Thread: decrypting

As I said before somewhere, the modulus operator is new to me so I struggle to work algebraically with it.

I input a number: 9876
I apply an encryption method that makes it: 6543

What I did was split that number into individual digits and applied the following: ( <number> + 7 ) % 10 = <answer>

Examples:
(6 + 7) % 10 = 3
(7 + 7) % 10 = 4

Now I am really struggling to reverse this for the decryption.

I have tried numerous ways and failed. How in the world do I input 3 to get 6 out as an answer? This must be a valid mathematical solution that I can apply in all cases. I thought decrypting it would be simple, obviously not.

Your suggestion makes sense. Here is the code, which does not decrypt correctly.

Code:

#include <iostream>
using namespace std;

// Prototype
void encrypt();
void decrypt();

int main()
{
   int choice;
   
   cout << "Digit ENCRYPTION & DECRYPTION Program By CC\n\n"
        << "Press 0 to quit\n"
        << "Press 1 to encrypt a 4-digit number\n"
        << "Press 2 to decrypt a number\n"
        << "Choice: ";
   cin >> choice;
   
   while ( choice != 0 )
   {
      switch (choice)
      {
         case 0:
                  break;
         case 1:
                  encrypt();
                  break;
         case 2:  
                  decrypt();
                  break;
         default:
                  cout << "Please enter a valid option...\n"
                       << "Choice: ";
                  cin >> choice;
                  break;
      }
      
      cout << "Press 0 to quit\n"
           << "Press 1 to encrypt a 4-digit number\n"
           << "Press 2 to decrypt a number\n"
           << "Choice: ";
      cin >> choice;
   }
   
   return 0;
}

void encrypt()
{
   int number;
   int split1, split2, split3, split4;
   
   cout << "\nEnter a four digit number to be encrypted: ";
   cin >> number;
   
   // Splits the numbers into indidual digits
   split1 = number % 10;
   number = number / 10;

   split2 = number % 10;
   number = number / 10;
   
   split3 = number % 10;
   number = number / 10;

   split4 = number % 10;
   
   // Resets the value of number for use later
   number = 0;

   // Let's now encrypt the numbers
   split1 += 7;
   split1 %= 10;

   split2 += 7;
   split2 %= 10;
   
   split3 += 7;
   split3 %= 10;
   
   split4 += 7;
   split4 %= 10;
   
   // Put the digits back together in a special order
   number += split3;
   number *= 10;
   number += split4;
   number *= 10;
   number += split1;
   number *= 10;
   number += split2;
   
   cout << "Encrypted number:\t" <<  number << "\n\n";
}

void decrypt()
{
   int number;
   int split1, split2, split3, split4;
   
   cout << "\nEnter a number to be decrypted: ";
   cin >> number;
   
   // Splits the numbers into indidual digits
   split1 = number % 10;
   number = number / 10;

   split2 = number % 10;
   number = number / 10;
   
   split3 = number % 10;
   number = number / 10;

   split4 = number % 10;
   
   // Resetting value of number
   number = 0;
   
   // Decrypting time
   if ( split1 < 7 )
   {
      split1 += 10;
      split1 -= 7;
   }
   
   if ( split2 < 7 )
   {
      split1 += 10;
      split1 -= 7;
   }
   
   if ( split3 < 7 )
   {
      split1 += 10;
      split1 -= 7;
   }
   
   if ( split4 < 7 )
   {
      split1 += 10;
      split1 -= 7;
   }
   
   // Rearrange numbers and join them back together
   number += split3;
   number *= 10;
   number += split4;
   number *= 10;
   number += split1;
   number *= 10;
   number += split2;
   
   cout << "Decrypted number:\t" <<  number << "\n\n";
}

Gotcha. My first attempt was to simply subtract 7 from the numbers and use absolute values to make sure that the numbers stay positive. It only worked for a few, which I now realized are for the one's greater than 7. At least you've shown me how to solve problems like this in the future.

Much appreciated.

Once you get this working, you might want to take a look at more advanced cryptography. It's a pretty interesting subject.

I would definitely like to dip my toe into this area. First let me finish going through my recently purchased Deitel book and from there I'll look into other more advanced areas.

What would you suggest CompiledMonkey?

try this code to decript 6543 to 9876.

Code:

#include<iostream>
#include<conio.h>

using std::cout;
using std::cin;
using std::endl;

int main()
{
    // declare variables
    
    int num,digit1,digit2,digit3,digit4,dcript1,dcript2,dcript3,dcript4;
    
    // initialize variables
    
    dcript1=0,dcript2=0,dcript3=0,dcript4=0;
    
    // read integer
    
    cout<<"\nEnter the integer to be decripted:\t";
    cin>>num;
    
    // separate integers
    
    digit1=num%10;
    num/=10;
    digit2=num%10;
    num/=10;
    digit3=num%10;
    num/=10;
    digit4=num%10;
    num/=10;
    
    // print decripted digit
    
    if((digit1<7||digit2<7)||(digit3<7||digit4<7))
    {
        dcript1=(10+digit1)-7;
        dcript2=(10+digit2)-7;
        dcript3=(10+digit3)-7;
        dcript4=(10+digit4)-7;
        
        cout<<"\nDecripted 4-digit integer:\t\t";
        cout<<dcript4<<dcript3<<dcript2<<dcript1;
    }
    else
        dcript1=(digit1-7)%10;
        dcript2=(digit2-7)%10;
        dcript3=(digit3-7)%10;
        dcript4=(digit4-7)%10;
           
getch();
}