JavaAlgorithms/ciphers/SimpleSubstitutionCipher.java

package ciphers;

import java.util.*;

/**
 *
 * The simple substitution cipher is a cipher that has been in use for many hundreds of years 
 * (an excellent history is given in Simon Singhs 'the Code Book'). 
 * It basically consists of substituting every plaintext character for a different ciphertext character. 
 * It differs from the Caesar cipher in that the cipher alphabet is not simply the alphabet shifted,
 * it is completely jumbled.
 * 
 * @author Hassan Elseoudy
 */

public class SimpleSubstitutionCipher {

    /**
     * Encrypt text by replacing each element with its opposite character.
     *
     * @param message
     * @param cipherSmall
     * @return Encrypted message
     */
    public static String encode(String message, String cipherSmall) {
        String encoded = "";

        // This map is used to encode
        Map<Character,Character> cipherMap = new HashMap<Character,Character>();

        char beginSmallLetter = 'a';
        char beginCapitalLetter = 'A';

        cipherSmall = cipherSmall.toLowerCase();
        String cipherCapital = cipherSmall.toUpperCase();

        // To handle Small and Capital letters
        for(int i = 0; i < cipherSmall.length(); i++){
            cipherMap.put(beginSmallLetter++,cipherSmall.charAt(i));
            cipherMap.put(beginCapitalLetter++,cipherCapital.charAt(i));
        }

        for(int i = 0; i < message.length(); i++){
            if(Character.isAlphabetic(message.charAt(i)))
                encoded += cipherMap.get(message.charAt(i));
            else
                encoded += message.charAt(i);
        }

        return encoded;
    }

    /**
     * Decrypt message by replacing each element with its opposite character in cipher.
     *
     * @param encryptedMessage
     * @param cipherSmall
     * @return message
     */
    public static String decode(String encryptedMessage, String cipherSmall) {
        String decoded = "";


        Map<Character,Character> cipherMap = new HashMap<Character,Character>();

        char beginSmallLetter = 'a';
        char beginCapitalLetter = 'A';

        cipherSmall = cipherSmall.toLowerCase();
        String cipherCapital = cipherSmall.toUpperCase();

        for(int i = 0; i < cipherSmall.length(); i++){
            cipherMap.put(cipherSmall.charAt(i),beginSmallLetter++);
            cipherMap.put(cipherCapital.charAt(i),beginCapitalLetter++);
        }

        for(int i = 0; i < encryptedMessage.length(); i++){
            if(Character.isAlphabetic(encryptedMessage.charAt(i)))
                decoded += cipherMap.get(encryptedMessage.charAt(i));
            else
                decoded += encryptedMessage.charAt(i);
        }

        return decoded;
    }

    /**
     *
     * TODO remove main and make JUnit Testing
     */
    public static void main(String[] args) {
        String a = encode("defend the east wall of the castle","phqgiumeaylnofdxjkrcvstzwb");
        String b = decode(a,"phqgiumeaylnofdxjkrcvstzwb");
        System.out.println(b);
    }

}