Caesar.java

package com.thealgorithms.ciphers;

/**
 * A Java implementation of Caesar Cipher. /It is a type of substitution cipher
 * in which each letter in the plaintext is replaced by a letter some fixed
 * number of positions down the alphabet. /
 *
 * @author FAHRI YARDIMCI
 * @author khalil2535
 */
public class Caesar {
 private static int normalizeShift(final int shift) {
 return ((shift % 26) + 26) % 26;
 }

 /**
 * Encrypt text by shifting every Latin char by add number shift for ASCII
 * Example : A + 1 -> B
 *
 * @return Encrypted message
 */
 public String encode(String message, int shift) {
 StringBuilder encoded = new StringBuilder();

 final int shiftChar = normalizeShift(shift);

 final int length = message.length();
 for (int i = 0; i < length; i++) {
 final char current = message.charAt(i);

 if (isCapitalLatinLetter(current)) {
 final int shifted = current + shiftChar;
 encoded.append((char) (shifted > 'Z' ? shifted - 26 : shifted)); // 26 = number of latin letters
 } else if (isSmallLatinLetter(current)) {
 final int shifted = current + shiftChar;
 encoded.append((char) (shifted > 'z' ? shifted - 26 : shifted)); // 26 = number of latin letters
 } else {
 encoded.append(current);
 }
 }
 return encoded.toString();
 }

 /**
 * Decrypt message by shifting back every Latin char to previous the ASCII
 * Example : B - 1 -> A
 *
 * @return message
 */
 public String decode(String encryptedMessage, int shift) {
 StringBuilder decoded = new StringBuilder();

 final int shiftChar = normalizeShift(shift);

 final int length = encryptedMessage.length();
 for (int i = 0; i < length; i++) {
 final char current = encryptedMessage.charAt(i);
 if (isCapitalLatinLetter(current)) {
 final int shifted = current - shiftChar;
 decoded.append((char) (shifted < 'A' ? shifted + 26 : shifted)); // 26 = number of latin letters
 } else if (isSmallLatinLetter(current)) {
 final int shifted = current - shiftChar;
 decoded.append((char) (shifted < 'a' ? shifted + 26 : shifted)); // 26 = number of latin letters
 } else {
 decoded.append(current);
 }
 }
 return decoded.toString();
 }

 /**
 * @return true if character is capital Latin letter or false for others
 */
 private static boolean isCapitalLatinLetter(char c) {
 return c >= 'A' && c <= 'Z';
 }

 /**
 * @return true if character is small Latin letter or false for others
 */
 private static boolean isSmallLatinLetter(char c) {
 return c >= 'a' && c <= 'z';
 }

 /**
 * @return string array which contains all the possible decoded combination.
 */
 public String[] bruteforce(String encryptedMessage) {
 String[] listOfAllTheAnswers = new String[27];
 for (int i = 0; i <= 26; i++) {
 listOfAllTheAnswers[i] = decode(encryptedMessage, i);
 }

 return listOfAllTheAnswers;
 }
}