PGP key spoofing

[christopher@nescio.zerberus.de (Christopher Creutzig)]

Hello, everybody,

 (Please note that I sent this to several mailinglists at once. I am not 
subscribed to cypherpunks any longer, so I won't see any replies there.)

 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:

- Get the real key you wish to mimic.

- Generate a fake key with the correct IDs.

- Send your bogus key to a person of which you know that
  - This person does not have the correct key yet.
  - This person is going to meet the correct key's owner.

 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. 
He will then, most probably, sign the bogus key without any further thought.

 Therefore, you should *always* check the fingerprint, even if you got 
the real key, at least if it has no valid signature from its alleged 
owner.

-- 
Christopher Creutzig # Im Samtfelde 19 # D-33098 Paderborn # V+49-5251-71873
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              Sammele Vorschl�ge zur Rettung vom Genitiv.