Ubuntu Security Notice USN-170-1 – 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.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/39528/USN-170-1.txt
Source: https://packetstormsecurity.com/files/39528/Ubuntu-Security-Notice-170-1.html