Abstract. In 1991 Lai, Massey and Murphy introduced the IPES (Improved Proposed Encryption Standard), later renamed IDEA (International Data Encryption Algorithm). In this paper we give two new attacks on a reduced number of rounds of IDEA. A truncated differential attack on IDEA reduced to 3.5 rounds and a differential-linear attack on IDEA reduced to 3 rounds. The truncated differential attack contains a novel method for determining the secret key.

Abstract. In this paper we evaluate the resistance of the block cipher RC5 against linear cryptanalysis. We describe a known plaintext attack that can break RC5-32 (blocksize 64) with 10 rounds and RC5-64 (blocksize 128) with 15 rounds. In order to do this we use techniques related to the use of multiple linear approximations. Furthermore the success of the attack is largely based on the linear hull-effect. To our knowledge, at this moment these are the best known plaintext attacks on RC5, which have negligible storage requirements and do not make any assumption on the plaintext distribution. Furthermore we discuss the impact of our attacking method on the AES-candidate RC6, whose design was based on RC5.

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