Chosen-Ciphertext Attacks Against MOSQUITO

Self-Synchronizing Stream Ciphers (SSSC) are a particular class of symmetric encryption algorithms, such that the resynchronization is automatic, in case of error during the transmission of the ciphertext. In this paper, we extend the scope of chosen-ciphertext attacks against SSSC. Previous work in this area include the cryptanalysis of dedicated constructions, like KNOT, HBB or SSS. We go further to break the last standing dedicated design of SSSC, i.e. the ECRYPT proposal MOSQUITO. Our attack costs about 270 computation steps, while a 96-bit security level was expected. It also applies to ΓΥ (an ancestor of MOSQUITO) therefore the only secure remaining SSSC are block-cipher-based constructions.