RFC 2144 CAST-128 Encryption Algorithms May 1997 2.2. Non-Identical Rounds Three different round functions are used in CAST-128. The rounds are as follows (where "D" is the data input to the f function and "Ia" - "Id" are the most significant byte through least significant byte of I, respectively). Note that "+" and "-" are addition and subtraction modulo 2**32, "^" is bitwise XOR, and "