Improved Collision Attack on OCB

In this paper we present an improvement of the collision attack [1] on the authenticated encryption mode of operation OCB. [1] presents a detection of collision method and a way to use the collision, and it is possible to use the information from a collision to change some blocks of the message unnoticed, if they have a special property. We found a way to use the information from a collision to change any future message in any position, without knowing anything about the plaintexts and the nonces. Once a collision is detected the probability of success of cheating is 1, and this part of the attack can be done by hand calculations. It must also be mentioned that our attack depends on the complexity of the detection of a collision, and this does not violate the security bounds given in [2, 3]. However once a collision is found the attack is simple and devastating.