crypto100 decoder

decoder.c

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

int main(int argc, char **argv) {
	FILE* input;
	FILE* output;
	char k[] = "VeryLongKeyYouWillNeverGuess";
	char c, p, t = 0;
	int i = 0;
	if (argc != 3) {
		printf("USAGE: %s INPUT OUTPUT\n", argv[0]);
		return 0;
	}

	input = fopen(argv[1], "rb");
	output = fopen(argv[2], "wb");
	i = 0;

	p = fgetc(input) & 0xFF;
	//printf("%0X", p);
	c = p - k[0];
	t = c;
	fputc(c, output);
	i++;
	while ((p = fgetc(input)) != EOF) {

		c = (p - (k[i % strlen(k)] ^ t) - i*i) & 0xff;;
		t = c;
		i++;
		fputc(c, output);
	}
	return 0;
}