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DESede算法详解

一、概述

3DES,即3重DES(Triple DES),也称为DESede,是一种对称密钥加密算法。DESede是通过对每个数据块应用三次DES加密算法来实现加密。由于DESede采用了多次加密的方法,因此算法更为安全,在现今网络通信安全中广泛应用。

二、原理

DESede的加密过程主要分为三个步骤:密钥的生成、加密和解密。

1. 密钥的生成

DESede算法使用长度为24个字节的密钥,其中包含3个单独的8字节密钥K1, K2, K3。密钥生成的过程如下:

private static byte[][] generateKey(byte[] key) throws Exception {
    byte[][] keys = new byte[3][8];
    byte[] keyBytes = paddingKey(key);
    DESedeKeySpec desKeySpec = new DESedeKeySpec(keyBytes);
    SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DESede");
    SecretKey secretKey = keyFactory.generateSecret(desKeySpec);
    byte[] secretKeyBytes = secretKey.getEncoded();
    System.arraycopy(secretKeyBytes, 0, keys[0], 0, 8);
    System.arraycopy(secretKeyBytes, 8, keys[1], 0, 8);
    System.arraycopy(secretKeyBytes, 16, keys[2], 0, 8);
    return keys;
}

2. 加密

加密是指将明文经过一定的方法转换为密文,使得第三方无法直接读取到明文。在DESede算法中,加密分为两个步骤:DES加密和反向DES加密。

(1)DES加密

首先将明文输入到第一轮DES加密中,使用密钥K1加密。得到的结果作为第二轮DES加密的明文,使用密钥K2加密。得到的结果作为第三轮DES加密的明文,使用密钥K3加密。加密过程如下:

private static byte[] encrypt(byte[] data, byte[] key) throws Exception {
    byte[][] keys = generateKey(key);
    Cipher cipher = Cipher.getInstance("DESede");
    cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keys[0], "DESede"));
    byte[] encrypted = cipher.doFinal(data);
    cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(keys[1], "DESede"));
    byte[] decrypted = cipher.doFinal(encrypted);
    cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keys[2], "DESede"));
    return cipher.doFinal(decrypted);
}

(2)反向DES加密

DESede算法采用了反向DES加密方式,即解密与加密使用的密钥相反。这样做的目的是增强算法的安全性。

3. 解密

解密是指将密文经过一定的方法还原为明文。DESede的解密过程与加密过程刚好相反,主要由反向DES加密和DES解密两部分组成。具体可参考DESede的加密过程。

三、应用

DESede算法的应用非常广泛,可以用于文件加密、数据传输加密以及数字签名等场景。在Java中,DESede算法可以通过Java Cryptography Extension (JCE) 来实现。以下是一个简单的DESede加解密示例:

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.DESedeKeySpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.Base64;

public class DESedeDemo {

    private static final String KEY = "my_key";

    public static void main(String[] args) throws Exception {
        String data = "hello world";
        String encrypted = encrypt(data, KEY);
        System.out.println(encrypted);
        String decrypted = decrypt(encrypted, KEY);
        System.out.println(decrypted);
    }

    private static byte[][] generateKey(byte[] key) throws Exception {
        byte[][] keys = new byte[3][8];
        byte[] keyBytes = paddingKey(key);
        DESedeKeySpec desKeySpec = new DESedeKeySpec(keyBytes);
        SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DESede");
        SecretKey secretKey = keyFactory.generateSecret(desKeySpec);
        byte[] secretKeyBytes = secretKey.getEncoded();
        System.arraycopy(secretKeyBytes, 0, keys[0], 0, 8);
        System.arraycopy(secretKeyBytes, 8, keys[1], 0, 8);
        System.arraycopy(secretKeyBytes, 16, keys[2], 0, 8);
        return keys;
    }

    private static byte[] paddingKey(byte[] key) throws Exception {
        MessageDigest md5 = MessageDigest.getInstance("MD5");
        byte[] md5Bytes = md5.digest(key);
        byte[] keyBytes = new byte[24];
        if (md5Bytes.length < 24) {
            System.arraycopy(md5Bytes, 0, keyBytes, 0, md5Bytes.length);
        } else {
            System.arraycopy(md5Bytes, 0, keyBytes, 0, 24);
        }
        return keyBytes;
    }

    private static String encrypt(String data, String key) throws Exception {
        byte[] dataBytes = data.getBytes(StandardCharsets.UTF_8);
        byte[] keyBytes = key.getBytes(StandardCharsets.UTF_8);
        byte[] encryptedBytes = encrypt(dataBytes, keyBytes);
        return Base64.getEncoder().encodeToString(encryptedBytes);
    }

    private static byte[] encrypt(byte[] data, byte[] key) throws Exception {
        byte[][] keys = generateKey(key);
        Cipher cipher = Cipher.getInstance("DESede");
        cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keys[0], "DESede"));
        byte[] encrypted = cipher.doFinal(data);
        cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(keys[1], "DESede"));
        byte[] decrypted = cipher.doFinal(encrypted);
        cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keys[2], "DESede"));
        return cipher.doFinal(decrypted);
    }

    private static String decrypt(String encrypted, String key) throws Exception {
        byte[] encryptedBytes = Base64.getDecoder().decode(encrypted);
        byte[] keyBytes = key.getBytes(StandardCharsets.UTF_8);
        byte[] decryptedBytes = decrypt(encryptedBytes, keyBytes);
        return new String(decryptedBytes, StandardCharsets.UTF_8);
    }

    private static byte[] decrypt(byte[] encrypted, byte[] key) throws Exception {
        byte[][] keys = generateKey(key);
        Cipher cipher = Cipher.getInstance("DESede");
        cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(keys[2], "DESede"));
        byte[] decrypted = cipher.doFinal(encrypted);
        cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keys[1], "DESede"));
        byte[] encrypted1 = cipher.doFinal(decrypted);
        cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(keys[0], "DESede"));
        return cipher.doFinal(encrypted1);
    }

}

四、总结

DESede算法是一种更为安全的对称加密算法,应用广泛。在实现DESede算法时,需要注意密钥的生成、加密和解密的流程,以及采用反向DES加密方式增强算法的安全性。