Java 3DES 实现（学术）

Question

My code so far is:

public class TripleDES {
    /**
     * @param args the command line arguments
     * @throws java.security.NoSuchAlgorithmException
     * @throws javax.crypto.NoSuchPaddingException
     * @throws java.security.InvalidKeyException
     * @throws javax.crypto.IllegalBlockSizeException
     * @throws javax.crypto.BadPaddingException
     */
    public static void main(String[] args) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
        //Encrypt: C = EK3(DK2(EK1(P)))
        //Decrypt: P = DK3(EK2(DK1(C)))
        
        Scanner sc = new Scanner(System.in);
        
        //Generate key for DES
        KeyGenerator keyGenerator = KeyGenerator.getInstance("DES");
        SecretKey secretKey = keyGenerator.generateKey();
        SecretKey secretKey2 = keyGenerator.generateKey();
        SecretKey secretKey3 = keyGenerator.generateKey();
        
        //Text Enc & Dec
        Cipher cipher = Cipher.getInstance("DES/ECB/PKCS5Padding");



        
        //enter msg
        System.out.print("Enter a string: ");
        String x= sc.nextLine();
        
//enc
        cipher.init(Cipher.ENCRYPT_MODE,secretKey);
        byte[] message = x.getBytes();//text
        byte[] messageEnc = cipher.doFinal(message);//encryption with key1
        cipher.init(Cipher.DECRYPT_MODE,secretKey2);
        byte[] deck2 = cipher.doFinal(messageEnc);//decryption with key2
        cipher.init(Cipher.ENCRYPT_MODE,secretKey3);
        byte[] messageEnc1 = cipher.doFinal(deck2);//encryption with key3
        System.out.println("Cipher Text: " + new String(messageEnc1));
        
        //dec
        cipher.init(Cipher.DECRYPT_MODE,secretKey3);
        byte[] dec = cipher.doFinal(messageEnc1);//decryption with key1
        cipher.init(Cipher.ENCRYPT_MODE,secretKey2);
        byte[] messageEnc2 = cipher.doFinal(dec);//encryption with key2
        cipher.init(Cipher.DECRYPT_MODE,secretKey);
        byte[] deck3 = cipher.doFinal(messageEnc2);//decryption with key3
        System.out.println("Plain Text: " + new String(deck3));
    }
}

I get the error:

Exception in thread "main" javax.crypto.BadPaddingException: Given final block not properly padded. Such issues can arise if a bad key is used during decryption.
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:991)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:847)
    at com.sun.crypto.provider.DESCipher.engineDoFinal(DESCipher.java:314)
    at javax.crypto.Cipher.doFinal(Cipher.java:2164)
    at tripledes.TripleDES.main(TripleDES.java:45)

My guess is that when I try decrypting with a different key at line 45 and it gives the error above, it's because the encrypted text is larger than the key generated, but I am not quite sure.

Can someone please help as I can't figure out the problem.

Answer 1

I am referring to the originally posted code with the cipher , cipher2 and cipher3 instances: The problem is the padding regarding cipher2 and cipher3 . Only cipher may use PKCS5Padding , cipher2 and cipher3 must apply NoPadding .

The generated ciphertext is then indeed identical to the one that would be generated by 3DES, provided that the concatenated bytes of secretKey , secretKey2 and secretKey3 are used as 3DES key.

ECB is an insecure mode, by the way, s. here .

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Regarding the comments:
For the decoding of a ciphertext with a charset encoding, see eg Problems when converting from byte array to string and back to byte array . Concerning the missing encoding specification, seg String.getBytes() in different default charsets .