Question

shift and affine cipher program in Java ND c

Answer 1

Affine Cipher problem :

Affine cipher is a type of monoalphabetic substitution cipher,

wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which.

The whole process relies on working modulo m (the length of the alphabet used).

In the affine cipher, the letters of an alphabet of size m are first mapped to the integers in the range 0 … m-1.
The ‘key’ for the Affine cipher consists of 2 numbers, we’ll call them a and b. The following discussion assumes the use of a 26 character alphabet (m = 26).

A should be chosen to be relatively prime to m (i.e. a should have no factors in common with m).

Instructions while running code :

Please save file name as AffineCipher.java

Compile it using command javac AffineCipher.java

After successful compilation execute it using java AffineCipher

Java Source Code :

import java.util.Scanner;

public class AffineCipher

{

    public static String encryptionMessage(String Msg,int a,int b)

    {

        String CTxt = "";        

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

        {

// encryption going on here

            CTxt = CTxt + (char) ((((a * Msg.charAt(i)) + b) % 26) + 65);

        }

        return CTxt;

    }

    public static String decryptionMessage(String CTxt,int a,int b)

    {

        String Msg = "";        

        int a_inv = 0;

        int flag = 0;

        for (int i = 0; i < 26; i++)

        {

            flag = (a * i) % 26;

            if (flag == 1)

            {

                a_inv = i;

                System.out.println(i);

            }

        }

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

        {

// message decryption going on here

            Msg = Msg + (char) (((a_inv * ((CTxt.charAt(i) - b)) % 26)) + 65);

        }

        return Msg;

    }

    public static void main(String[] args)

    {

        Scanner sc = new Scanner(System.in);

int a,b;

        System.out.println("Enter the message: ");

        String message = sc.next();

        System.out.println("Enter values of a and b :");
        a=sc.nextInt();
        b=sc.nextInt();
        System.out.println("Message is :" + message);

        System.out.println("Encrypted Message is : "

                + encryptionMessage(message,a,b));

        System.out.println("Decrypted Message is: "

                + decryptionMessage(encryptionMessage(message,a,b)));

        sc.close();

    }

}

// end of java source code

C source code :

#include<stdio.h>
#include<string.h>
#include<ctype.h>
#include<stdlib.h>

int CalcGCD(int);
void main()
{
 int i,j,k,gcd,a,b,numstr[100],numcipher[100];
 char str[100],cipher[100];
 printf("Enter a string\n");
 gets(str);
 //converting entered string to Capital letters
 for(i=0,j=0;i<strlen(str);i++)
 {
  if(str[i]!=' ')
  {
   str[j]=toupper(str[i]);   
   j++;
  }
  else
  {
   str[j]=' ';
   j++;
  }
 }
 str[j]='\0';
 printf("Entered string is : %s \n",str);
 printf("Enter a value and must be between 1 and 25 both included\n");
 scanf("%d",&a); 
 //Checking consitions
 if(a<1 || a>25)
 {
  printf("a should lie in between 1 and 25\nSorry Try again !\n");
  exit(0);
 } 
 gcd=CalcGCD(a);
 if(gcd!=1)
 {
  printf("gcd(a,26)=1 but \n gcd(%d,26)=%d\nSorry Try again !\n",a,gcd);
  exit(0);
 }
 printf("Enter b value and must be between 0 and 25 both included\n");
 scanf("%d",&b);
 if(b<0 || b>25)
 {
  printf("b value should lie between 0 and 25\nSorry Try again !\n");
  exit(0);
 }
 //Conditions Over
 //Program Starts
 //Storing string in terms of ascii and to restore spaces I used -20
 for(i=0;i<strlen(str);i++)
 {
  if(str[i]!=' ')
  numstr[i]=str[i]-'A';
  else
  numstr[i]=-20;
 }
 //Ciphering Process
    //If numcipher is more than 25 .We need to convert and ensure that lie in between 0 and 25.(indicating Alphabets)
    //A-0,B-1,C-2,.....Y-24,Z-25
    printf("Affine Cipher text is\n");   
    for(i=0;i<strlen(str);i++)
    {
     if(numstr[i]!=-20)
     {
     numcipher[i]=((alpha*numstr[i])+b)%26;
          printf("%c",(numcipher[i]+'A'));         
     }
     else
     {
          printf(" ");      
     }
    }
printf("\n");
 
}

int CalcGCD(int a)
{
 int x;
   int temp1=a;
   int temp2=26;

     while(temp2!=0)
      {
        x=temp2;
        temp2=temp1%temp2;
        temp1=x;
      }
     return(temp1);
}

// end of cpp source code