On the Security of IV Dependent Stream Ciphers

Almost all the existing stream ciphers are using two inputs: a secret key and an initial value (IV). However recent attacks indicate that designing a secure IV-dependent stream cipher and especially the key and IV setup component of such a cipher remains a difficult task. In this paper we first formally establish the security of a well known generic construction for deriving an IV-dependent stream cipher, namely the composition of a key and IV setup pseudo-random function (PRF) with a keystream generation pseudo-random number generator (PRNG). We then present a tree-based construction allowing to derive a IV-dependent stream cipher from a PRNG for a moderate cost that can be viewed as a subcase of the former generic construction. Finally we show that the recently proposed stream cipher quad [3] uses this tree-based construction and that consequently the security proof for quad’s keystream generation part given in [3] can be extended to incorporate the key and IV setup.