Question

Java programming problem. I've managed to get through the basic level of encryption, but the advanced and military grade sections are beyond me. If someone can show me what the hell this code is supposed to look like I'd greatly appreciate it. In other words I need to write a code that can decrypt a string message by the following parameters.

Basic: Using Caesar Cipher with a static shift of 1-25. This is easy to decrypt as there are only 25 possibilities.

Advanced: They use two different shift values, one for even index characters, and another value for the odd characters.

Military Grade: They use a variable shift that starts at a certain value and increases by a second variable after encrypting each character. So the first character may be encrypted with a shift of 3, the second with a shift of 5 (3 + 2) and the third with 7 (3 + 2 + 2) and so on. We don’t have any clue of these numbers ahead of time

We also need to be able to receive a message and decrypt it. The Basic, Advanced, and military versions can print all possibilities to the screen.

Answer 1

Thanks for the question, here are the 2 methods you will be needing. I have commented the code so that you can follow the code and the logic.

It would have been great if you could have shared some test cases so that I could have tested it. Hope you find this helpful. Let me know for any help with any other questions.

thanks 

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public class Encryption {

    // method takes in the encrypted text, the starting key and the increment value
    // mehtod returns the decrypted string
    public static String decryptUsingMilitaryRule(String encrypted, int startKey, int increment) {
       
        
        StringBuffer decrypted = new StringBuffer();

        for (int i = 0; i < encrypted.length(); i++) {
            char letter = (char) (encrypted.charAt(i) +startKey); // add startKey to the letter
            startKey+=increment; // add increment to the startKey for the next letter 
            decrypted.append(letter);// we will append the decrypted letter to the string
        }
        return decrypted.toString(); // return the decrypted string back
        
    }
    // method takes in the encrypted text, the odd position key value, the even position key value
    // method returns the decrypted string
    public static String decryptUsingAdvancedRule(String encrypted, int oddKey, int evenKey) {

        StringBuffer decrypted = new StringBuffer();

        for (int i = 0; i < encrypted.length(); i++) {
            char letter = encrypted.charAt(i);
            // when i is even we will add oddKey value to the letter
            // when i is odd we will add evenKey to the letter
            letter += i % 2 == 0 ? oddKey : evenKey;
            decrypted.append(letter); // we will append the decrypted letter to the string
        }
        return decrypted.toString();// return the decrypted string back
    }
}

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