"""Device pairing and derivation of encryption keys."""

import logging
import binascii
from collections import namedtuple

from pyatv import const, exceptions, net
from pyatv.interface import PairingHandler
from pyatv.airplay.srp import (SRPAuthHandler, new_credentials)
from pyatv.airplay.auth import DeviceAuthenticator

_LOGGER = logging.getLogger(__name__)

AuthData = namedtuple('AuthData', 'identifier seed credentials')


class AirPlayPairingHandler(PairingHandler):
    """Base class for API used to pair with an Apple TV."""

    def __init__(self, config, session, _):
        """Initialize a new MrpPairingHandler."""
        super().__init__(session, config.get_service(const.PROTOCOL_AIRPLAY))
        self.srp = SRPAuthHandler()
        self.http = net.HttpSession(
            session,
            'http://{0}:{1}/'.format(config.address, self.service.port))
        self.auther = DeviceAuthenticator(self.http, self.srp)
        self.auth_data = self._setup_credentials()
        self.srp.initialize(binascii.unhexlify(self.auth_data.seed))
        self.pairing_complete = False
        self.pin_code = None

    def _setup_credentials(self):
        credentials = self.service.credentials

        # If service has credentials, use those. Otherwise generate new.
        if self.service.credentials is None:
            identifier, seed = new_credentials()
            credentials = '{0}:{1}'.format(identifier, seed.decode().upper())
        else:
            split = credentials.split(':')
            identifier = split[0]
            seed = split[1]
        return AuthData(identifier, seed, credentials)

    @property
    def has_paired(self):
        """If a successful pairing has been performed."""
        return self.pairing_complete

    async def begin(self):
        """Start pairing process."""
        _LOGGER.debug('Starting AirPlay pairing with credentials %s',
                      self.auth_data.credentials)
        self.pairing_complete = False
        return await self.auther.start_authentication()

    async def finish(self):
        """Stop pairing process."""
        if not self.pin_code:
            raise exceptions.DeviceAuthenticationError('no pin given')

        if await self.auther.finish_authentication(self.auth_data.identifier,
                                                   self.pin_code):
            self.service.credentials = self.auth_data.credentials
            self.pairing_complete = True

    def pin(self, pin):
        """Pin code used for pairing."""
        self.pin_code = pin

    @property
    def device_provides_pin(self):
        """Return True if remote device presents PIN code, else False."""
        return True

@@ -2544,19 +2544,33 @@
 _code = 
-pin
+str(pin).zfill(4)
 %0A%0A    @p

Support leading zeros for PINs in AirPlay