CODEWITHSUNDEEP
java
lio
lio

Reputation: 75

Java Ascii to String Decryption

Original Question (Problem 3)

I'm completing a problem for class that requires converting a string message from ascii with a key to a decoded String message. I've tried both accessing the string from message.charAt(i) and by converting it to a char array but both times I get this weird out put on the console.

screenshot of console

This is the method I have running

public static char[] decrypt(String message) {
    char[] decoded = new char[message.length()];
    char[] newmessage = message.toCharArray();
    int ascii;
    for(int key=0; key<=100; key++) {
        for(int i=0; i<message.length(); i++) {
            ascii = ( (int)newmessage[i] + 127) - 32;
            if(ascii > 126)
                decoded[i] = (char)((int)newmessage[i] - key);
            else
                decoded[i] = (char)((((int)newmessage[i] - key) +127) -32);
        }
    }
    System.out.println(decoded);
    return decoded;
}

This is where I called it in main

    System.out.println("Problem 3");
    String message =  ":mmZ\\dxZmx]Zpgy";
    System.out.println("Message Received: ");
    System.out.println(message);
    decrypt(message);

I can't seem to figure out where I went wrong with it. The expected output is for each key to be printed with the corresponded decoded message. The 88th key will show the message is "Attack at Dawn!".

Upvotes: 1

Views: 5639

Answers (2)

Gatusko
Gatusko

Reputation: 2608

Okey first you need to now wich Key decode the message of :mmZ\\dxZmx]Zpgy So for go for the easy part try to decode first the Hey message with your algorithm that you made, and you forgot the most escenssial part:

if(originalChar + key > 126) then
     EncryptedChar = 32 + ((originalChar + key) - 127)
else
     EncryptedChar = (originalChar + key)

So you in your if statement you are missing the minus key 

 if(ascii - key > 126)

Always go and re read your question. 

    for(int key=0; key<=100; key++) {
        for(int i=0; i<message.length(); i++) {
            ascii = ( (int)newmessage[i] + 127) - 32;
            if(ascii - key > 126)
                decoded[i] = (char)((int)newmessage[i] - key);
            else
                decoded[i] = (char)((((int)newmessage[i] - key) +127) -32);
        }
        System.out.println("Decoded with i=" +key +":"+new String(decoded)); // For check what is the correct message. 
    }

Spoiler The MEssage is: Attacck at dawn!

Upvotes: 1

bobah
bobah

Reputation: 18864

If all you need is decode from ASCII to Java String, then one of correct ways of doing it is

char [] output = StandardCharsets.US_ASCII.decode(ByteBuffer.wrap(input)).array();

Based on your performance requirements, you may want to preallocate buffers, and may be the decoder (in the above example Charset::decode will create one on the fly).

Upvotes: 2

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