On IV Setup of Pomaranch

Pomaranch is a synchronous bit-oriented stream cipher submitted to eSTREAM, the ECRYPT Stream Cipher Project. Following the recently published chosen IV [1] and correlation [7] key-recovery attacks, the authors changed the configuration of jump registers and introduced two new key-IV setup procedures for the cipher. We call the updated version as Tweaked Pomaranch vs. Original Pomaranch [4]. In this paper we use the findings of [7] to mount a chosen IV key-recovery attack on the Original Pomaranch with computational complexity of O(2). The attack is also applicable to the first key-IV setup proposal for Tweaked Pomaranch with computational complexity of O(2). The alternative key-IV setup for Tweaked Pomaranch is immune against our attack. Both versions of Pomaranch deal with 128 bit keys.