Cryptanalysis of the FLIP Family of Stream Ciphers.

At Eurocrypt 2016, Méaux et al. proposed FLIP, a new family of stream ciphers intended for use in Fully Homomorphic Encryption systems. Unlike its competitors which either have a low initial noise that grows at each successive encryption, or a high constant noise, the FLIP family of ciphers achieves a low constant noise thanks to a new construction called filter permutator. In this paper, we present an attack on the early version of FLIP that exploits the structure of the filter function and the constant internal state of the cipher. Applying this attack to the two instantiations proposed by Méaux et al. allows for a key recovery in $$2^{54}$$ basic operations resp. $$2^{68}$$, compared to the claimed security of $$2^{80}$$ resp. $$2^{128}$$.