使用AES和MAC的Java加密

Question

• 加密有三个阶段:
• 生成16字节随机数据作为CBC模式所需的初始向量(IV)
• 应用AES密码，使用PKCS5填充方案在CBC模式下加密文件内容。
• 应用MAC密码（例如“HMACSHA1”）来计算封装IV和密文的MAC
• 加密文件将是以下数据的级联：16字节IV密文20字节HMAC

我写的代码是这样的，它成功地加密了一个文本文件。这是我的应用程序的全部代码。

import java.io.FileNotFoundException;
import java.io.*;
import java.util.Scanner;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.security.AlgorithmParameters;
import java.security.SecureRandom;
import java.security.spec.KeySpec;

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;

public class AESFileEncryption {

/*public AESFileEncryption(String nameoffile){

}
public String FileReturn(String filename){
    String fl = filename;       
    return fl;      
}*/

public static void main(String[] args) throws Exception {

    File f = new File("plainfile.txt");
    File g = new File("plainfile.txt.8102");
    File fl = new File("plainfile.txt.8102");

    if(g.exists() && !g.isDirectory()){
        System.out.println("The file is already encrypted...");
        String fname = fl.getAbsolutePath();
        System.out.print("Absolute Encrypted File Pathname => "+ fname);
        System.exit(0);
    }       
    else if(f.exists() && !f.isDirectory()) { 
         System.out.println(" The file is found.The encryption process is going to begin...");

    }       
    else{
         System.out.println(" The file is missing!!!!");
         System.exit(0);
    }

    // file to be encrypted
    FileInputStream inFile = new FileInputStream("plainfile.txt");       

    // encrypted file
    FileOutputStream outFile = new FileOutputStream("plainfile.txt.8102");


    // password to encrypt the file
    Scanner scan= new Scanner(System.in);
    System.out.println("Enter the password : => ");
    String password= scan.nextLine();

    //String password = "javapapers";

    // password, iv and salt should be transferred to the other end
    // in a secure manner

    // salt is used for encoding
    // writing it to a file
    // salt should be transferred to the recipient securely
    // for decryption
    byte[] salt = new byte[8];
    SecureRandom secureRandom = new SecureRandom();
    secureRandom.nextBytes(salt);
    FileOutputStream saltOutFile = new FileOutputStream("salt.enc");
    saltOutFile.write(salt);
    saltOutFile.close();

    SecretKeyFactory factory = SecretKeyFactory
            .getInstance("PBKDF2WithHmacSHA1");
    KeySpec keySpec = new PBEKeySpec(password.toCharArray(), salt, 65536,
            256);
    SecretKey secretKey = factory.generateSecret(keySpec);
    SecretKey secret = new SecretKeySpec(secretKey.getEncoded(), "AES");

    //
    Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    cipher.init(Cipher.ENCRYPT_MODE, secret);
    AlgorithmParameters params = cipher.getParameters();

    // iv adds randomness to the text and just makes the mechanism more
    // secure
    // used while initializing the cipher
    // file to store the iv
    FileOutputStream ivOutFile = new FileOutputStream("iv.enc");
    byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();
    ivOutFile.write(iv);
    ivOutFile.close();

    //file encryption
    byte[] input = new byte[64];
    int bytesRead;

    while ((bytesRead = inFile.read(input)) != -1) {
        byte[] output = cipher.update(input, 0, bytesRead);
        if (output != null)
            outFile.write(output);
    }

    byte[] output = cipher.doFinal();
    if (output != null)
        outFile.write(output);

    inFile.close();
    outFile.flush();
    outFile.close();

    System.out.println("File Encrypted.");

    }

}

Answer 1

教师的意思是，应该应用HMAC来为密文创建身份验证标记。这被称为Encrypt-then-MAC。HMAC是一个有密钥的散列函数，它为拥有正确密钥的接收者提供完整性/真实性检查。由于它本质上是一个哈希函数，所以它通过更新内部状态来工作。

Mac mac = Mac.getInstance("HmacSHA1");
SecretKeySpec macKey = new SecretKeySpec(macKeyBytes, "HmacSHA1");
mac.init(macKey);
mac.update(iv); // update for IV
...
mac.update(output); // update for each ciphertext chunk
...
byte[] authTag = mac.doFinal();
outfile.write(authTag); // at the very end

一个问题仍然存在，那就是mackeybytes的生成。它不应与您通过PBKDF2从密码生成的加密密钥相同。您应该使用您生成的密钥来分别导出加密和MAC密钥。这通常是用HKDF完成的，但您也可以再次使用HMAC。伪代码：

byte[] encKeyBytes = hmacSha256(key, "Encryption");
byte[] macKeyBytes = hmacSha256(key, "Authentication");

该指令没有说明任何关于密钥或密码的内容，所以我将假设它应该是一个静态密钥（出于测试目的）。但是使用PBKDF2的方式是可以的，但是salt也必须写入到文件中。否则，接收者将无法在没有随机盐的情况下导出相同的密钥。此外，salt可能应该是16字节长。