Tampered message with HMac, encryption with AES in CTR mode : Advanced Encryption Standard « Security « Java Tutorial

1. Java Tutorial
2. Security
3. Advanced Encryption Standard

import java.security.Key;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.SecureRandom;
import java.security.Security;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class MainClass {
  public static void main(String[] args) throws Exception {
    Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());

    SecureRandom random = new SecureRandom();
    IvParameterSpec ivSpec = createCtrIvForAES();
    Key key = createKeyForAES(256, random);
    Cipher cipher = Cipher.getInstance("AES/CTR/NoPadding", "BC");
    String input = "12345678";

    Mac hMac = Mac.getInstance("HmacSHA1", "BC");
    Key hMacKey = new SecretKeySpec(key.getEncoded(), "HmacSHA1");

    System.out.println("input : " + input);

    cipher.init(Cipher.ENCRYPT_MODE, key, ivSpec);

    byte[] cipherText = new byte[cipher.getOutputSize(input.length() + hMac.getMacLength())];

    int ctLength = cipher.update(input.getBytes(), 0, input.length(), cipherText, 0);

    hMac.init(hMacKey);
    hMac.update(input.getBytes());

    ctLength += cipher.doFinal(hMac.doFinal(), 0, hMac.getMacLength(), cipherText, ctLength);

    cipherText[9] ^= '0' ^ '9';

    cipher.init(Cipher.DECRYPT_MODE, key, ivSpec);

    byte[] plainText = cipher.doFinal(cipherText, 0, ctLength);
    int messageLength = plainText.length - hMac.getMacLength();

    hMac.init(hMacKey);
    hMac.update(plainText, 0, messageLength);

    byte[] messageHash = new byte[hMac.getMacLength()];
    System.arraycopy(plainText, messageLength, messageHash, 0, messageHash.length);
    System.out.println("plain : " + new String(plainText) + " verified: "
        + MessageDigest.isEqual(hMac.doFinal(), messageHash));
  }

  public static SecretKey createKeyForAES(int bitLength, SecureRandom random)
      throws NoSuchAlgorithmException, NoSuchProviderException {
    KeyGenerator generator = KeyGenerator.getInstance("AES", "BC");

    generator.init(128, random);

    return generator.generateKey();
  }

  public static IvParameterSpec createCtrIvForAES() {
    return new IvParameterSpec("1234567812345678".getBytes());
  }

}