Question

Playing with encryption:

Write a program that will read a four-digit integer entered by the user and encrypt it as follows: Replace each digit with the result of adding 7 to the digit and getting the remainder after dividing the new value by 10. Then swap the second digit with the fourth. Finally, print the original number and its encrypted one. Now reverse the process. Read an encrypted integer and decrypt it by reversing the algorithm to obtain the original number. Print out both the encrypted and decrypted integer.

Answer 1

Programming Language is not Mentioned so I am doing it in c :

C Language Code:

#include<stdio.h>
int main(){
   int num,rem,option,dNum,oNum=0,n=1000,sec,four;
   //variables for storing the number,remainder , reading option and duplicate number dNum for storing num output number oNum
   printf("Enter 1.Encryption 2.Decryption");
   //reading which thing we do encryption or decryption
   scanf("%d",&option);
   printf("Enter four digit Number");
   scanf("%d",&num);
   dNum=num;
   if(option==1){
       while(dNum!=0){
           //getting the first number and add 7 to it and get remainter after that when divided by 10
           rem=dNum/n;
           rem=(rem+7)%10;
           oNum=oNum*10+rem;
           dNum=dNum%n;
          
           if(n==100){
               //storing second number
               sec=rem;
           }else if(n==1){
               //storing fourth number
               four=rem;
           }
           n=n/10;
       }
           printf("%d\t %d",sec,four);
       printf(" Number is %d\n",oNum);
       //making the second number zero and making fourth number there in that place
       oNum=oNum-(sec*100)+(four*100);
       //making the fourth number zero and making second number there in that place
       oNum=oNum-(four*1)+sec;
      
       printf("Original Number %d\n",num);
       printf("Encrypted Number is %d",oNum);
      
      
   }else{
           while(dNum!=0){
           //getting the first number and add 7 to it and get remainter after that when divided by 10
           rem=dNum/n;
           //reason for adding the +3 and substracting seven is explained in the pic
           if(rem>=7)
           rem=(rem-7)%10;
           else{
               rem=rem+3;
           }
           oNum=oNum*10+rem;
           dNum=dNum%n;
      
           if(n==100){
               //storing second number
               sec=rem;
           }else if(n==1){
               //storing fourth number
               four=rem;
           }
               n=n/10;
}
       //making the second number zero and making fourth number there in that place
       oNum=oNum-(sec*100)+(four*100);
       //making the fourth number zero and making second number there in that place
       oNum=oNum-(four*1)+sec;
      
       printf("Decrypted Number %d\n",num);
       printf("Original Number is %d",oNum);
      
   }
   return 0;
}

Reason for adding +3 and substracting -7 Explanation:

OUTPUT:

DECRYPTION OUTPUT: