  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 an elastic cipher for any message
length greater than L with the same properties of security as Cook's elastic
cipher.
