*     neither fast nor secure. As of January 2017, we suggest:
   *     <ul>
   *       <li>For security:
   *           {@link Hashing#sha256} or a higher-level API.
   *       <li>For speed: {@link Hashing#goodFastHash}, though see its docs for caveats.
   *     </ul>
   */
  @Deprecated
  public static HashFunction md5() {
    return Md5Holder.MD5;
  }

  private static class Md5Holder {