Question

Write a cryptographic program that works similarly to my crypto program:• Prompt the user to enter a secret key and use it to compute the seed of a random number generator • Prompt the user to give the names of an input file and the output coded/decoded file• Create the sequence of random bytes from a random number generator with the seed obtained from user’s secret key. • Perform the coding of bit x using a random bit r as follows: x⊕ = r • Since ∀r ∈{0,1}, r ⊕r = 0, perform decoding using the same sequence of random bytes and the same operation x⊕ = r . Decoding is based on the operation x⊕r ⊕r = x⊕(r ⊕r) = x⊕0 = x

Below is the code that works for encryption but when it come to decryption, the cipher text contains only 3 characters. I have not been able to figure why the decoding does not work. I am using Dev-C++. Your help will be very appreciated.

  #include<iostream>
  #include<conio.h>
  #include<math.h>
  #include<stdlib.h>
  #include<fstream>

  using namespace std;


  int main()
  {   
    char  n, line[1024],  keystring[100]; char FitoEncr[100], NewFiCrypt[100];
    char FiletobeDecrypted[100];
    ifstream IS ("FitoEncr.txt", ios::in);
    ofstream OS ("NewFiCrypt.txt", ios::out); 

    unsigned int psswd=0, number;
    cout<<"Please, enter a secret key :";
    cin.getline(keystring, 100);
    for( int i=0;keystring[i]!='\0'; i++)

      psswd=(psswd+3)*keystring[i];
    cout<<"initial password: "<<keystring<<endl;
    cout<<"encrypted password: "<<psswd<<endl;

    cout<<"please, enter the name of the input file: ";
    cin.getline(FitoEncr,20);
    cout<<"please, enter the name of the output file: ";
    cin.getline(NewFiCrypt,20);

    srand(psswd);    //not sure about creating the sequence of random bytes from a...
    number=rand() % 255; //random number generator with the seed obtained from user's secret key

   //ENCRYPTION

    while(IS.get(n))
     {
         if(IS.get(n))
         {      
           n^=rand();
           OS<<n;
           cout<<"character coded:"<<n;   
         }

     }
     IS.close();
     OS.close();

    //DECRYPTION

     ifstream IS1;
     ofstream OS1; 

     IS1.open("NewFiCrypt.txt", ios::in);
     OS1.open("FilDecr.txt", ios::out); 

     while(IS1.get(n))
     {
       if(IS1.get(n))
       {      
         n^=rand();  //I think the problem starts on this line...
         OS1<<n;
         cout<<"character coded:"<<n;   
       }

     }       
     IS1.close();
     OS1.close();     

     getch();

     return 0;
  }

Answer 1

You seed the random number generator before encrypting, but not before decrypting, so the series of random numbers will be different; obviously they would have to be the same for this to work. If you add another

srand(psswd);  

before the decryption second, that will get you closer.

On closer inspection, though, there are some significant other problems. For example, each time you call get(n), you consume a character from your input file. You’re calling get(n) in your while loop conditions, but then immediately calling it again in your if; that means you’ll effectively be skipping every other character on input and again on output; the decrypted file would then end up with just 25% of the characters from the original file! Just remove the if statements altogether, and your loop will be right. Hopefully having fixed both of these things your program will work.