ceaser cipher

SUBSTITUTION & TRANSPOSITION TECHNIQUES

 CAESAR CIPHER

AIM

To write a program for encrypting a plain text and decrypting a cipher text using Caesar Cipher (shift cipher) substitution technique

ALGORITHM DESCRIPTION

· 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. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on.

· The method is named after Julius Caesar, who used it in his private correspondence.

· The transformation can be represented by aligning two alphabets; the cipher alphabet is the plain alphabet rotated left or right by some number of positions.

· The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1, Z = 25.

· Encryption of a letter x by a shift n can be described mathematically as,

En(x) = (x + n) mod26

· Decryption is performed similarly,

Dn (x)=(x - n) mod26

PROGRAM

import java.io.*;

importjava.util.Scanner;

public class CaeserCipher

 {

public static void main(String[] args)

{            

Scanner s=new Scanner(System.in);

System.out.println("Input Data to encypt:");

        String str = s.nextLine();

System.out.println("Input the key");

int key =s.nextInt();

         String encrypted = encrypt(str, key);

System.out.println("Encrypted Data :" + encrypted);

        String decrypted = decrypt(encrypted, key);

System.out.println("Decrypted Data:" + decrypted);

    }

public static String encrypt(String str, int key)

    {

       String encrypted = "";

for(int i = 0; i <str.length(); i++)

        {

int c = str.charAt(i);

if (Character.isUpperCase(c))

            {                

                c = c + (key % 26);

if (c > 'Z')

                c = c - 26;

            }

else if (Character.isLowerCase(c))

            {

                c = c + (key % 26);

if (c > 'z')

                c = c - 26;

            }

encrypted += (char) c;

        }

return encrypted;

    }

public static String decrypt(String str, int key)

    {

        String decrypted = "";

for(int i = 0; i <str.length(); i++)

        {

int c = str.charAt(i);

if (Character.isUpperCase(c))

            {

                c = c - (key % 26);

if (c < 'A')

                c = c + 26;

           }

else if (Character.isLowerCase(c))

            {

                c = c - (key % 26);

if (c < 'a')

                c = c + 26;

            }

decrypted += (char) c;

        }

return decrypted;

    }

}

OUTPUT

Input Data to encrypt:

 Hello How wre you

Input the key

3

Encrypted data: khoor krz zuh brx

Decrypted data : hello how wre you