5 - USN-170-1

A security issue affects the following Ubuntu releases:

Ubuntu 4.10 (Warty Warthog) Ubuntu 5.04 (Hoary Hedgehog)

The following packages are affected:

gnupg

The problem can be corrected by upgrading the affected package to version 1.2.4-4ubuntu2.1 (for Ubuntu 4.10), or 1.2.5-3ubuntu5.1 (for Ubuntu 5.04). In general, a standard system upgrade is sufficient to effect the necessary changes.

Details follow:

Serge Mister and Robert Zuccherato discovered a weakness of the symmetrical encryption algorithm of gnupg. When decrypting a message, gnupg uses a feature called "quick scan"; this can quickly check whether the key that is used for decryption is (probably) the right one, so that wrong keys can be determined quickly without decrypting the whole message.

A failure of the quick scan will be determined much faster than a successful one. Mister/Zuccherato demonstrated that this timing difference can be exploited to an attack which allows an attacker to decrypt parts of an encrypted message if an "oracle" is available, i. e. an automatic system that receives random encrypted messages from the attacker and answers whether it passes the quick scan check.

However, since the attack requires a huge amount of oracle answers (about 32.000 for every 16 bytes of ciphertext), this attack is mostly theoretical. It does not have any impact on human operation of gnupg and is not believed to be exploitable in practice.

The updated packages disable the quick check, which renders this timing attack impossible.