 The block is divided into 256 32-bit subblocks, which are permuted at the beginning.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.