Answer for https://stackoverflow.com/questions/43293902/cryptography-universal-forgery-attack-to-compute-mac-without-knowing-key

MacForging.java

private int BlockSize = 8;

private byte[] xor(byte[] in1, byte[] in2) {
  byte[] out = new byte[in1.length];
  for(int i = 0; i < in1.length; i++) {
    out[i] = in1[i] ^ in2[i];
  }
  return out;
}

public byte[] crack(final byte[] msg) {
  byte[] mac = new byte[BlockSize];
  byte[] previousMac = new byte[BlockSize];
  final byte[] mac0_1 = oracle.mac0(1);
  final byte[] mac0_2 = oracle.mac0(2);
  final byte[] o2For0 = xor(mac0_1, mac0_2);
  
  for(int i = 0; i * BlockSize < msg.length; i++) {
    byte[] block = Arrays.copyOfRange(msg, i * BlockSize, (i+1) * BlockSize);
    block = xor(block, o2For0);
    block = xor(block, previousMac);
    
    final byte[] blocksForMac3 = new byte[3 * BlockSize];
    System.arraycopy(block, 0, blocksForMac3, 2 * BlockSize, BlockSize);
    
    byte[] macForBlocks = oracle.mac3(blocksForMac3);
    macForBlocks = xor(macForBlocks, mac0_2);
    previousMac = macForBlocks;
    mac = xor(mac, macForBlocks);
  }
  
  return mac;
}