手动实施3DES（学术性）

Question

For a course I am taking we are manually implementing the 3DES scheme, which is pretty straight-forward on paper (Two key, with EDE encryption). I have chosen Java as the implementation language but have run into an issue with how it handles encryption/decryption with differing keys. I keep receiving a javax.crypto.BadPaddingException error when attempting to apply the second round (ie "decryption" with K2). The default DES Cipher uses PKCS5Padding and I assume this is the problem, but I'm not sure how to work around it. My code for encryption is below (I hope it is not too straight-forward, less I overlooked something simple). Thank you in advance.

Key Definition (pretty basic and I will look to improve it as I've seen some different approaches while browsing around)

        KeyGenerator kgen = KeyGenerator.getInstance("DES");
        SecretKey sk_1 = kgen.generateKey(); 
        SecretKey sk_2 = kgen.generateKey();
        byte[] raw_1 = sk_1.getEncoded();
        byte[] raw_2 = sk_2.getEncoded();

        spec_1 = new SecretKeySpec(raw_1, "DES"); //key 1
        spec_2 = new SecretKeySpec(raw_2, "DES"); //key 2

        cipher = Cipher.getInstance("DES"); //standard mode is ECB which is block-by-block w/PKCS5Padding
        cipher2 = Cipher.getInstance("DES");


    protected byte[] get3DESEncryption(byte[] plaintext) throws Exception{
        byte[] output = new byte[plaintext.length];
        System.out.println("output len init: " + output.length);
        cipher.init(Cipher.ENCRYPT_MODE, spec_1);
        cipher2.init(Cipher.DECRYPT_MODE, spec_2);

        //first encryption round, key 1 used
        output = cipher.doFinal(plaintext);
        //second "encryption" round, key 2 used but decrypt run
        output = cipher2.doFinal(output);
        //third encryption round, key 1 used
        output = cipher.doFinal(output);

        //return ciphertext
        return output;
    } 

Answer 1

The problem is that you should not use any padding on second (decrypting) and third (encrypting) steps. When you actually apply EDE you should pad only the plain text.

A transformation is of the form:

"algorithm/mode/padding" or "algorithm" (in the latter case, provider-specific default values for the mode and padding scheme are used).

So, you should explicitly tell it not to use padding on cipher2 & cipher3 (you did not created the latter one yet).

Thus, you should have three cipher objects:

• cipher1 DES/ECB/PKCS5Padding
• cipher2 DES/ECB/NoPadding
• cipher3 DES/ECB/NoPadding

[EXTRA HINT]

For decryption you should initialize the ciphers differently and you should reorder the ciphers as well.