Crab (cipher)

In cryptography, Crab is a block cipher proposed by Burt Kaliski and Matt Robshaw at the first Fast Software Encryption workshop in 1993.

Not really intended for use, Crab was developed to demonstrate how ideas from hash functions could be used to create a fast cipher.

Then the algorithm makes four passes over the data, each time applying one of four transformations adapted from MD5.

A brief note on the cryptanalysis of Crab is included in Markku-Juhani Saarinen's paper on block ciphers based on SHA-1 and MD5, published at FSE 2003.

The author demonstrates a weakness in Crab that permits a distinguisher using no more than a dozen chosen plaintexts, and speculates that this can be converted into a full key-recovery attack using no more than 216 chosen plaintexts.