On the Security of the IDEA Block Cipher

IDEA is an iterated block cipher proposed by Lai and Massey and is based on the design concept of “mixing operations from different algebraic groups”. New arithmetic properties of the basic operations used in the round function are found and investigated with respect to the security of this block cipher. Evidence is given that these properties can be exploited in the cryptanalysis of the first 2 rounds of IDEA but that they are of no assistance in the cryptanalysis of the full IDEA block cipher containing 8 rounds.