IDEA is a 64-bit block cipher with 128-bit keys introduced by Lai and 
Massey in 1991. IDEA is one of the most widely used block ciphers, due 
to its inclusion in several cryptographic packages, such as PGP and SSH. 
The cryptographic strength of IDEA relies on a combination of three 
incompatible group operations - XOR, addition and modular multiplication. 
Since its introduction in 1991, IDEA has withstood extensive cryptanalytic 
effort, but no attack was found on the full variant of the cipher. In this 
talk we go over past attempts at cryptanalysis of IDEA, and describe sevreal 
new results: 
- The first known non-trivial relation that involves all the three 
operations of IDEA. 
- A linear attack on 5-round IDEA that uses 2^{19} known plaintexts and has 
a time complexity of 2^{103} encryptions. 
- A related-key linear attack on 7.5-round IDEA that uses 2^{43.5} known 
plaintexts and jas a time complexity equivalent to 2^{115.1} encryptions. 

This is a joint work with Eli Biham and Nathan Keller.