code: public static String computeEtag(long lastModification, ApplicationConfiguration configuration, Crypto crypto) {
          boolean useEtag = configuration.getBooleanWithDefault(HTTP_USE_ETAG,
                  HTTP_USE_ETAG_DEFAULT);
          if (!useEtag) {
              return null;
          }
          String raw = Long.toString(lastModification);
          return crypto.hexSHA1(raw);
      }
docs: Computes the ETAG value based on the last modification date passed as parameter.
  
  @param lastModification the last modification (must be valid)
  @param configuration    the configuration
  @param crypto           the crypto service
  @return the encoded etag