Msb returns the most significant bit of the supplied data (which must be non-empty).

PadBlock pads a string of bytes less than 16 bytes long to a full block size by appending a one bit followed by zero bits.

ShiftLeft shifts the binary string left by one bit, causing the most-signficant bit to disappear and a zero to be introduced at the right.

Xor computes `a XOR b`, as defined by RFC 4493.