A note on Cook's elastic block cipher

"VIL Block Cipher" is a kind of block cipher that supports Variable Input-Lengths. It was first proposed by Bellare, etc. and since then several constructions have been given, among which Cook's elastic block cipher is a special one. In this paper we present a security model called "MIL model" for VIL block ciphers, investigating their security when a fixed secret key is used for multiple input-lengths. Our results show that if the key schedule is not well designed, Cook's elastic block cipher is vulnerable when processing multiple-length inputs under a fixed secret key. Thus, further considerations are needed to use Cook's elastic block cipher safely in practice.