Perhaps my notation was a bit too terse. GF2(8) is a Galois field of order Pow(2, 8). The xor operation is addition in a Galois field, so 8-bit xor with 'M' i

Perhaps my notation was a bit too terse. GF2(8) is a Galois field of order Pow(2, 8). The xor operation is addition in a Galois field, so 8-bit xor with 'M' is the same thing as a constant additive shift by 'M' in a Galois field of order Pow(2, 8). So, it's a Caesar shift cipher using the GF2(8) definition of byte addition rather than the standard modulo 256 definition.