@Test
public void testSign() {

    String expectedSig = "587d4b472eeef7d07aafcd0b049640b0bb3f39784118c2e2b73a04fa2f64c9c538b4b2d0f5335e968a480021fdc23e98c0ddf424cb15d8131df8cb6c4bb58309";
    KeyPair keypair = KeyPair.fromSecretSeed(Util.hexToBytes(SEED));
    String data = "hello world";
    byte[] sig = keypair.sign(data.getBytes());
    Assert.assertArrayEquals(Util.hexToBytes(expectedSig), sig);
}