decoding an encrypted string in java

Question

I am trying to decode the string encoded by the function below. I tried to reverse the encryption by doing c -= c-i, but it doesn't seem to work. Can anyone please explain how does this work?

public static String encode(String message){
        StringBuilder b = new StringBuilder();
        for(int i = 0;i<message.length();i++){
            char c = message.charAt(i);
            b.append(c +=  c+i);
        }
        return b.reverse().toString();
    }

Arithmetic on character codes is a bit sketchy (@Elliot alludes to unstated conditions for this algorithm). That's why encryption libraries take in bytes and put out bytes. If you want to convert text to bytes, you'd apply a character encoding (such as UTF-8), and visa versa. If you want to handle encryped bytes (or any non-text bytes) as text, you'd use Base64 or similar. — Tom Blodget
– Tom Blodget, Commented Apr 27, 2018 at 23:26

Elliott Frisch · Accepted Answer · 2018-04-25 23:32:56Z

5

Your encoding is reversible (for small messages and some set of the charset), but your formula is incorrect. First, reverse the message (to decode). Then iterate the characters. Subtract i from the current value, and then divide by 2. Like,

public static String decode(String message) {
    StringBuilder b = new StringBuilder(message).reverse();
    StringBuilder o = new StringBuilder();
    for (int i = 0; i < b.length(); i++) {
        char c = b.charAt(i);
        c -= i;
        c /= 2;
        o.append(c);
    }
    return o.toString();
}

Math: c += c + i; is equivalent to c = (2 * c) + i

answered Apr 25, 2018 at 23:32

Elliott Frisch

202k20 gold badges166 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Dookoto_Sea Over a year ago

Thanks for the explanation. Now i understand the math behind it

Collectives™ on Stack Overflow

decoding an encrypted string in java

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related