I observed some inconsistency, if not errors, in the directory server specs [1]:

1. Outline

   Every authority has a very-secret, long-term "Authority Identity Key".

   This is stored encrypted and/or offline, and is used to sign "key

   certificate" documents.  Every key certificate contains a medium-term

   (3-12 months) "authority signing key", that is used by the authority to

   sign other directory information.

   

2.1. Router descriptor format

   "fingerprint" fingerprint NL

       [At most once]

       A fingerprint (a HASH_LEN-byte of asn1 encoded public key, encoded in

       hex, with a single space after every 4 characters) for this router's

       identity key. A descriptor is considered invalid (and MUST be

       rejected) if the fingerprint line does not match the public key.

  

    "signing-key" NL a public key in PEM format

       [Exactly once]

       The OR's long-term identity key.  It MUST be 1024 bits.

According to the outline, the long-term identity key should be different from the 

signing key, which changes every 3-12 months. Then why should the signing key

become the identity key in the descriptor format, and fingerprint become the hash

of the identity key?

Cheers,

Beck

[1] https://gitweb.torproject.org/torspec.git?a=blob_plain;hb=HEAD;f=dir-spec.txt