Tampered message, plain encryption, 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.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 = "input1234567";
    cipher.init(Cipher.ENCRYPT_MODE, key, ivSpec);

    byte[] cipherText = cipher.doFinal(input.getBytes());

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

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

    byte[] plainText = cipher.doFinal(cipherText);

    System.out.println("plain : " + new String(plainText));
  }

  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);
  }

}