Schneier et al Report PGP Vulnerability

SpaceTaxi writes: "Researchers reported that they were able to intercept and modify a PGP encrypted message so that, IF it is sent back to the attacker, the original message could be read by the attacker." The paper comes from Kahil Jallad, Jonathan Katz, and Bruce Schneier. Here is the Yahoo! article.

Comments

This is a very specialized attack

[Featureless]

It hinges on being able to intercept a message, add some random data to the encrypted blocks containing its payload, and then for the recipient to decrypt it, and respond "hey Ed, what's with this garbled message you just sent me?" with that decrypted message quoted below. And, naturally, for the attacker to be able to intercept that response as well.

The basic idea of a "chosen cyphertext" attack is that if you can see a decryption of blocks you mangle, you can work backwards to get the plaintext in the unmangled blocks. You might consider this an attack on the user interface or the protocol rather than the algorithm. You should just never be quoting failed decryptions...

The talk about compression preventing the attack is not referring to the compression of cyphertext by you (i.e. ZIP'ing the payload before sending). That doesn't make a difference. It involves the DEFLATE compression the PGP/GPG software applies (and it generally does so only for uncompressed plaintext) both before and after encryption. You may already be realizing, randomizing compressed data will cause the decompression to fail with an error; that will make it much less likely for the user to disclose the failed decryption.

Fixing this is a good idea. Until it is fixed, if someone sends you garbage, don't reply, or if you do, don't quote their message in your reply. However, this is not the end of the world. The foundation is still sound, the attack is only useful on a per-message basis, and your keys are not affected by this strategy.

I do have a question for the crowd; it seems to me that this is an attack on "encrypted" messages, as opposed to "encrypted and signed" messages. I am assuming that the use of signatures will also foil this attack, but I would welcome comments from others on that subject.