On Related-Key Attacks and KASUMI: The Case of A5/3
P. Q. Nguyen, M. Robshaw, and H. Wang.

Due to its widespread deployment in mobile telephony, the block cipher KASUMI is a prominent target for cryptanalysts. While the cipher offers excellent resistance to differential and linear cryptanalysis, in the related-key model there have been several impressive cryptanalytic results. In this paper we revisit these related-key attacks and highlight a small, but important, detail in the specification of KASUMI for the algorithm A5/3; namely that a 64- and not a 128-bit session key is used. We show that existing related-key attacks on KASUMI in the literature are (negatively) impacted by this feature and we provide evidence that repairing these attacks will be difficult.