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.
   *
   * @return Encrypted message
   */
  public static String encode(String message, String cipherSmall) {
    StringBuilder encoded = new StringBuilder();

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

    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.append(cipherMap.get(message.charAt(i)));
      else encoded.append(message.charAt(i));
    }

    return encoded.toString();
  }

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

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

    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.append(cipherMap.get(encryptedMessage.charAt(i)));
      else decoded.append(encryptedMessage.charAt(i));
    }

    return decoded.toString();
  }

  /** 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);
  }
}