Décodage Cesar

cesar.c

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

/*
 *
 * N = [0 ; 25]
 *
 */
#define ALPHA_MAX	25

static char get_prev_cahr(char, int);
static char get_next_char(char, int);

static char *encrypt(const char *, int);
static char *decrypt(const char *, int);
static void bruteforce(const char *);

static const char *alpha = "abcdefghijklmnopqrstuvwxyz";

static char get_next_char(char c, int offset) {
	int pos = tolower(c) - 'a';

	/* Follow circular encoding, z -> a */
	if ((pos + offset) > ALPHA_MAX) {
		offset = (pos + offset) - ALPHA_MAX - 1;
		pos = 0;
	}

	return alpha[pos + offset];
}

static char get_prev_char(char c, int offset) {
	int pos = tolower(c) - 'a';

	/* Follow circular encoding, a -> z */
	if ((pos - offset) < 0) {
		offset = -(pos - offset) - 1;
		pos = ALPHA_MAX;
	}

	return alpha[pos - offset];
}

static char *encrypt(const char *src, int offset) {
	char *dst = NULL;
	int i = 0;

	dst = malloc(strlen(src) + 1);

	/* Don't encrypt non alpha chars */
	for (; *src != '\0'; i++, src++)
		dst[i] = isalpha(*src) ? get_next_char(*src, offset) : *src;

	dst[i] = '\0';

	return dst;
}

static char *decrypt(const char *src, int offset) {
	char *dst = NULL;
	int i = 0;

	dst = malloc(strlen(src) + 1);

	/* Don't encrypt non alpha chars */
	for (; *src != '\0'; i++, src++)
		dst[i] = isalpha(*src) ? get_prev_char(*src, offset) : *src;

	dst[i] = '\0';

	return dst;
}

static void bruteforce(const char *hash) {
	int offset = 0;
	char *msg = NULL;

	msg = malloc(strlen(hash) + 1);

	/* Test all encryption cases. */
	for (; offset <= ALPHA_MAX; offset++) {
		msg = decrypt(hash, offset);
		printf("N=%02d %s = %s\n", offset, hash, msg);
	}

	free(msg);
	msg = NULL;
}

int main(void) {
	const char *msg = "C wizardry is cool.";

	char *str = encrypt(msg, 2);

	printf("%s = %s\n", msg, str);

	char *rstr = decrypt(str, 2);

	printf("%s = %s\n", str, rstr);

	printf("Hash to crack: %s (encrypted with N=2)\n", str);
	bruteforce(str);

	free(str);
	str = NULL;

	free(rstr);
	rstr = NULL;

	return 0;
}