New related key attacks on the RAKAPOSHI stream cipher

Abstract

RAKAPOSHI is a hardware oriented stream cipher designed by Cid et al. in 2009. It is based on Dynamic Linear Feedback Shift Registers, with a simple and potentially scalable design, and is particularly suitable for hardware applications with restricted resources. The RAKAPOSHI stream cipher offers 128-bit security. In this paper, we point out some mistakes existing in the related key attack on RAKAPOSHI by Isobe et al., and propose a new related key attack on RAKAPOSHI, which recovers the 128-bit secret key with a time complexity of 2 56, requiring one related key and 2 55 chosen IVs. Furthermore, an improved key recovery attack on RAKAPOSHI in the multiple related key setting is proposed with a time complexity of 2 33, requiring 2 12.58 chosen IVs. As confirmed by the experimental results, our new attack can recover all 128 key bits of RAKAPOSHI in less than 1.5 hours on a PC.