add the pad bytes to the passed in block, returning the
number of bytes added.
Note: this assumes that the last block of plain text is always
passed to it inside in. i.e. if inOff is zero, indicating the
entire block is to be overwritten with padding the value of in
should be the same as the last block of plain text. The reason
for this is that some modes such as "trailing bit compliment"
base the padding on the last byte of plain text.
