Check out my code to decrypt a message encrypted with Caesar Cipher algorithm without knowing the key, just using a dictionary o words in English.
package security;
import java.io.BufferedReader;
import java.io.FileReader;
import java.util.ArrayList;
public class CaesarCipher {
/*
* Encrypt messages using Caesar Cipher algorithm and decrypt
* the message without the key, only using a dictionary of words
*/
public static String encryptMessage(String str, int k){
String encrypted = "";
char letter;
for(int i = 0; i < str.length(); i++){
letter = (char)((str.charAt(i)-'a' + k)%26 + 'a');
encrypted= encrypted + letter;
}
return encrypted;
}
public static String decryptMessage(String str, ArrayList<String> words){
String decryptLine = "", message = "";
char letter;
int k = 0, currentTimes = 0, max = 0;
for(int j=0; j<26; j++){
for(int i = 0; i < str.length(); i++){
if(str.charAt(i)-'a' - j<0){
letter = (char)(26 - Math.abs((str.charAt(i) - 'a' - j)) +'a');
}else{
letter = (char)((str.charAt(i)-'a' - j)%26 + 'a');
}
decryptLine= decryptLine + letter;
}
for(int i = 0; i < words.size(); i++){ if(decryptLine.contains(words.get(i))){ currentTimes++; } } if(currentTimes > max){
max = currentTimes;
message = decryptLine;
k = j;
}
currentTimes = 0;
decryptLine = "";
}
System.out.println("Decrypted text : " + message + "\nK is : " + k );
return message;
}
private static void buildDictinary(ArrayList<String> words){
try{
BufferedReader buf = new BufferedReader(new FileReader("words.txt"));
String lineJustFetched = null;
while(true){
lineJustFetched = buf.readLine();
if(lineJustFetched == null){
break;
}else{
words.add(lineJustFetched);
}
}
buf.close();
}catch(Exception e){
e.printStackTrace();
}
}
public static void main(String[] args) {
ArrayList<String> words = new ArrayList<>();
buildDictinary(words);
String plainText = "amoderndaywarriormeanmeanstridetodaystomsawyermeanmeanpridethoughhismindisnotforrentdon7tputhimdownasarroganthisreserveaquietdefenseridingoutthedayseventstheriver";
System.out.println("Text to encrypt : " + plainText);
String encrypteText = encryptMessage(plainText, 10);
System.out.println("Encrypted text : " + encrypteText);
decryptMessage(encrypteText, words);
}
}
GitHub repository: Caesar Cipher Decryption
CRACK THE NETWORK 