Question

So I’m trying to making a file encrypting program in C (by the way I’m kind of new to C) so I wrote this simple XOR file encryption code:

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

int main()
{
char fileName1[35] = {'\0'};        //Name of original file
char fileName2[35] = {'\0'};        //Name of encrypted file
char keyString[20] = {'\0'};        //Key determines result encryption
FILE* originalFile;                 //File to be encrypted
FILE* cryptedFile;                  //The encrypted file
int c;                              //byte read from original file


printf("Enter file location followed by name of the file you want to encrypt: ");
scanf("%s", fileName1);

printf("Enter file location followed by name of the encrypted file: ");
scanf("%s", fileName2);

printf("Enter your key (Encryption changes based on Key): ");
scanf("%s", keyString);

originalFile = fopen(fileName1, "rb"); //rb to read file bytes
cryptedFile = fopen(fileName2, "wb");  //wb to write bytes to file


if(originalFile != NULL && cryptedFile != NULL){
    while( (c = getc(originalFile)) != EOF ){
        int x;
        for(x=0; x<strlen(keyString); x++){
            c ^= keyString[x];
            putc(c, cryptedFile);
        }
    }
    fclose(originalFile);
    fclose(cryptedFile);
}

return 0;
}

So to test this program I created a file called file1.txt and run the encryption program giving it the second file as file2.txt and key as secret. Then I ran the program again but this time on the encrypted file2.txt and created a file3.txt with the same key secret. Since it was the same key the file3.txt should be the same as file1.txt but file3.txt has random contents in it. So what am I doing wrong?

Answer 1

Your program outputs too much data; try checking the sizes of file1.txt, file2.txt and file3.txt. The problem in is this section:

while( (c = getc(originalFile)) != EOF ){
        int x;
        for(x=0; x<strlen(keyString); x++){
            c ^= keyString[x];
            putc(c, cryptedFile);
        }
    }

There’s two nested loops, so the whole inner loop is executed for each input character. Try a new c = getc() inside the inner loop and break out of both loops if EOF is reached, or flatten the loops into one, using a c ^= keyString[(x++) % strlen(keyString)];.