This Google Apps Script function returns a string representing the 16-byte MD5 digest of a given message. It was originally written as an answer to StackOverflow question http://stackoverflow.com/questions/16216868/get-back-a-string-representation-from-computedigestalgorithm-value-byte. It's been refactored to support adaptation to other digest …

digest.js

/**
 * Return string representation of MD5 digest of the given message.
 * 
 * @param {String} message Message to be encoded.
 * 
 * @return {String} 16-byte digest value
 */
function signMd5(message){
  return digest(Utilities.DigestAlgorithm.MD5, message);
}

/**
 * Return string representation of SHA_256 digest of the given message.
 * 
 * @param {String} message Message to be encoded.
 * 
 * @return {String} 16-byte digest value
 */
function signSHA_256(message){
  return digest(Utilities.DigestAlgorithm.SHA_256, message);
}

/**
 * Return string representation of digest of the given string,
 * using the indicated digest algorithm.
 * 
 * @see {link https://developers.google.com/apps-script/reference/utilities/digest-algorithm|
 *       Enum DigestAlgorithm}
 * 
 * @param {DigestAlgorithm} 
 * @param {String} message Message to be encoded.
 * 
 * @return {String} 16-byte digest value
 */
function digest(algorithm,aStr) {
  algorithm = algorithm || Utilities.DigestAlgorithm.MD5; // default MD5
  aStr = aStr || "";  // default to empty string
  var signature = Utilities.computeDigest(algorithm, aStr,
                                      Utilities.Charset.US_ASCII)
  //Logger.log(signature);
  var signatureStr = '';
    for (i = 0; i < signature.length; i++) {
      var byte = signature[i];
      if (byte < 0)
        byte += 256;
      var byteStr = byte.toString(16);
      // Ensure we have 2 chars in our byte, pad with 0
      if (byteStr.length == 1) byteStr = '0'+byteStr;
      signatureStr += byteStr;
    }   
  //Logger.log(signatureStr);
  return signatureStr;
}

Note that Google Apps Script also supports Utilities.Charset.UTF_8 as a Charset representing the input character set. Any benefit of not using it as a default nowadays?