KASUMI

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In cryptography, KASUMI, also termed A5/3, is a block cipher used in the confidentiality (f8) and integrity algorithms (f9) for 3GPP mobile communications. KASUMI was designed by the Security Algorithms Group of Experts (SAGE), part of the European standards body ETSI. Rather than invent a cipher from scratch, an existing algorithm, MISTY1, was selected by SAGE and slightly optimised for implementation in hardware. Hence, both MISTY1 and KASUMI are very similar — kasumi (霞) is the Japanese word for "misty" — and the cryptanalysis of one is likely to be readily adaptable to the other.

KASUMI has a block size of 64 bits and a key size of 128 bits. It is a Feistel cipher with eight rounds, and like MISTY1 and MISTY2, it has a recursive structure, with subcomponents also having a Feistel-like form.

In 2001, an impossible differential attack on six rounds of KASUMI was presented by Kühn (2001).

In 2005, Israeli researchers Eli Biham, Orr Dunkelman and Nathan Keller published a related-key rectangle (boomerang) attack on KASUMI that can break all 8 rounds faster than exhaustive search. The attack requires 2^54.6 chosen plaintexts, each of which has been encrypted under one of four related keys, and has a time complexity equivalent to 2^76.1 KASUMI encryptions. While this is not a practical attack, it invalidates some proofs about the security of the 3GPP protocols that had relied on the presumed strength of KASUMI.

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References

ETSI/SAGE, "KASUMI Specification, Part of the Specification of the 3GPP Confidentiality and Integrity Algorithms", 1999.
Kühn, U. "Cryptanalysis of Reduced Round MISTY", EUROCRYPT 2001.
Eli Biham, Orr Dunkelman and Nathan Keller, A Related-Key Rectangle Attack on the Full KASUMI, ASIACRYPT 2005, 443–461

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