We discuss the elastic versions of block ciphers whose round function processes subsets of bits from the data block differently, such as occurs in a Feistel network and in MISTY1. We focus on how specific bits are selected to be swapped after each round when forming the elastic version, using an elastic version of MISTY1 and differential cryptanalysis to illustrate why this swap step must be carefully designed. We also discuss the benefit of adding initial and final key dependent permutations in all elastic block ciphers. The implementation of the elastic version of MISTY1 is analyzed from a performance perspective.
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