Tampered message, encryption with digest, 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.SecretKey;
import javax.crypto.spec.IvParameterSpec;

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(1, random);
    Key key = createKeyForAES(256, random);
    Cipher cipher = Cipher.getInstance("AES/CTR/NoPadding", "BC");
    String input = "12345678";
    MessageDigest hash = MessageDigest.getInstance("SHA-1", "BC");

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

    byte[] cipherText = new byte[cipher.getOutputSize(input.length() + hash.getDigestLength())];

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

    hash.update(input.getBytes());

    ctLength += cipher.doFinal(hash.digest(), 0, hash.getDigestLength(), cipherText, ctLength);

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

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

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

    hash.update(plainText, 0, messageLength);

    byte[] messageHash = new byte[hash.getDigestLength()];
    System.arraycopy(plainText, messageLength, messageHash, 0, messageHash.length);

    System.out.println("plain : " + new String(plainText) + " verified: "
        + MessageDigest.isEqual(hash.digest(), 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(int messageNumber, SecureRandom random) {
    byte[] ivBytes = new byte[16];
    random.nextBytes(ivBytes);
    ivBytes[0] = 1;
    ivBytes[1] = 2;
    ivBytes[2] = 3;
    ivBytes[3] = 4;

    for (int i = 0; i != 7; i++) {
      ivBytes[8 + i] = 0;
    }
    ivBytes[15] = 1;
    return new IvParameterSpec(ivBytes);
  }
}