How to finish creating Caesar cipher?

Question

Here is my code so far. The program is designed to take a message and encrypt or decrypt a message by shifting each letter a certain number of letters in the alphabet. I managed to get the programs talking to each other (there are two methods here by the way) and the first prompt works, however I am not sure how to get the decryption working as well as with either upper or lower case? Any help is greatly appreciated.

public class CaesarCipher
{
    public static String encrypt(String message, int key)
    {
        String alphaLower = "abcdefghijklmnopqrstuvwxyz";
        String alphaUpper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        int sum = 0;
        String encryptedMessage = "";

        //Find the index of every char in the string "message" 
        //Add 1 to every index.
        //Take that index and turn it back into a char. 

        for (int i = 0; i < message.length(); i++)
           {
            if (alphaLower.indexOf(message.charAt(i)) > -1)
            {char letter = message.charAt(i);

            int index =(alphaLower.indexOf(message) + key)%26;


            char newLetter = alphaLower.charAt(index);
            encryptedMessage += newLetter;
           }
           else if (alphaUpper.indexOf(message.charAt(i)) )
           {char letter = message.charAt(i);

            int index = alphaUpper.indexOf(message) + key;


            char newLetter = alphaUpper.charAt(index);
            encryptedMessage += newLetter;

           }
           else 
           {
               encryptedMessage
            }
        }

        return encryptedMessage;
    }

    public static String decrypt(String message, int key)
    {
        String decryptedMessage;
        decryptedMessage = encrypt(message, -key)
        return decryptedMessage;
    }
}


public class CaesarTester
{
    public static void main(String[] args)
    {

        System.out.println("Test 1 - simple encrypt:");
        System.out.println("Expected: j offe npofz");
        System.out.println("Actual: " + CaesarCipher.encrypt("i need money", 1));
        System.out.println();

        System.out.println("Test 2 - simple decrypt:");
        System.out.println("Expected: i need money");
        System.out.println("Actual: " + CaesarCipher.decrypt("j offe npofz", 1));
        System.out.println();

        System.out.println("Test 3 - simple encrypt w/ casing:");
        System.out.println("Expected: Fyx M jsvksx qc TMR jsv xli EXQ");
        System.out.println("Actual: " + CaesarCipher.encrypt("But I forgot my PIN for the ATM", 4));
        System.out.println();

        System.out.println("Test 4 - simple decrypt w/ casing:");
        System.out.println("Expected: But I forgot my PIN for the ATM");
        System.out.println("Actual: " + CaesarCipher.decrypt("Fyx M jsvksx qc TMR jsv xli EXQ", 4));
        System.out.println();

        System.out.println("Test 5 - simple encrypt w/ casing and symbols:");
        System.out.println("Expected: Un xomnz. VOT oy 1234!");
        System.out.println("Actual: " + CaesarCipher.encrypt("Oh right. PIN is 1234!", 6));
        System.out.println();

        System.out.println("Test 6 - simple decrypt w/ casing and symbols:");
        System.out.println("Expected: Oh right. PIN is 1234!");
        System.out.println("Actual: " + CaesarCipher.decrypt("Un xomnz. VOT oy 1234!", 6));
        System.out.println();

        System.out.println("Test 7 - complex encrypt:");
        System.out.println("Expected: Hiq, qbun'm gs qczc jummqilx?!");
        System.out.println("Actual: " + CaesarCipher.encrypt("Now, what's my wifi password?!", 20));
        System.out.println();

        System.out.println("Test 8 - complex decrypt:");
        System.out.println("Expected: Now, what's my wifi password?!");
        System.out.println("Actual: " + CaesarCipher.decrypt("Hiq, qbun'm gs qczc jummqilx?!", 20));
        System.out.println();
    }
}

Answer 1

You are attempting to reverse the encryption by passing in -key as the key here:

decryptedMessage = encrypt(message, -key);

However, this line will go wrong because of that.

int index =(alphaLower.indexOf(message) + key)%26;

This is because in Java, a negative operand to the % operator will result in a negative value. This can occur with a low enough character value and a negative key of high enough magnitude.

You can rectify this by passing in 26 - key as your decryption "key".

decryptedMessage = encrypt(message, 26 - key);

This will ensure that the operand to the % operator remains positive.