Arbitrary precision integers MUST be encoded as big-endian signed two's
complement binary strings. Henceforth, "btwoc" is a function that takes
an arbitrary precision integer and returns its shortest big-endian two's
complement representation. All integers that are used with
Diffie-Hellman Key Exchange are positive. This means that the left-most
bit of the two's complement representation MUST be zero. If it is not,
implementations MUST add a zero byte at the front of the string.