Caesar in CBC / How to XOR in Java?

Question

I have been trying out to ''improve'' the simple Caesar encryption by encrypting in the CBC mode.

As I understand, the first character has to be XORed by an initialization vector and then by the key, the output then is the first character of the encrypted text. This will then be XORed by the second char, then again XORed by the key, … and so forth.

I don't quite understand how the XORing should work.

Let us have the translation table given (only space and A-Z): /s: 0, A: 1, B: 2, …, Z: 26, key: 1, Init.vector: 5

Using the simple Caesar, ''HELLO'' -> {8,5,12,12,20} -> {9,6,13,13,21} -> ''IFMMP''

But how would I get to encrypt using CBC?

It'd be especially helpful if you could show me how to implement it in Java. Thanks!

Answer 1

Hmm I interpret your question like you think you want to xor by the key to your cipher, this is wrong:

You xor by the previous result from the cipher. Something like this:

// Untested code
// The code below need to be adjusted for it to print meaningful 
// characters in the encrypted string as the xor function produces
// integers outside the range of standard ascii characters 

private void cbcCaesar(){

    int key = 1;
    String message = "java";
    int initialisationVector = 5; // the IV is not super necessarily hidden from an attacker but should be different for each message

    StringBuilder encryptedMessageBuilder = new StringBuilder();

    char[] charArray = message.toCharArray();

    int encryptedLetter = initialisationVector;
    for (int letter : charArray){
        int xorApplied = letter ^ encryptedLetter;
        encryptedLetter = applyCaesarCipher(xorApplied, key);
        encryptedMessageBuilder.append((char) encryptedLetter);
    }

    System.out.println(encryptedMessageBuilder.toString());
}

private int applyCaesarCipher(int xorApplied, int key) {
    return (xorApplied+ key) % 26;
}

The easiest way to convert the above snippet to something usable would be to map letters to the numbers 0-26 and use that instead of the char's ascii encoding

I found this resource to be pretty good https://www.youtube.com/watch?v=0D7OwYp6ZEc