docs: private int hmacSignature(byte[] workBytes) {
      try {
        Mac integrityHmac = createMac();
        integrityHmac.init(keys.getIntegrityKey());
        integrityHmac.update(workBytes, PAYLOAD_BASE, workBytes.length - OVERHEAD_SIZE);
        integrityHmac.update(workBytes, INITV_BASE, INITV_SIZE);
        return Ints.fromByteArray(integrityHmac.doFinal());
      } catch (InvalidKeyException e) {
        throw new IllegalStateException(e);
      }
    }