Java, How to implement a Shift Cipher (Caesar Cipher)

Question

I want to implement a Caesar Cipher shift to increase each letter in a string by 3.

I am receiving this error:

possible loss of precision required char; found int

Here is my code so far:

import java.util.Scanner;
import java.io.*;

public class CaesarCipher
{
    public static void main (String [] args) {

        char[] letters = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 
            'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 
            'w', 'x', 'y', 'z'};

        char[] message = "onceuponatime".toCharArray();
        char[] eMessage = new char[message.length];
        char shift = 3;

        //encrypting message
        for(int i = 0; i <= message.length; ++i)
        {
            eMessage[i] = (message[i] + shift) % (char) letters.length;
            System.out.println(x);               
        }              
    }
}

What causes this error? How can I implement a caesar Cipher shift to increase each letter in a string by 3?

Christychristye

Sidenote: your code doesn't seem to be using either import statement, so you should remove them. – Gynarchy

I see other errors like in " eMessage[i] = (message[i] + shift) % (char) letters.length;" error- Type mismatch: cannot convert from int to char which is obvious and in System.out.println(x); Here x is undefined. First try to remove these errors... – Petronella

Answer 1

Java Shift Caesar Cipher by shift spaces.

Restrictions:

1. Only works with a positive number in the shift parameter.
2. Only works with shift less than 26.
3. Does a += which will bog the computer down for bodies of text longer than a few thousand characters.
4. Does a cast number to character, so it will fail with anything but ascii letters.
5. Only tolerates letters a through z. Cannot handle spaces, numbers, symbols or unicode.
6. Code violates the DRY (don't repeat yourself) principle by repeating the calculation more than it has to.

Pseudocode:

1. Loop through each character in the string.
2. Add shift to the character and if it falls off the end of the alphabet then subtract shift from the number of letters in the alphabet (26)
3. If the shift does not make the character fall off the end of the alphabet, then add the shift to the character.
4. Append the character onto a new string. Return the string.

Function:

String cipher(String msg, int shift){
    String s = "";
    int len = msg.length();
    for(int x = 0; x < len; x++){
        char c = (char)(msg.charAt(x) + shift);
        if (c > 'z')
            s += (char)(msg.charAt(x) - (26-shift));
        else
            s += (char)(msg.charAt(x) + shift);
    }
    return s;
}

How to invoke it:

System.out.println(cipher("abc", 3));  //prints def
System.out.println(cipher("xyz", 3));  //prints abc

Familiar

If you need to use negative numbers, you can just subtract them from 26 (shift of -3 is 23). – Secant

This is good, but should probably use a StringBuilder rather than appending to your String s. See: #4645520 – Sn

Answer 2

Below code handles upper and lower cases as well and leaves other character as it is.

import java.util.Scanner;

public class CaesarCipher
{
    public static void main(String[] args)
    {
    Scanner in = new Scanner(System.in);
    int length = Integer.parseInt(in.nextLine());
    String str = in.nextLine();
    int k = Integer.parseInt(in.nextLine());

    k = k % 26;

    System.out.println(encrypt(str, length, k));

    in.close();
    }

    private static String encrypt(String str, int length, int shift)
    {
    StringBuilder strBuilder = new StringBuilder();
    char c;
    for (int i = 0; i < length; i++)
    {
        c = str.charAt(i);
        // if c is letter ONLY then shift them, else directly add it
        if (Character.isLetter(c))
        {
        c = (char) (str.charAt(i) + shift);
        // System.out.println(c);

        // checking case or range check is important, just if (c > 'z'
        // || c > 'Z')
        // will not work
        if ((Character.isLowerCase(str.charAt(i)) && c > 'z')
            || (Character.isUpperCase(str.charAt(i)) && c > 'Z'))

            c = (char) (str.charAt(i) - (26 - shift));
        }
        strBuilder.append(c);
    }
    return strBuilder.toString();
    }
}

Petronella

Answer 3

The warning is due to you attempting to add an integer (int shift = 3) to a character value. You can change the data type to char if you want to avoid that.

A char is 16 bits, an int is 32.

char shift = 3;
// ...
eMessage[i] = (message[i] + shift) % (char)letters.length;

As an aside, you can simplify the following:

char[] message = {'o', 'n', 'c', 'e', 'u', 'p', 'o', 'n', 'a', 't', 'i', 'm', 'e'}; 

To:

char[] message = "onceuponatime".toCharArray();

Childbearing

I changed the line to (char)letters.length; and I still receive the same error. – Christychristye

Did you change the data-type of shift to char? – Childbearing

Answer 4

Two ways to implement a Caesar Cipher:

Option 1: Change chars to ASCII numbers, then you can increase the value, then revert it back to the new character.

Option 2: Use a Map map each letter to a digit like this.

A - 0
B - 1
C - 2
etc...

With a map you don't have to re-calculate the shift every time. Then you can change to and from plaintext to encrypted by following map.

Lir

Answer 5

Hello...I have created a java client server application in swing for caesar cipher...I have created a new formula that can decrypt the text properly... sorry only for lower case..!

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class ceasarserver extends JFrame implements ActionListener {
    static String cs = "abcdefghijklmnopqrstuvwxyz";
    static JLabel l1, l2, l3, l5, l6;
    JTextField t1;
    JButton close, b1;
    static String en;
    int num = 0;
    JProgressBar progress;

    ceasarserver() {
        super("SERVER");
        JPanel p = new JPanel(new GridLayout(10, 1));
        l1 = new JLabel("");
        l2 = new JLabel("");
        l3 = new JLabel("");
        l5 = new JLabel("");
        l6 = new JLabel("Enter the Key...");
        t1 = new JTextField(30);
        progress = new JProgressBar(0, 20);
        progress.setValue(0);
        progress.setStringPainted(true);
        close = new JButton("Close");
        close.setMnemonic('C');
        close.setPreferredSize(new Dimension(300, 25));
        close.addActionListener(this);
        b1 = new JButton("Decrypt");
        b1.setMnemonic('D');
        b1.addActionListener(this);
        p.add(l1);
        p.add(l2);
        p.add(l3);
        p.add(l6);
        p.add(t1);
        p.add(b1);
        p.add(progress);
        p.add(l5);
        p.add(close);
        add(p);
        setVisible(true);
        pack();
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == close)
            System.exit(0);
        else if (e.getSource() == b1) {
            int key = Integer.parseInt(t1.getText());
            String d = "";
            int i = 0, j, k;
            while (i < en.length()) {
                j = cs.indexOf(en.charAt(i));
                k = (j + (26 - key)) % 26;
                d = d + cs.charAt(k);
                i++;
            }
            while (num < 21) {
                progress.setValue(num);
                try {
                    Thread.sleep(100);
                } catch (InterruptedException ex) {
                }
                progress.setValue(num);
                Rectangle progressRect = progress.getBounds();
                progressRect.x = 0;
                progressRect.y = 0;
                progress.paintImmediately(progressRect);
                num++;
            }
            l5.setText("Decrypted text: " + d);
        }
    }

    public static void main(String args[]) throws IOException {
        new ceasarserver();
        String strm = new String();
        ServerSocket ss = new ServerSocket(4321);
        l1.setText("Secure data transfer Server Started....");
        Socket s = ss.accept();
        l2.setText("Client Connected !");
        while (true) {
            Scanner br1 = new Scanner(s.getInputStream());
            en = br1.nextLine();
            l3.setText("Client:" + en);
        }
    }

The client class:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class ceasarclient extends JFrame {
    String cs = "abcdefghijklmnopqrstuvwxyz";
    static JLabel l1, l2, l3, l4, l5;
    JButton b1, b2, b3;
    JTextField t1, t2;
    JProgressBar progress;
    int num = 0;
    String en = "";

    ceasarclient(final Socket s) {
        super("CLIENT");
        JPanel p = new JPanel(new GridLayout(10, 1));
        setSize(500, 500);
        t1 = new JTextField(30);
        b1 = new JButton("Send");
        b1.setMnemonic('S');
        b2 = new JButton("Close");
        b2.setMnemonic('C');
        l1 = new JLabel("Welcome to Secure Data transfer!");
        l2 = new JLabel("Enter the word here...");
        l3 = new JLabel("");
        l4 = new JLabel("Enter the Key:");
        b3 = new JButton("Encrypt");
        b3.setMnemonic('E');
        t2 = new JTextField(30);
        progress = new JProgressBar(0, 20);
        progress.setValue(0);
        progress.setStringPainted(true);
        p.add(l1);
        p.add(l2);
        p.add(t1);
        p.add(l4);
        p.add(t2);
        p.add(b3);
        p.add(progress);
        p.add(b1);
        p.add(l3);
        p.add(b2);
        add(p);
        setVisible(true);
        b1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                try {
                    PrintWriter pw = new PrintWriter(s.getOutputStream(), true);
                    pw.println(en);
                } catch (Exception ex) {
                }
                ;
                l3.setText("Encrypted Text Sent.");
            }
        });
        b3.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String strw = t1.getText();
                int key = Integer.parseInt(t2.getText());
                int i = 0, j, k;
                while (i < strw.length()) {
                    j = cs.indexOf(strw.charAt(i));
                    k = (j + key) % 26;
                    en = en + cs.charAt(k);
                    i++;
                }
                while (num < 21) {
                    progress.setValue(num);
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException exe) {
                    }
                    progress.setValue(num);
                    Rectangle progressRect = progress.getBounds();
                    progressRect.x = 0;
                    progressRect.y = 0;
                    progress.paintImmediately(progressRect);
                    num++;
                }
            }
        });
        b2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        });
        pack();
    }

    public static void main(String args[]) throws IOException {
        final Socket s = new Socket(InetAddress.getLocalHost(), 4321);
        new ceasarclient(s);
    }
}

Jury

Answer 6

//we can use char as integer as Java store every character as ascii integer

String rotationalCipher(String input, int rotationFactor){

    char [] inputArr = input.toCharArray();

    String resultStr = "";

    for (char ch: inputArr ) {
        if(ch < 58 && ch>48){
            int withRotetionFactor = ch + rotationFactor;
            if(withRotetionFactor < 57){
              resultStr = resultStr + (char) withRotetionFactor;
           }else {
                 int looprotation =  withRotetionFactor - 10;
                resultStr = resultStr + (char)looprotation;
            }
        }else  if(ch < 91 && ch>63){
            int withRotetionFactor = ch + rotationFactor;
            if(withRotetionFactor < 91){
                resultStr = resultStr + (char) withRotetionFactor;
            }else {
                int looprotation =  withRotetionFactor - 26;
                resultStr = resultStr + (char)looprotation;
            }
        }else if(ch < 123 && ch>96){
            int withRotetionFactor = ch + rotationFactor;
            if(withRotetionFactor < 123){
                resultStr = resultStr + (char) withRotetionFactor;
            }else {
                int looprotation =  withRotetionFactor - 26;
                resultStr = resultStr + (char)looprotation;
            }
        }else {
            resultStr = resultStr + ch;
        }



    }
    return resultStr;

}

Littlest

I would recommend changing the "magic numbers" to their corresponding ASCII character. Numbers like 58 and 123 are not as obvious as ':' and '{'. – Acclamation

Answer 7

Here is my Java Stream API implementation of a program that can encrypt using Caesar cipher:

public String encrypt(String plaintext) {
  return plaintext.toLowerCase().chars()
    .mapToObj(plaintextLetter -> plaintextLetter == ' ' ? ' ' : applyEncryptionAlgorithm(plaintextLetter))
    .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
    .toString()
    .toUpperCase();  // convention - ciphertext is in uppercase
}

private int applyEncryptionAlgorithm(int plaintextLetter) {
  plaintextLetter -= 'a';
  int ciphertextLetter = (plaintextLetter + 3) % 26;
  return ciphertextLetter + 'a';
}

And a unit test:

@Test
public void testEncrypt() {
  String plaintext = "meet me after the toga party";
  String ciphertext = "PHHW PH DIWHU WKH WRJD SDUWB";

  String result = caesarCipher.encrypt(plaintext);

  assertEquals(ciphertext, result);
}

Hylomorphism