A Generalization of Linear Cryptanalysis andthe

Matsui's linear cryptanalysis for iterated block ciphers is generalized by replacing his linear expressions with I/O sums. For a single round, an I/O sum is the XOR of a balanced binary-valued function of the round input and a balanced binary-valued function of the round output. The basic attack is described and conditions for it to be successful are given. A procedure for nding eeective I/O sums, i.e., I/O sums yielding successful attacks, is given. A cipher contrived to be secure against linear cryptanalysis but vulnerable to this generalization of linear cryptanalysis is given. Finally, it is argued that the ciphers IDEA and SAFER K-64 are secure against this generalization.