# Stem Docs

## Checking Descriptor Digests

Tor relay information is provided by multiple documents. Signed descriptors transitively validate others by inclusion of their digest. For example, our consensus references server descriptor digest, and server descriptors in turn cite extrainfo digests.

To illustrate, here’s a diagram from Iain…

Stem can calculate digests from server, extrainfo, microdescriptor, and consensus documents. For instance, to validate an extrainfo descriptor…

    import sys

    import stem.descriptor.remote
    import stem.util.tor_tools

    def download_descriptors(fingerprint):
      """
      Downloads the descriptors we need to validate this relay. Downloads are
      parallelized, providing the caller with a tuple of the form...

        (router_status_entry, server_descriptor, extrainfo_descriptor)
      """

      conensus_query = stem.descriptor.remote.get_consensus()
      server_desc_query = stem.descriptor.remote.get_server_descriptors(fingerprint)
      extrainfo_query = stem.descriptor.remote.get_extrainfo_descriptors(fingerprint)

      router_status_entries = filter(lambda desc: desc.fingerprint == fingerprint, conensus_query.run())

      if len(router_status_entries) != 1:
        raise IOError("Unable to find relay '%s' in the consensus" % fingerprint)

      return (
        router_status_entries[0],
        server_desc_query.run()[0],
        extrainfo_query.run()[0],
      )

    if __name__ == '__main__':
      fingerprint = raw_input("What relay fingerprint would you like to validate?\n")
      print('')  # blank line

      if not stem.util.tor_tools.is_valid_fingerprint(fingerprint):
        print("'%s' is not a valid relay fingerprint" % fingerprint)
        sys.exit(1)

      try:
        router_status_entry, server_desc, extrainfo_desc = download_descriptors(fingerprint)
      except Exception as exc:
        print(exc)
        sys.exit(1)

      if router_status_entry.digest == server_desc.digest():
        print("Server descriptor digest is correct")
      else:
        print("Server descriptor digest invalid, expected %s but is %s" % (router_status_entry.digest, server_desc.digest()))

      if server_desc.extra_info_digest == extrainfo_desc.digest():
        print("Extrainfo descriptor digest is correct")
      else:
        print("Extrainfo descriptor digest invalid, expected %s but is %s" % (server_desc.extra_info_digest, extrainfo_desc.digest()))

    % python check_digests.py
    What relay fingerprint would you like to validate?
    3BB34C63072D9D10E836EE42968713F7B9325F66

    Server descriptor digest is correct
    Extrainfo descriptor digest is correct