code: private byte[] hmac(byte[] key, byte[] str) throws SmackSaslException {
          try {
              return scramHmac.hmac(key, str);
          }
          catch (InvalidKeyException e) {
              throw new SmackSaslException(getName() + " Exception", e);
          }
      }
docs: RFC 5802 § 2.2 HMAC(key, str)
  
  @param key
  @param str
  @return the HMAC-SHA1 value of the input.
  @throws SmackException