Elastic Block Ciphers: Method, Security and Instantiations

The authors introduce the concept of an elastic block cipher, which refers to stretching the supported block size of a block cipher to any length up to twice the original block size while incurring a computational workload that is proportional to the block size. The method uses the round function of an existing block cipher as a black box and inserts it into a substitution-permutation network. The method is designed to enable one to form a reduction between the elastic and the original versions of the cipher. Using this reduction, they prove that the elastic version of a cipher is secure against key-recovery attacks if the original cipher is secure against such attacks.