On the construction of variable-input-length ciphers

Abstract

Whereas a block cipher enciphers messages of some one par- ticular length (the blocklength), a variable-input-length cipher takes mes- sages of varying (and preferably arbitrary) lengths. Still, the length of the ciphertext must equal the length of the plaintext. This paper intro- duces the problem of constructing such objects, and provides a prac- tical solution. Our VIL mode of operation makes a variable-input-length cipher from any block cipher. The method is demonstrably secure in the provable-security sense of modern cryptography: we give a quantita- tive security analysis relating the dificulty of breaking the constructed (variable-input-length) cipher to the dificulty of breaking the underlying block cipher.