http://rzuwtppveur6yh3n2in7trmddgrkuhww4eb2znykgjwubihfpljiyuid.onion
Each directory authority additionally has a "directory signing key".
The directory authorities provide a signed list of all the known relays, and in that list are a set of certificates from each relay (self-signed by their identity key) specifying their keys, locations, exit policies, and so on.