code: public static String generateKey(final MultifactorAuthenticationTrustRecord r) {
          val builder = new StringBuilder(r.getPrincipal());
          return builder.append('@')
              .append(r.getRecordDate())
              .append('@')
              .append(r.getDeviceFingerprint())
              .toString();
      }
docs: Generate key.
  
  @param r the r
  @return the key for this trust record