GfnDouble computes 2 * input in the field of 2^n elements.

RightXor XORs smaller input (assumed Y) at the right of the larger input (assumed X).

ShiftBytesLeft outputs the byte array corresponding to x << 1 in binary.

ShiftNBytesLeft puts in dst the byte array corresponding to x << n in binary.

SliceForAppend takes a slice and a requested number of bytes.

XorBytes assumes equal input length, puts X XOR Y into Z.

XorBytesMut assumes equal input length, replaces X with X XOR Y.