everseeker,
thank you for pointing to the crypt32.dll. That was exactly what I am looking for. Your code is running without errors, when you replace the line
DllStructSetData($tInput, 1, $input_string & 0)
within the function _Base64Decode_MS($input_string) by
DllStructSetData($tInput, 1, $input_string & chr(0))
i.e. $input_string has to be terminated by a null-byte, not a string "0".