The Cube Attack on Stream Cipher Trivium and Quadraticity Tests

In 2008 I. Dinur and A. Shamir presented a new type of algebraic attack on symmetric ciphers named cube attack. The method has been applied to reduced variants of stream ciphers Trivium and Grain128, reduced variants of the block ciphers Serpent and CTC and to a reduced version of the keyed hash function MD6. Independently a very similar attack named AIDA was introduced by M. Vielhaber. In this paper we develop quadraticity tests within the cube attack and apply them to a variant of stream cipher Trivium reduced to 709 initialization rounds. Using this method we obtain the full 80-bit secret key. In this way it eliminates the stage of brute force search of some secret key bits which occured in previous cube attacks.