Question

I would like to XOR a very big file (~50 Go).

More precisely, I would like to do so by XORing each block of 32 bytes of a plaintext file (because of lack of memory) with the key 3847611839 and create (block after block) a new cipher file.

Thank You for any help!!

Answer 1

This sounded like fun, and doesn’t sound like a homework assignment.

I don’t have a previously xor-encrypted file to try with,but if you convert one back and forward, there’s no diff.

That I tried atleast. Enjoy! 🙂 This xor’s every 4 bytes with 0xE555E5BF, I presume that’s what you wanted.

Here’s bloxor.c

// bloxor.c - by Peter Boström 2009, public domain, use as you see fit. :)

#include <stdio.h>

unsigned int xormask = 0xE555E5BF; //3847611839 in hex.

int main(int argc, char *argv[])
{
    printf("%x\n", xormask);
    if(argc < 3)
    {
        printf("usage: bloxor 'file' 'outfile'\n");
        return -1;
    }

    FILE *in = fopen(argv[1], "rb");
    if(in == NULL)
    {
        printf("Cannot open: %s", argv[2]);
        return -1;
    }

    FILE *out = fopen(argv[2], "wb");

    if(out == NULL)
    {
        fclose(in);
        printf("unable to open '%s' for writing.",argv[2]);
        return -1;
    }
    char buffer[1024]; //presuming 1024 is a good block size, I dunno...

    int count;

    while(count = fread(buffer, 1, 1024, in))
    {
        int i;
        int end = count/4;
        if(count % 4)
            ++end;

        for(i = 0;i < end; ++i)
        {
            ((unsigned int *)buffer)[i] ^= xormask;
        }
        if(fwrite(buffer, 1, count, out) != count)
        {
            fclose(in);
            fclose(out);

            printf("cannot write, disk full?\n");

            return -1;
        }
    }

    fclose(in);
    fclose(out);

    return 0;
}