xorer.c | gcc -O2 -o xorer xorer.c

/*  by xerpi (c) 2013 */

#include <stdio.h>
#include <stdlib.h>

typedef unsigned char byte;

void print_usage();
int xorer(char* filename_input, char *key_file, char* filename_output);
int xordata(byte *input, byte* key, byte* output, unsigned int size);

int main(int argc, char *argv[])
{
    if(argc < 3) {
        print_usage();
    } else if(argc == 3) {
        if(!xorer(argv[1], argv[2], "xored.bin")) printf("error\n");
    } else {
        if(!xorer(argv[1], argv[2], argv[3])) printf("error\n");
    }

    return 0;
}

void print_usage()
{
    printf("usage: xorer file keyfile [output]\n");
}


int xorer(char* filename_input, char *key_file, char* filename_output)
{
    byte *input_buf, *output_buf, *key_buf;
    FILE *finput, *fkey, *foutput;
    int input_readed, key_readed, steps, key_len, input_len, i;

    if(!(finput  = fopen(filename_input, "rb")))  return 0;
    if(!(fkey    = fopen(key_file, "rb")))        return 0;
    if(!(foutput = fopen(filename_output, "wb"))) return 0;

    //Get key length
        fseek(fkey, 0, SEEK_END);
        key_len = ftell(fkey);
        fseek(fkey, 0, SEEK_SET);

    //Get input file length
        fseek(finput, 0, SEEK_END);
        input_len = ftell(finput);
        fseek(finput, 0, SEEK_SET);

    //Calculate number of for cycles to do
        steps = (int)(input_len/key_len);
        if(input_len%key_len) steps++;

    //Allocate buffers
        key_buf    = (byte *)malloc(key_len * sizeof(byte));
        input_buf  = (byte *)malloc(key_len * sizeof(byte));
        output_buf = (byte *)malloc(key_len * sizeof(byte));
    //Read key
        key_readed = fread(key_buf, 1, key_len, fkey);

    //XOR data
        for(i = 0; i < steps; i++) {
            input_readed = fread(input_buf, 1, key_len, finput);
            xordata(input_buf, key_buf, output_buf, input_readed);
            fwrite(output_buf, 1, input_readed, foutput);
        }

    fclose(finput);
    fclose(fkey);
    fflush(foutput);
    fclose(foutput);
    free(key_buf);
    free(input_buf);
    free(output_buf);
    return 1;
}

int xordata(byte *input, byte* key, byte* output, unsigned int size)
{
    int i;
    for(i = 0; i < size; i++) {
        output[i] = input[i] ^ key[i];
    }
}