Question

I need a simple code for the shift cipher

i find this wb page , it explains the encryption and decryption process but i dont know how to implement this in Matlab

https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/shift-cipher

Please write the code and i need in 2 hours

Answer 1

Caesar's cypher is the simplest encryption algorithm. It adds a fixed value to the ASCII (unicode) value of each character of a text. In other words, it shifts the characters. Decrypting a text is simply shifting it back by the same amount,

Process Plain Text

function output = encCsar(word,key)

numWord = uint8(word);

numKey = uint8(lower(key)) - 96;

for i = 1:numel(numWord)

if ( numWord>=65 & numWord<=90 )

numWord=mod(numWord-65+numKey,26);

output(i,1)=char(numWord+65);

elseif ( numWord>=97 & numWord<=122 )

numWord=mod(numWord-97+numKey,26);

output(i,1)=char(numWord+97);

else

error('Please enter a string and a number key.')

end

end

Pseudo Code for Caesar Cipher

1. Input : String plaintext
2. Input : An integer between 0 and 25 representing the right shift of the character or,
3. an integer between -25 and -1 representing the left shift of the characters.
4. Traverse each character in the plaintext one at a time
5. Transform the given character depending on encryption or decryption.
6. print the ciphertext.

try once understanding in java

import java.util.*;
public class CaesarCipherProgram {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        System.out.println(" Input the plaintext message : ");
        String plaintext = sc.nextLine();
        System.out.println(" Enter the value by which 
        each character in the plaintext         
        message gets shifted : ");
        int shift = sc.nextInt();
        String ciphertext = "";
        char alphabet;
        for(int i=0; i < plaintext.length();i++) 
        {
             // Shift one character at a time
            alphabet = plaintext.charAt(i);
            
            // if alphabet lies between a and z 
            if(alphabet >= 'a' && alphabet <= 'z') 
            {
             // shift alphabet
             alphabet = (char) (alphabet + shift);
             // if shift alphabet greater than 'z'
             if(alphabet > 'z') {
                // reshift to starting position 
                alphabet = (char) (alphabet+'a'-'z'-1);
             }
             ciphertext = ciphertext + alphabet;
            }
            
            // if alphabet lies between 'A'and 'Z'
            else if(alphabet >= 'A' && alphabet <= 'Z') {
             // shift alphabet
             alphabet = (char) (alphabet + shift);    
                
             // if shift alphabet greater than 'Z'
             if(alphabet > 'Z') {
                 //reshift to starting position 
                 alphabet = (char) (alphabet+'A'-'Z'-1);
             }
             ciphertext = ciphertext + alphabet;
            }
            else {
             ciphertext = ciphertext + alphabet;   
            }
        
    }
    System.out.println(" ciphertext : " + ciphertext);
  }
}

output:

Input the plaintext message : Java Hungry Blog
Enter the value by which each character in the plaintext message gets shifted : 2
ciphertext : Lcxc Jwpita Dnqi

Decryption

import java.util.*;
public class CaesarCipherProgram {
        public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        System.out.println(" Input the ciphertext message : ");
        String ciphertext = sc.nextLine();
        System.out.println(" Enter the shift value : ");
        int shift = sc.nextInt();
        String decryptMessage = "";
        for(int i=0; i < ciphertext.length();i++)  

        {
            // Shift one character at a time
            char alphabet = ciphertext.charAt(i);
            // if alphabet lies between a and z 
            if(alphabet >= 'a' && alphabet <= 'z')
            {
                // shift alphabet
                alphabet = (char) (alphabet - shift);
            
                // shift alphabet lesser than 'a'
                if(alphabet < 'a') {
                    //reshift to starting position 
                    alphabet = (char) (alphabet-'a'+'z'+1);
                }
                decryptMessage = decryptMessage + alphabet;
            }    
                // if alphabet lies between A and Z
            else if(alphabet >= 'A' && alphabet <= 'Z')
            {
             // shift alphabet
                alphabet = (char) (alphabet - shift);
                
                //shift alphabet lesser than 'A'
                if (alphabet < 'A') {
                    // reshift to starting position 
                    alphabet = (char) (alphabet-'A'+'Z'+1);
                }
                decryptMessage = decryptMessage + alphabet;            
            }
            else 
            {
             decryptMessage = decryptMessage + alphabet;            
            } 
        }
        System.out.println(" decrypt message : " + decryptMessage);
    }
}

output:

Input the ciphertext message : Lcxc Jwpita Dnqi
Enter the shift value : 2
decrypt message : Java Hungry Blog