@Test
public void sValue() throws Exception {

    // Check that we never generate an S value that is larger than half the curve order. This avoids a malleability
    // issue that can allow someone to change a transaction [hash] without invalidating the signature.
    final int ITERATIONS = 10;
    ListeningExecutorService executor = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(ITERATIONS));
    List<ListenableFuture<ECKey.ECDSASignature>> sigFutures = Lists.newArrayList();
    final ECKey key = new ECKey();
    for (byte i = 0; i < ITERATIONS; i++) {
        final Sha256Hash hash = Sha256Hash.create(new byte[]{i});
        sigFutures.add(executor.submit(new Callable<ECKey.ECDSASignature>() {
            @Override
            public ECKey.ECDSASignature call() throws Exception {
                return key.sign(hash);
            }
        }));
    }
    List<ECKey.ECDSASignature> sigs = Futures.allAsList(sigFutures).get();
    for (ECKey.ECDSASignature signature : sigs) {
        assertTrue(signature.s.compareTo(ECKey.HALF_CURVE_ORDER) <= 0);
    }
}