Cryptanalysis of Pomaranch (CJCSG)

Pomaranch is a synchronous stream cipher submitted to eSTREAM, the ECRYPT Stream Cipher Project. It uses 128-bit keys and IVs with different lengths. The cipher is constructed as a cascade clock control sequence generator, which is based on the notion of jump registers. Each jump register can be considered as a non-autonomous finite state machine which the input sequence is called jump control sequence. In this paper we show that a jump register with a balanced identically distributed binary jump control sequence can be modeled as a non-autonomous linear finite state machine with an additive unbalanced input sequence. Using this result we mount a correlation based key-recovery attack on Pomaranch with computational complexity around 2 using about 2 bits of the output sequence.