code: def allow_identity(self, fingerprint):
          
          if self.site_control == SITE_CONTROL_NONE:
              raise TypeError(
                  METAPOLICY_ERROR.format("allow access from signed documents")
              )
          if fingerprint not in self.identities:
              self.identities.append(fingerprint)
docs: Allows access from documents digitally signed by the key with
          ``fingerprint``.
  
          In theory, multiple algorithms can be added in the future for
          calculating ``fingerprint`` from the signing key, but at this
          time only one algorithm -- SHA-1 -- is supported by the
          cross-domain policy specification.