BITXOR(string1[,[string2][,pad]])

This function performs a bitwise-XOR on its two string arguments.  If
the string2 argument is omitted, an empty string is used for string2
instead.  Before the operation, the shorter of the two string arguments
is padded with the pad character.  If no pad character is given, then
'00'x is used, so that excess characters in the longer string remain
unchanged after the operation.  The result of the operation is the
string of characters such that the nth character in the string is the
bitwise-XOR of the nth character of string1 and the nth character of
string2.