NEW TYPES OF CRYPTANALYTIC ATTACKS USING RELATED KEYS

In this paper we study the influence of key-scheduling algorithms on t he strength of blockciphers. We show that the key-scheduling algorithm s of many blockciphers inherit obvious relationships between keys, and use these key relations to attack the blockciphers. Two new types of attacks are described: New chosen plaintext reductions of the complexi ty of exhaustive search attacks (and the faster variants based on comp lementation properties), and new low-complexity chosen key attacks. Th ese attacks are independent of the number of rounds of the cryptosyste ms and of the details of the F-function and may have very small comple xities. These attacks show that the key-scheduling algorithm should be carefully designed and that its structure should not be too simple. T hese attacks are applicable to both variants of LOKI and to Lucifer. D ES is not vulnerable to the related keys attacks since the shift patte rn in the key-scheduling algorithm is not the same in all the rounds.