An extension of Cook’s elasti cipher

Abstract

Given a block cipher of length L Cook's elastic cipher allows to encrypt messages of variable length from L to 2L. Given some conditions on the key schedule, Cook's elastic cipher is secure against any key recovery attack if the underlying block cipher is, and it achieves complete diffusion in at most q + 1 rounds if the underlying block cipher achieves it in q rounds. We extend Cook's construction inductively, obtaining the first elastic cipher for any message length greater than L with the same properties of security as Cook's elastic cipher, and whose complexity of the encryption/decryption grows polynomially with the size of the message.

Keywords:

• variable length block cipher
• elastic cipher
• Cook