我正在尝试找到一种方法来完成我的 Java 课程作业。我应该加密一个文件,但老实说我并没有真正理解这些说明。我实现了文件加密和解密,但是我怀疑它是否按照说明进行操作。说明如下:
我写的代码是这样的,成功加密了一个文本文件。这是我的应用程序的完整代码。
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.");
}
}
导师的意思是应该应用HMAC来为密文创建一个认证标签。这称为“先加密后 MAC”。 HMAC 是一种密钥哈希函数,它为拥有正确密钥的接收者提供真实性(和完整性,因为没有完整性的真实性没有任何意义)检查。由于它本质上是一个哈希函数,因此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
仍然存在一个问题,那就是
macKeyBytes
的产生。它不应与您通过 PBKDF2 从密码生成的加密密钥相同。您应该使用生成的密钥分别导出加密和 MAC 密钥。这通常是使用
HKDF完成的,但您也可以再次使用 HMAC 来完成。伪代码:
byte[] encKeyBytes = hmacSha256(key, "Encryption");
byte[] macKeyBytes = hmacSha256(key, "Authentication");
该指令没有提及任何有关密钥或密码的信息,因此我假设它应该是静态密钥(用于测试目的)。您使用 PBKDF2 的方式是可以的,但盐也必须写入文件中。否则,如果没有随机盐,接收者将无法导出相同的密钥。另外,salt 的长度应该是 16 个字节。当前代码的另一个问题是您将 IV 写入单独的文件中。不要那样做。只需将 IV 写入密文文件的开头即可。 IV 与 CBC 模式下的块大小相同,而 AES 的块大小固定为 16 字节。接收者将始终知道 IV 需要读取多少字节。