Re: PGP key spoofing

[Hal <hfinney@shell.portal.com>]

From: christopher@nescio.zerberus.de (Christopher Creutzig)
>  I think I have realized a serious flaw in PGPs key-handling. This may 
> lead to people using and signing bogus keys despite the usual security 
> measures.
> 
>  The problem is that PGP fails to differentiate between two keys sharing 
> the same 64-bit-Key-ID. It is not a real problem to generate a key with a 
> given key-ID (just take a prime, invert the desired key-ID modulo this 
> prime and look for another prime whose lower bits are the same as in the 
> number you just calculated), so the following attack would be possible:

PGP checks specifically for the case of keys whose IDs match but the
keys themselves differ.  It has always been obvious that keys can easily
be synthesized with given IDs.  I added this warning in version 2.0
about four years ago, in the keyadd code:

"\n\007Warning: Key ID %s matches key ID of key already on
key ring '%s', but the keys themselves differ.
This is highly suspicious.  This key will not be added to ring.
Acknowledge by pressing return: "

>  If the owner of the correct key does not give a fingerprint, but rather 
> a disk with the correct key to the person you are trying to fool, his or 
> her pgp won't ring alarm bells when reading the key (apart from possibly 
> a failed signature), but rather will tell him the key is already there. 

As you can see, it does in fact literally ring an alarm bell - the "\007"
above is the ASCII bell character.

Disclaimer: I have not worked on PGP since version 2.0 so possibly my
code has been changed or eliminated, but I think that is unlikely.

Hal Finney