cracker.c

#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <assert.h>

#include "misc.h"
#include "filtered_string.h"
#include "mfreq_analysis.h"
#include "kasiski.h"
#include "cracker.h"



/* Initialize a struct cracker. */
void ck_init(struct cracker *state, struct fs_ctx *str) {
	memset(state, 0, sizeof(*state));
	state->str = str;

	state->ka_minlen = 3;
}



/* Free everything one of the ck_* function allocated */
void ck_fini(struct cracker *state) {
	if (state->key != NULL)
		free(state->key);

	if (state->ka_done)
		ka_fini(&state->ka);

	if (state->mfa_done)
		mfa_fini(&state->mfa);

	memset(state, 0, sizeof(*state));
}



void ck_set_length(struct cracker *state, size_t len) {
	state->klen = len;

	if (state->key != NULL)
		free(state->key);

	state->key = malloc((len + 1) * sizeof(*state->key));
	if (state->key == NULL)
		system_error("malloc");

	memset(state->key, 0, (len + 1) * sizeof(*state->key));
}



void ck_length(struct cracker *state) {
	size_t i, nlen;
	size_t bestlength, bestscore;

	nlen = strlen(state->str->norm);
	if (nlen < 3)
		custom_error("Can't break key length of a text with only %lu "
		             "signficant characters", nlen);

	/* Restart the kasiski analysis if specifically asked to. */
	if (state->ka_done)
		ka_fini(&state->ka);

	ka_init(&state->ka, state->str->norm, state->ka_minlen);
	ka_analyze(&state->ka);

	bestlength = 2;
	bestscore = state->ka.score[bestlength];

	for (i = 3; i < nlen; i++) {
		size_t score = state->ka.score[i];
		if (score > bestscore) {
			bestscore = score;
			bestlength = i;
		}
	}

	ck_set_length(state, bestlength);
	state->ka_done = 1;
}



static void key_from_mfa_shift(struct cracker *state) {
	size_t i;
	const struct charset *cs;

	assert(state->mfa_done);

	cs = state->str->charset;

	for (i = 0; i < state->klen; i++) {
		/* Substract because we're undoing a Vigenère. */
		size_t j = cs->length - state->mfa.shift[i];
		state->key[i] = cs_chr(cs, j % cs->length);
	}
}



void ck_freq(struct cracker *state) {
	if (state->klen == 0)
		custom_error("Call either ck_length or ck_set_length "
		             "before calling ck_freq.");

	/* Restart the multi-freq analysis if specifically asked to. */
	if (state->mfa_done)
		mfa_fini(&state->mfa);

	mfa_init(&state->mfa, state->str->norm, state->klen, state->str->charset, freq_en);
	state->mfa_done = 1;

	mfa_analyze(&state->mfa);
	key_from_mfa_shift(state);
}



void ck_crack(struct cracker *state) {
	if (state->klen == 0)
		ck_length(state);

	if (!state->mfa_done)
		ck_freq(state);
}