code: private String generateHmac() throws HawkException {
  
  		String baseString = getBaseString();
  
  		Mac mac;
  		try {
  			mac = Mac.getInstance(getAlgorithm().getMacName());
  		} catch (NoSuchAlgorithmException e) {
  			throw new HawkException("Unknown algorithm "
  					+ getAlgorithm().getMacName(), e);
  		}
  
  		SecretKeySpec secret_key = new SecretKeySpec(getKey().getBytes(
  				Charsets.UTF_8), getAlgorithm().getMacName());
  		try {
  			mac.init(secret_key);
  		} catch (InvalidKeyException e) {
  			throw new HawkException("Key is invalid ", e);
  		}
  
  		return new String(Base64.encodeBase64(mac.doFinal(baseString
  				.getBytes(Charsets.UTF_8))), Charsets.UTF_8);
  	}
docs: Generate an HMAC from the context ts parameter.
  
  @return
  @throws HawkException