Below are both messages once they have been verified. Notice that the
signature block along with the comment field are not present in the verified
messages, and your original message appears unaltered.

*** PGP Signature Status: good
*** Signer: [Segmen] <dontpanic999yahoo.com> (Invalid)
*** Signed: 10/15/2001 10:16:15 AM
*** Verified: 10/15/2001 2:05:53 PM
*** BEGIN PGP VERIFIED MESSAGE ***

Hello, meeting cancelled, speak to you soon.

*** END PGP VERIFIED MESSAGE ***

I could change this to :

*** PGP Signature Status: good
*** Signer: [Segmen] <dontpanic999yahoo.com> (Invalid)
*** Signed: 10/15/2001 10:16:15 AM
*** Verified: 10/15/2001 2:05:53 PM
*** BEGIN PGP VERIFIED MESSAGE ***

Hello, meeting cancelled, speak to you soon.

*** END PGP VERIFIED MESSAGE ***

-----Original Message-----
From: [Segmen] [mailto:dontpanic999yahoo.com]
Sent: Monday, October 15, 2001 10:28 AM
To: vuln-devsecurityfocus.com; bugtraqsecurityfocus.com
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.

Example :

A generic message I could have got hold of :

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hello, meeting cancelled, speak to you soon.

-----BEGIN PGP SIGNATURE-----
Version: PGP 7.0.4

iQA/AwUBO8r9v9nrfc+JfUO6EQLrEACgv6+C07aWgAO+Dna0MHgEDaoDMxEAoJ2P
7gojqeCRqKqTkbFMkHCToxtq
=lki3
-----END PGP SIGNATURE-----

I could change this to :

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hello, meeting cancelled, speak to you soon.

-----BEGIN PGP SIGNATURE-----
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Please Send the Confidential Files from the planned meeting to
My colleague Instead at mehost.com . He will now be dealing with
this matter.
Speak to you soon, victim.

-----BEGIN PGP SIGNATURE-----
Version: PGP 7.0.3

iQA/AwUBO8r9v9nrfc+JfUO6EQLrEACgv6+C07aWgAO+Dna0MHgEDaoDMxEAoJ2P
7gojqeCRqKqTkbFMkHCToxtq
=lki3
-----END PGP SIGNATURE-----

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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