title: 图解密码技术-对CFB模式的攻击
tags: '密码技术,读书笔记'
abbrlink: 3573874495
date: 2023-08-14 22:40:07

CFB模式简介

密文反馈模式

CFB模式的解密

文字步骤如下：

1. 解密端获得密文块C1、C2、C3...以及一个初始向量IV。

2. 将IV进行加密,获得第一个密钥块K1。(使用的密钥和加密算法与加密端相同)

3. 将密钥块K1与密文块C1进行异或,得到明文块P1。

4. 将刚刚解密得到的明文块P1再次进行加密,生成下一个密钥块K2。

5. 将K2与下一个密文块C2进行异或,得到明文块P2。

6. 依此类推,重复步骤4和5,直到所有密文全部解密。

图示：

cfb 解密

CFB模式的攻击

Java代码模拟重放攻击

package com.yt.mvcframework.v2.CFBDemo;
​
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import java.security.Key;
import java.security.SecureRandom;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
​
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import java.security.SecureRandom;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
​
public class ReplayAttackExample {
​
    private static class Server {
        private Cipher cipher;
        private SecretKey key;
        private Map<Long, byte[]> seenMessages = new HashMap<>();
​
        public Server(SecretKey key, IvParameterSpec iv) throws Exception {
            this.key = key;
            this.cipher = Cipher.getInstance("AES/CFB/NoPadding");
            cipher.init(Cipher.DECRYPT_MODE, key, iv);
        }
​
        public void receiveMessage(long timestamp, byte[] ciphertext) throws Exception {
            if (seenMessages.containsKey(timestamp) &&
                    Arrays.equals(seenMessages.get(timestamp), ciphertext)) {
                System.out.println("Replay attack detected!");
                return;
            }
​
            byte[] plaintext = cipher.doFinal(ciphertext);
            System.out.println("Received message: " + new String(plaintext));
​
            seenMessages.put(timestamp, ciphertext);
            cipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(cipher.getParameters().getParameterSpec(IvParameterSpec.class).getIV()));
        }
    }
​
    private static class Client {
        private Cipher cipher;
        private SecretKey key;
​
        public Client(SecretKey key, IvParameterSpec iv) throws Exception {
            this.key = key;
            this.cipher = Cipher.getInstance("AES/CFB/NoPadding");
            cipher.init(Cipher.ENCRYPT_MODE, key, iv);
        }
​
        public byte[] sendMessage(String message) throws Exception {
            byte[] ciphertext = cipher.doFinal(message.getBytes());
            cipher.init(Cipher.ENCRYPT_MODE, key, new IvParameterSpec(cipher.getParameters().getParameterSpec(IvParameterSpec.class).getIV()));
            return ciphertext;
        }
    }
​
    public static void main(String[] args) throws Exception {
        KeyGenerator kg = KeyGenerator.getInstance("AES");
        SecretKey key = kg.generateKey();
​
        SecureRandom random = new SecureRandom();
        IvParameterSpec iv = new IvParameterSpec(random.generateSeed(16));
​
        Server server = new Server(key, iv);
        Client client = new Client(key, iv);
​
        long timestamp = System.currentTimeMillis();
        byte[] ciphertext = client.sendMessage("Hello world");
​
        server.receiveMessage(timestamp, ciphertext);
        server.receiveMessage(timestamp, ciphertext);  // Attempt replay attack
    }
}
​
​

碰撞攻击

Java代码模拟碰撞攻击

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
​
public class CFBAttack {
​
  public static void main(String[] args) throws Exception {
​
    // 生成密钥
    KeyGenerator kg = KeyGenerator.getInstance("AES");
    SecretKey key = kg.generateKey();
​
    // 初始化IV
    IvParameterSpec iv = new IvParameterSpec(new byte[16]);
​
    // 设置加密模式为CFB,并获得Cipher实例
    Cipher cipher = Cipher.getInstance("AES/CFB/NoPadding");
    cipher.init(Cipher.ENCRYPT_MODE, key, iv);
​
    // 构造明文block1和block2完全相同
    byte[] block1 = "HelloWorld".getBytes();
    byte[] block2 = "HelloWorld".getBytes();
    
    // 加密两块明文
    byte[] cipherBlock1 = cipher.doFinal(block1);
    byte[] cipherBlock2 = cipher.doFinal(block2);
​
    // 两个密文块由于明文相同也完全相同
    // 这可被攻击者利用来获取明文信息
    if (java.util.Arrays.equals(cipherBlock1, cipherBlock2)) {
      System.out.println("Collision attack succeeded!");
    }
  }
}