The Pseudorandomness of Elastic Block Ciphers

We investigate elastic block ciphers, a method for constructing variable length block ciphers, from a theoretical perspective. We view the underlying structure of an elastic block cipher as a network, which we refer to as an elastic network, and analyze the network in a manner similar to the analysis performed by Luby and Rackoff on Feistel networks. We prove that a three round elastic network is a pseudorandom permutation and a four round network is a strong pseudorandom permutation when the round functions are independently chosen pseudorandom permutations. As a result, the elastic network allows for the creation of pseudorandom permutations and strong pseudorandom permutations with variable length inputs from PRPs with fixed length inputs.