Question

My code comparing 2 file sizes seems to behave as if keyfile > sourcefile. Is there something missing from the code below and what can I change? The two files I’m using for testing are 3kb keyfile and 14kb sourcefile, which should activate the last if statement provided below.

/* Get size of sourcefile. */
if((sourcefile = fopen(argv[1], "rb"))== NULL)
{
printf("Can't open source file.\n");
printf("Please enter a valid filename.\n");
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return (1);
}

fflush(sourcefile);
fstat(fileno(sourcefile), &statbuf);
fclose(sourcefile);

/* Get size of keyfile. */
if((keyfile = fopen(argv[3], "rb"))== NULL)
{
printf("Can't open keyfile.\n");
printf("Please enter a valid filename.\n"); 
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return(1);

fflush(keyfile);
fstat(fileno(keyfile), &keybuf);
fclose(keyfile);
}

/* Open necessary files. */
keyfile=fopen(argv[3], "rb");
sourcefile=fopen(argv[1], "rb");
destfile=fopen(argv[2], "wb");

/* Check if keyfile is the same size as, or bigger than the sourcefile */
if((keybuf.st_size) < (statbuf.st_size))
{
printf("Source file is larger than keyfile.\n");
printf("This significantly reduces cryptographic strength.\n");
printf("Do you wish to continue? (Y/N)\n");
scanf("%c", &ans);
if(ans == 'n' || ans == 'N')
    {
    return (1);
    }
if(ans == 'y' || ans == 'Y')
    {
    printf("Proceeding with Encryption/Decryption.\n");
    }
}   

Answer 1

It’s because you only fstat the keyfile if it fails to open. Move your curly brace up. Also, as an optimisation of forms, don’t open the file, stat it, close it and re-open it. Just don’t close it and carry on.

So, since you asked

/* Get size of sourcefile. */
if((sourcefile = fopen(argv[1], "rb"))== NULL)
{
printf("Can't open source file.\n");
printf("Please enter a valid filename.\n");
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return (1);
}

fflush(sourcefile);
//fstat(fileno(sourcefile), &statbuf);   // <-- this is not needed
//fclose(sourcefile);                    // <-- this is not needed

/* Get size of keyfile. */
if((keyfile = fopen(argv[3], "rb"))== NULL)
{
printf("Can't open keyfile.\n");
printf("Please enter a valid filename.\n"); 
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return(1);
    }                                     // <-- this brace is new (well, moved) (1)

fflush(keyfile);
//fstat(fileno(keyfile), &keybuf);    // <-- not needed
//fclose(keyfile);                    // <-- not needed
//}                                     // <-- this brace has moved up 4 lines to (1)

/* Open necessary files. */
keyfile=fopen(argv[3], "rb");
sourcefile=fopen(argv[1], "rb");
destfile=fopen(argv[2], "wb");

/* Check if keyfile is the same size as, or bigger than the sourcefile */
if((keybuf.st_size) < (statbuf.st_size))
{
printf("Source file is larger than keyfile.\n");
printf("This significantly reduces cryptographic strength.\n");
printf("Do you wish to continue? (Y/N)\n");
scanf("%c", &ans);
if(ans == 'n' || ans == 'N')
    {
    return (1);
    }
if(ans == 'y' || ans == 'Y')
    {
    printf("Proceeding with Encryption/Decryption.\n");
    }
}