Saving to a file from an array in C ?
 

Hi there,

I'm trying to write a simple encryption program that reads normal text from a txt file - encrypts it using a Vigenère Table - then outputs the results to another file. This process should obviously be reversible as well.

I have written this for the encryption part of the program:

 

            input = fopen(argv[3], "r");

            output = fopen(argv[4], "w");

    

          flag = 0;



               while(flag==0)

                {

                   for(i=0;i<MAX;i++)

                    {

                         if((c = getc(input)) != EOF)

                         {

                           file[i] = c; 

                         }

                         else

                         {

                           file[i] = -1;

                           break;

                         }

                            

                    }

                

                  encrypt(key,file,cipher);

    

                   for(i=0;i<MAX;i++)

                   { 

                        if(cipher[i]!=-1)

                        {

                        fprintf(output,"%c",cipher[i]);

                        }

                        else

                        {

                         fprintf(output,"%c",cipher[i]);

                         i = MAX;

                         flag = 1;  //this a flag that exits a while loop not included in this code.

                        } 

                   }        

                }  

                fclose(input);

                fclose(output);

The 'encrypt' function looks like this:

 void encrypt(char *key,char *plainTxt,char *cipherTxt)

{

 int i,vig[MAX][MAX];

 char a,b;



 createVig(vig);            // Creates a standard Vig 128 x 128 table 

  

     for(i=0;i<MAX;i++)         // Loops for 128 characters

     {

      a = key[i];               // Assigns the value of key[i] to a    

      b = plainTxt[i];

      

      if(b!=-1)

      {            

      cipherTxt[i] = vig[a][b]; // Finds the new cipherTxt character and assigns it to cipherTxt[i]        

      }

      else

      {

      cipherTxt[i]=-1;

      i = MAX;

      }

     } 

}

I decrypt in almost the same way with a few differences. The problem im having is when I decrypt from the cipher text file it only gets about 200 characters in and then finds an EOF (-1) in the text file although there isnt one there.

I tested my algorithms by encrypting and decrypting without writing them to a file and they worked fine. I believe this has narrowed it down to how it is written to the file however, I don't know of any other ways in which to do it.

I hope that made sense to some one out there. Any help would be greatly appreciated.

Many Thanks

Craig

Re: Saving to a file from an array in C ?
 

Try reading and writing in binary mode

I know from experience, that encryption programs have problems if you're using just .txt files.

Reading and writing in binary mode with a .dat file worked for me.

Re: Saving to a file from an array in C ?
 

Since only you know how the encryption works, we can only guess. And my guess is:
If your encryption changes a valid character to 0x1A, you get EOF in your file. To solve this, change your encrypted file to a binary file instead of text and that should solve your problem.

Re: Saving to a file from an array in C ?
 

Hiya,

Thanks for your replies. I can say that i did try changing the extension to '.dat' however, I didn't realize that you could change the way the file is written. How do I change to writing in binary mode?

Regards

Craig

Re: Saving to a file from an array in C ?
 

First of all you open the file in binary mode:

input = fopen(argv[3], "rb");
output = fopen(argv[4], "wb");

and then using fwrite function.

Re: Saving to a file from an array in C ?
 

Thanks for all the help...

I'm new to the fread and fwrite functions however, i have managed to get most of it working to try and keep it simple for testing i have made this algorithm.

 while(fread(file,128,1, input))

                 {

                      

                       encrypt(key,file,cipher); //function that encrypts 128 bits at a time

                       

                       decrypt(key,plain,cipher);//function that decrypts128 bits at a time



                      fwrite(plain, 128, 1,output);



                }

this works however,cuts about 100 bits of the end. I thought it was because the while loop exits before the last bytes have been written so i added a fwrite outside of the loop but it writes the full 128 bites that were in the array. Is there a way of finding out how many bytes there are left before it reached the EOF marker so that i can set a variable? Or have a gone completely in the wrong direction and there is a super easy way to make this work?

thanks again.

Re: Saving to a file from an array in C ?
 

Try something like this after the loop:

printf("current position = %d\n", curr = ftell(file));

fseek(file, SEEK_END, 1); /* go to end of file */

printf("last position = %d\n", last = ftell(file));

printf("left = %d\n", last - curr);

Re: Saving to a file from an array in C ?
 

Thanks Andor and everybody else..... got it working now.. much appreciated.