Question

I have a problem with AES encryption and the customer showed me their PHP server code for “decrypting”. Curiously enough the exact code has been taken from S.O. (not surprising). I found this out because the code the customer passed me had the exact same comment! 🙂

Anyway, it’s a piece of PHP code taken from this S.O. question.

I am trying to do the same with Java but I don’t know what this exact line is adding:

$key = 'a16byteslongkey!';

$padded_key = $key . str_repeat(chr(0x00), 16); // Argh!

(note the // Argh! comment was not mine 😉

Is it trying to add chr(0x00) to make a 32 bytes key (because the $key is 16?) if so, how would I do the same in Java?

Answer 1

As Gareth stated this returns the character with ASCII code 0. Using this we can make a function which repeats a string:

public static String strRepeat(String toRepeat, int reps){
    //Sanity checks go here!
    StringBuilder sb = new StringBuilder();
    for(int x = 0; x < reps; x++){
        sb.append(toRepeat);
    }
    return sb.toString();
}

Now the line can be replaced with:

String paddedKey = key + strRepeat('\0', 16); // Argh!