A coming attack on PGP, and a way to mitigate it
dshaw at jabberwocky.com
Mon May 4 04:41:49 CEST 2009
On May 2, 2009, at 1:42 AM, Daniel Franke wrote:
> A necessary precondition for this attack to succeed is that the
> of the packet that Trent signs be predictable. Right now, they are.
> Guessing the signature creation time could be problematic, but if
> relying on that, then that's a hell of a way to run a cryptosystem.
> However, there's no reason this has to be so. Without breaking
> compatibility, GnuPG could include in every signature packet a hashed
> 'Notation' subpacket containing a random nonce. Now the SHA-1 digest
> that Trent signs is no longer predictable in advance, so if Mallory
> wants to find a collision, he needs a preimage attack rather than
> just a
> birthday attack. This is much more difficult.
I rather like this idea. I was thinking that a good way to tie it
into the GPG way of doing things would be another one of the %-
expandos. So you could do something like set a notation for "random at example.com
=%10r" for 10 digits of random hex, %15r for 15 digits, etc.
Before we go down this road, however, it might be worth waiting the
(should be short) period of time before the new SHA-1 paper is
formally published and people can give it a good read over. The
slides are good, but I think it's healthy to get a better notion of
the attack before we implement a countermeasure (even one as harmless
to compatibility as this).
> This seems like a worthwhile precaution regardless of the strength of
> the hash algorithm being used.
True. It's a very neat idea.
More information about the Gnupg-devel