static byte[] addChecksum(byte[] input) {
   int inputLength = input.length;
   byte[] checksummed = new byte[inputLength + 4];
   System.arraycopy(input, 0, checksummed, 0, inputLength);
   byte[] checksum = Sha256Hash.hashTwice(input);
   System.arraycopy(checksum, 0, checksummed, inputLength, 4);
   return checksummed;
 }
 /**
  * Returns private key bytes, padded with zeros to 33 bytes.
  *
  * @throws java.lang.IllegalStateException if the private key bytes are missing.
  */
 public byte[] getPrivKeyBytes33() {
   byte[] bytes33 = new byte[33];
   byte[] priv = getPrivKeyBytes();
   System.arraycopy(priv, 0, bytes33, 33 - priv.length, priv.length);
   return bytes33;
 }