Overtaking VEST

VEST is a set of four stream cipher families submitted by S. O’Neil, B. Gittins and H. Landman to the eSTREAM call for stream cipher proposals of the European project ECRYPT. The state of any family member is made of three components: a counter, a counter diffusor and a core accumulator. We show that collisions can be found in the counter during the IV Setup. Moreover they can be combined with a collision in the linear counter diffusor to form collisions on the whole cipher. As a consequence, it is possible to retrieve 53 bits of the keyed state of the stream cipher by performing a chosen IV attack. For the default member of a VEST family, we present a “long” IV attack which requires 2 IV setups, and a “short” IV attack which requires 2 IV setups on average. The 53 bits retrieved can be used to reduce the complexity of the exhaustive key search. The chosen IV attack can be turned into a chosen message attack on a MAC based on VEST.