Xor.java

import java.nio.charset.StandardCharsets;

public class Xor {

  // This is the cipher text, encoded as a hex string. If you translate every two
  // characters of this string into a byte you will have an array of bytes which
  // can be decoded by xor'ing the bytes with bits from the key, taking first the 8
  // least significant bits of the key for the 0th byte, the the next 8, and so on,
  // looping back around to the least significant bits every four bytes.
  private static final String CIPHERTEXT =
    "452dbb017333a6456328a64f6064a6522722ba4f26";

  // This is the key. It is just a random int, so 32 random bits. It was used to
  // encode my secret message converted to bytes with msg.getBytes("UTF-8")
  private static final int KEY = 567231495;

  public static void main(String[] argv) throws Exception {
    System.out.println(new Xor().decode(CIPHERTEXT, KEY));
  }

  /*
   * Convert a string representing a hex-encoded array of bytes to an actual
   * array of bytes.
   */
  public byte[] fromHexString(String s) {
    var bytes = new byte[s.length() / 2];
    for (var i = 0; i < bytes.length; i++) {
      bytes[i] = (byte) Integer.parseInt(s.substring(i * 2, (i + 1) * 2), 16);
    }
    return bytes;
  }

  /*
   * Encrypt/decrypt plaintext bytes using an int key. (It's symmetric.)
   */
  public byte[] xor(byte[] plaintext, int key) {
    var ciphertext = new byte[plaintext.length];
    for (var i = 0; i < plaintext.length; i++) {
      ciphertext[i] = (byte) (plaintext[i] ^ (key >>> (8 * (i % 4))));
    }
    return ciphertext;
  }

  /*
   * Decode hex string and then decrypt the resulting bytes and make into a String.
   */
  public String decode(String encoded, int key) {
    return new String(xor(fromHexString(encoded), key), StandardCharsets.UTF_8);
  }
}