OMAC is a generic name for OMAC1 and OMAC2. OMAC1 is described first. OMAC1 is equivalnent to CMAC. Before using OMAC1, you have to choose a blockcipher E and a tag length t . E may be the AES, Camellia, Triple-DES, or whatever blockcipher you like. Suppose that the key length of E is k bits and the block length is n bits. The tag length t may be any integer between 1 and n , depending on your app