ShiftCryptoSystem.java

package project1;

import java.util.Random;

public class ShiftCryptoSystem {
    private int key;

    public static int additive(int a, int m) {
        int normalised = ((a % m) + m) % m;
        return (m - normalised) % m;
    }

    public static String encrypt(String plaintext, int key) {
        // if key is greater than 26 then the shift is moduled
        key = (key % 26) % 26;
        StringBuilder sb = new StringBuilder();
        for (char c : plaintext.toCharArray()) {
            // clean while encrypting (we just shift alphabetical chars)
            if (Character.isLetter(c)) {
                char base = Character.isUpperCase(c) ? 'A' : 'a';
                sb.append((char)(base + (c - base + key) % 26));
            } else {
                sb.append(c);
            }
        }
        return sb.toString();
    }

    public static String decrypt(String ciphertext, int key) {
        // Decryption in shift cipher is the same as encrypting
        // but shifts backwards
        return encrypt(ciphertext, additive(key, 26));
    }

    public static String[] bruteForce(String ciphertext) {
        // try all shifts to decrypt the cipher
        String[] results = new String[26];
        for (int key = 0; key < 26; key++) {
            // store current attempt of decrypting
            results[key] = decrypt(ciphertext, key);
        }
        return results;
    }
}