code: def unlock(ctx, pin):
      
  
      controller = ctx.obj['controller']
      if not controller.is_fips:
          ctx.fail('This is not a YubiKey FIPS, and therefore'
                   ' does not support a U2F PIN.')
  
      if pin is None:
          pin = _prompt_current_pin('Enter your PIN')
  
      _fail_if_not_valid_pin(ctx, pin, True)
      try:
          controller.verify_pin(pin)
      except ApduError as e:
          if e.code == SW.VERIFY_FAIL_NO_RETRY:
              ctx.fail('Wrong PIN.')
          if e.code == SW.AUTH_METHOD_BLOCKED:
              ctx.fail('PIN is blocked.')
          if e.code == SW.COMMAND_NOT_ALLOWED:
              ctx.fail('PIN is not set.')
  
          logger.error('PIN verification failed', exc_info=e)
          ctx.fail('PIN verification failed.')
docs: Verify U2F PIN for YubiKey FIPS.
  
      Unlock the YubiKey FIPS and allow U2F registration.