Descriptions: I am suppose to come up with a program that will break encrypted messages that used a Caesar Cipher algorithm. Sounds easy enough the problem is you have to figure out what position was used to make the encrypted message in order to decrypt the coded message.
So I have a method called public void train(String trainingFileName)
that reads in a file with a lot of text in it to determine the frequency of each lowercase alphabetical English character (a - z)
that are stored in a double array []
. That method works great so it's not necessary to look at unless you want to, but I do use a good majority of that code in the method that I'm having trouble on which is my public int decrypt(String cipherTextFileName, String outputFileName)
method.
The code works beautifully when the Caesar Cipher is set to 3 positions, but anything else it gives me a lot of problems.
I have a do-while
loop in my public int decrypt(String cipherTextFileName, String outputFileName)
method that will decrypt the coded message starting with 0 positions and then using the "distance" formula that I'm using (NOTE: I cannot use any other formula) to find the minimum distance between the knownFrequencies
and the observedFreq
in my encrypted message. Right now I have my do-while
loop set to where if the distance
is less than 0.6
then stop the loop. In theory when I have the correct number of positions in the Caesar Cipher the distance should be below that value.
Problem: Program works great when numberOfPositions
is 3, but when I use an encrypted message that is not using 3 positions in the Caesar Cipher the distance
never falls below 1 and while in debug mode when I set the numberOfPositions
to what it should be to decrypt the message, the message is still encrypted.
Question: How can I implement this method better so I am not testing my distance
on a "hard" value to stop the do-while
loop? I tried using Math.min()
, but that doesn't work. Why can't I decode a message with the positions of the Caesar Cipher other than 3.
I will show you my code now. If you want to test it on your system. You will need 3 text files. One file has to be long with a bunch of words in it... at least 1000. That file will be read in the train
method. You need a file with an encrypted message and another file for the program to write the decrypted message.
Here is an encrypted message first using 3 positions of the Caesar Cipher and then 5 positions.
Wkh surjudp zdv krvwhg eb dfwru Slhufh Eurvqdq dqg kdg frpphqwdub iurp pdqb Kroobzrrg dfwruv dqg iloppdnhuv Prylh txrwdwlrqv wkdw ylhzhuv xvh lq wkhlu rzq olyhv dqg vlwxdwlrqv
Ymj uwtlwfr bfx mtxyji gd fhytw Unjwhj Gwtxsfs fsi mfi htrrjsyfwd kwtr rfsd Mtqqdbtti fhytwx fsi knqrrfpjwx Rtanj vztyfyntsx ymfy anjbjwx zxj ns ymjnw tbs qnajx fsi xnyzfyntsx
When decrypted it should say:
The program was hosted by actor Pierce Brosnan and had commentary from many Hollywood actors and filmmakers Movie quotations that viewers use in their own lives and situations
Alright here is the class I wrote (you will need all the imports) and I would like to thank anyone who helps in advance:
public class CodeBreaker {
public final int NUMBER_OF_LETTERS = 26;
private double[] knownFrequencies = new double[NUMBER_OF_LETTERS];
public double[] getKnownFrequencies() {
return knownFrequencies;
}
public void setKnownFrequencies(double[] knownFrequencies) {
this.knownFrequencies = knownFrequencies;
}
/**
* Method reads in a file with a lot of text in it and
* then use that to figure out the frequencies of each character
*
* @param trainingFileName
*/
public void train(String trainingFileName) {
try {
Scanner fileIO = new Scanner(new File(trainingFileName));
int total = 0;
String temp = "";
while (fileIO.hasNext()) {
//reading into file and storing it into a string called temp
temp += fileIO.next().toLowerCase().replaceAll("[ -,!?';:.]+", "");
//converting temp string into a char array
}
char[] c = temp.toCharArray();
total += c.length; // how many characters are in text
int k = (int) 'a'; // int value of lowercase letter 'a'
int[] counter = new int[NUMBER_OF_LETTERS];
for (int j = 0; j < total; j++) {
for (int i = k - k; i < knownFrequencies.length; i++) {
char[] d = new char[knownFrequencies.length];
d[i] = (char) (k + i);
if (c[j] == d[i]) {//checking to see if char in text equals char in d array
counter[i]++;
knownFrequencies[i] = (double) counter[i] / total;
}
}
}
fileIO.close();
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
System.err.println(e);
System.exit(0);
}
}
/**
* Main decryption method used to take coded text from a file, figure out the positions in the CaesarCipher
* and then decode it onto another file.
*
* @param cipherTextFileName
* @param outputFileName
* @return
*/
public int decrypt(String cipherTextFileName, String outputFileName) {
Scanner fileIO;
int numberOfPositions = 0;
double distance = 0.000000;
try {
fileIO = new Scanner(new File(cipherTextFileName));
PrintWriter writer = new PrintWriter(new File(outputFileName));
String temp = "";
while (fileIO.hasNext()) {
//reading into file and storing it into a string called temp
temp += fileIO.next().toLowerCase().replaceAll(" ", "");
}
fileIO.close();
do {
distance = 0.0;
int total = 0;
double[] observedFreq = new double[NUMBER_OF_LETTERS];
temp = decrypt(temp, numberOfPositions);
char[] c = temp.toCharArray(); //store decrypted chars into an array
total += c.length; // how many characters are in text
int k = (int) 'a'; // int value of lowercase letter 'a'
int[] counter = new int[NUMBER_OF_LETTERS]; //use to count the number of characters in text
for (int j = 0; j < total; j++) {
for (int i = k - k; i < observedFreq.length; i++) {
char[] d = new char[observedFreq.length];
d[i] = (char) (k + i);
if (c[j] == d[i]) { //checking to see if char in text equals char in d array
counter[i]++;
observedFreq[i] = (double) counter[i] / total;
}
}
}
//Formula for finding distance that will determine the numberOfPositions in CaesarCipher
for (int j = 0; j < knownFrequencies.length; j++) {
distance += Math.abs(knownFrequencies[j] - observedFreq[j]); //This is the part of the code I am having trouble with
}
numberOfPositions = numberOfPositions + 1;
} while (distance > 0.6); //This is the part of the code I am having trouble with
Scanner fileIO2 = new Scanner(new File(cipherTextFileName));
while (fileIO2.hasNextLine()) {
//reading into file and storing it into a string called temp
temp = fileIO2.nextLine();
writer.println(decrypt(temp, numberOfPositions));
}
writer.close();
fileIO2.close();
System.out.println(distance);
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
System.err.println(e);
System.exit(0);
}
return numberOfPositions;
}
/**
* CaesarCipher decrypt and encrypt methods
*
* @param ciphertext
* @param numberOfPositions
* @return
*/
public String decrypt(String ciphertext, int numberOfPositions) {
// TODO Auto-generated method stub
return encrypt(ciphertext, -numberOfPositions);
}
public String encrypt(String msg, int offset) {
offset = offset % 26 + 26;
StringBuilder encoded = new StringBuilder();
for (char i : msg.toCharArray()) {
if (Character.isLowerCase(i)) {
int j = (i - 'a' + offset) % 26;
encoded.append((char) (j + 'a'));
} else if (Character.isUpperCase(i)) {
int h = (i - 'A' + offset) % 26;
encoded.append((char) (h + 'A'));
} else {
encoded.append(i);
}
}
return encoded.toString();
}
// barebones main method to test your code
public static void main(String[] args) {
// args[0] contains the filename of the training file
// args[1] contains the filename of the cipher text file
// args[2] contains the filename of the output file
CodeBreaker cb = new CodeBreaker();
cb.train(args[0]);
System.out.println(cb.decrypt(args[1], args[2]));
}
}