We introduce a new approach for cryptanalysis of key agreement protocols based on noncommutative groups. This approach uses functions that estimate the distance of a group element to a given subgroup. We test it against the Shpilrain-Ushakov protocol, which is based on Thompson’s group F , and show that it can break about half the keys within a few seconds on a single PC.

Batch Di e-Hellman key agreement schemes are described, motivated, and analyzed for security and e ciency.

An attack is demonstrated on a previously proposed class of key agreement protocols. Analysis of the attack reveals that a small change in the construction of the protocols is suucient to prevent the attack. The insight gained allows a generalisation of the class to a new design for conference key agreement protocols.

This paper presents some new unknown key-share attacks on STS-MAC, the version of the STS key agreement protocol which uses a MAC algorithm to provide key confirmation. Various methods are considered for preventing the attacks.