SIGNATURE-----). If people don't bother to check the signature (very very
very common!) then it doesn't matter much. Iused to sign all my email with
PGP for a while, then started forging them and no-one complained. Hell, I've
seen security alerts with totally messed up MD5 sums/signatures/etc/etc and
after notifying the appropriate people typically gotten a "yeah we made a
mistake, but only 3 people noticed". One reason for X.509 instead of PGP for
email, clients automatically check stuff and display a nasty warning (in
outlook anyways) if it isn't signed right, has been modified/etc. Also
another reason why you should ENCRYPT sensitive data aswell as sign it.

BTW PGP key ID's can easily be faked, you can make arbitrary keys with any
PGP key ID you want. Don't forget to include the fingerprint (at least then
it's only mostly useless as opposed to completely useless).

Kurt Seifried, kurt () seifried org
A15B BEE5 B391 B9AD B0EF
AEB0 AD63 0B4E AD56 E574
http://www.seifried.org/security/



----- Original Message -----
From: "[Segmen]" <dontpanic999 () yahoo com>
To: <vuln-dev () securityfocus com>; <bugtraq () securityfocus com>
Sent: Monday, October 15, 2001 9:27 AM
Subject: PGP Signed Messages


> It occurred to me today what a bad idea the Comment Field is in PGP signed
> messages. Altering the Comment filed does not affect the validity of the
> signature, but to the non experienced PGP/GPG user it certainly appears to
> be part of the message.
>
> well, you get the idea. The signature is still valid.
>
> Agreed that only the beginner crypto user would fall for this, but if they
> were to read the message and then just use PGP to check the validity, they
> could be tricked into believing that the extra lines were part of the
> verified message.
> Does anybody else think this is quite a bad idea?
>
>
> --
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