code: public KeyIdentifier getAuthKeyId() throws IOException {
          AuthorityKeyIdentifierExtension aki = getAuthKeyIdExtension();
          if (aki != null) {
              KeyIdentifier keyId = (KeyIdentifier)aki.get(
                      AuthorityKeyIdentifierExtension.KEY_ID);
              return keyId;
          } else {
              return null;
          }
      }
docs: return the AuthorityKeyIdentifier, if any.
  
  @returns AuthorityKeyIdentifier or null
  (if no AuthorityKeyIdentifierExtension)
  @throws IOException on error