def A1(self, algorithm, auth_pass):
        """
         If 'algorithm' is "MD5" or unset, A1 is:
         A1 = unq(username-value) ":" unq(realm-value) ":" passwd

         if 'algorithm' is 'MD5-Sess', A1 is:
         A1 = H( unq(username-value) ":" unq(realm-value) ":" passwd )
          ":" unq(nonce-value) ":" unq(cnonce-value)

        """

        username = self.params["username"]
        if algorithm == 'MD5' or not algorithm:
            return "%s:%s:%s" % (username, self.realm, auth_pass)
        elif algorithm == 'MD5-Sess':
            return self.apply_checksum('%s:%s:%s:%s:%s' % \
                                       (username,
                                       self.realm,
                                       auth_pass,
                                       self.params['nonce'],
                                       self.params['cnonce']))