Known Plaintext Cryptanalysis of Tree-structured Block Ciphers

In this Letter we examine the cryptanalysis of a class of block ciphers referred to as substitution-permutation networks or SPNs. Speciically, we present a novel attack applicable to tree-structured SPNs. Because it uses a known plaintext approach, the attack is preferable to previously outlined chosen plaintext attacks. As well, it is shown that the attack is applicable to networks which are simple extensions of tree-structured SPNs. Introduction: The concepts of \confusion" and \diiusion" introduced by Shannon 1] form the basis for a class of private key block ciphers referred to as substitution-permutation networks or SPNs. These networks, rst suggested by Feistel 2], encrypt by passing an N-bit data block through R rounds of substitutions followed by permutations of the bit positions. The substitutions are accomplished by dividing the block into small n-bit sub-blocks and using N=n n-bit mappings referred to as S-boxes.