code: function calculateAuthKeyValue(context) {
      var ga = context.serverDHInnerData.g_a;
      var b = context.b;
      var dhPrime = context.serverDHInnerData.dh_prime;
      var authKeyValue = utility.modPow(ga, b, dhPrime);
      if (logger.isDebugEnabled()) {
          logger.debug('authKeyValue(%s) = %s', authKeyValue.length, authKeyValue.toString('hex'));
      }
      context.authKeyValue = authKeyValue;
      return context;
  }
docs: Calculate the authentication key value