A: Several messages RSA-encrypted with small (< 513 bits) keys have been cracked publicly. Further effort is still ongoing, RSA Security offers prizes for their RSA factoring challenges.

First was the RSA-129 key. The inventors of RSA published a message encrypted with a 129-digits (430 bits) RSA public key, and offered $100 to the first person who could decrypt the message. In 1994, an international team coordinated by Paul Leyland, Derek Atkins, Arjen Lenstra, and Michael Graff successfully factored this public key and recovered the plaintext. The message read: " THE MAGIC WORDS ARE SQUEAMISH OSSIFRAGE "

They headed a huge volunteer effort in which work was distributed via E-mail, fax, and regular mail to workers on the Internet, who processed their portion and sent the results back. About 1600 machines took part, with computing power ranging from a fax machine to Cray supercomputers. They used the best known factoring algorithm of the time; better methods have been discovered since then, but the results are still instructive in the amount of work required to crack a RSA-encrypted message.

The coordinators have estimated that the project took about eight months of real time and used approximately 5000 MIPS-years of computing time.

What does all this have to do with PGP? The RSA-129 key is approximately equal in security to a 426-bit PGP key. This has been shown to be easily crackable by this project. PGP used to recommend 384-bit keys as "casual grade" security; recent versions offer 768 bits as a recommended minimum security level.

Note that this effort cracked only a single RSA key. If this had been a PGP key, it would have allowed them to decrypt all messages encrypted to that key. Nothing was discovered during the course of the experiment to cause any other keys to become less secure than they had been, i.e. it would not make it any easier to read messages encrypted to other keys.

A year later, the first real PGP key was cracked. It was the infamous Blacknet key, a 384-bits key for the anonymous entity known as "Blacknet". A team consisting of Alec Muffett, Paul Leyland, Arjen Lenstra and Jim Gillogly managed to use enough computation power (approximately 1300 MIPS) to factor the key in three months. It was then used to decrypt a publicly-available message encrypted with that key.

The most important thing in this attack is that it was done in almost complete secrecy. Unlike with the RSA-129 attack, there was no publicity on the crack until it was complete. Most of the computers only worked on it in spare time, and the total power is well within reach of a large, perhaps even a medium sized organization.