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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _challenge_response(self, challenge, mode, slot, variable, may_block):
 """ Do challenge-response with a YubiKey > 2.0. """

         # Check length and pad challenge if appropriate
        if mode == 'HMAC':
            if len(challenge) > yubikey_defs.SHA1_MAX_BLOCK_SIZE:
                raise yubico_exception.InputError('Mode HMAC challenge too big (%i/%i)' \
                                                      % (yubikey_defs.SHA1_MAX_BLOCK_SIZE, len(challenge)))
            if len(challenge) < yubikey_defs.SHA1_MAX_BLOCK_SIZE:
                pad_with = b'\0'
                if variable and challenge[-1:] == pad_with:
                    pad_with = b'\xff'
                challenge = challenge.ljust(yubikey_defs.SHA1_MAX_BLOCK_SIZE, pad_with)
            response_len = yubikey_defs.SHA1_DIGEST_SIZE
        elif mode == 'OTP':
            if len(challenge) != yubikey_defs.UID_SIZE:
                raise yubico_exception.InputError('Mode OTP challenge must be %i bytes (got %i)' \
                                                      % (yubikey_defs.UID_SIZE, len(challenge)))
            challenge = challenge.ljust(yubikey_defs.SHA1_MAX_BLOCK_SIZE, b'\0')
            response_len = 16
        else:
            raise yubico_exception.InputError('Invalid mode supplied (%s, valid values are HMAC and OTP)' \
                                                  % (mode))

        try:
            command = _CMD_CHALLENGE[mode][slot]
        except:
            raise yubico_exception.InputError('Invalid slot specified (%s)' % (slot))

        frame = yubikey_frame.YubiKeyFrame(command=command, payload=challenge)
        self._device._write(frame)
        response = self._device._read_response(may_block=may_block)
        if not yubico_util.validate_crc16(response[:response_len + 2]):
            raise YubiKeyUSBHIDError("Read from device failed CRC check")
        return response[:response_len]