The Security of Elastic Block Ciphers Against Key -Recovery Attacks

We analyze the security of the recently proposed method of Elastic Block Cipher. The method creates a variable-length block cipher from any existing fixed-length block cipher. The basic step in the elastic design is to allow any length up to double the original block size, while employing the original fixed-length block cipher's round function to be used throughout (as a design goal). In this work we form a reduction between the elastic and the original versions of the cipher, exploiting the structure of the design goal. We prove that the elastic version of a cipher is secure against key-recovery attacks if the original cipher is secure against such attacks. The proof technique allows us to demonstrate under what properties of the elastic design and of the original cipher, the security of the elastic version can be directly related to that of the original fixed-length version under the above attacks (which are the typical, general attacks considered against block ciphers). We note that while reduction-based proofs of security are a cornerstone of cryptographic analysis, they are typical when complete components are used as sub-components in a bigger design. We are not aware of use of such techniques in the case of concrete block cipher designs.