codebreakparty/test.cpp

#include <algorithm>
#include <array>
#include <cmath>
#include <cstring>
#include <fstream>
#include <iostream>
#include <queue>
#include <random>
#include <stdio.h>
#include <stdlib.h>
#include <tuple>

#include "markov.hpp"

using Corpus = Markov<char, 6>;

/* Manage a lookup table for monoalphabetic substitution */
struct Mapping {
  char map[26]{};
  Mapping() = default;

  char &operator[](size_t i) { return map[i]; }

  constexpr auto begin() { return &map[0]; }

  constexpr auto cbegin() const { return &map[0]; }

  constexpr auto cend() const { return &map[26]; }

  /* Create the LUT. Maps all characters except a-z to themselves, uses map for
     the rest */
  constexpr void create_map(char *output) const {
    for (size_t i = 0; i < 256; i++) {
      output[i] = i;
    }
    for (size_t i = 0; i < 26; i++) {
      output['a' + i] = map[i];
      output['A' + i] = map[i];
    }
  }

  constexpr auto end() { return &map[26]; }

  /* Swap two output characters */
  bool swap(char a, char b) {
    auto left = std::find(begin(), end(), a);
    auto right = std::find(begin(), end(), b);
    if (left != end() && right != end()) {
      std::swap(left, right);
      return true;
    }
    return false;
  }
};
class Decrypt {
  /* Markov model of the plaintext */
  Corpus corpus;

  /* Cipher text */
  char const *cipher_text;

  /* Temporary buffer for decryption attempts */
  char *plain_text;

  /* Length of the cipher text */
  size_t len;

public:
  Decrypt(char const *corpus, char const *data)
      : corpus(create_corpus(corpus)), cipher_text(data),
        plain_text(static_cast<char *>(malloc(strlen(data) + 1))),
        len(strlen(cipher_text)) {
    plain_text[strlen(data)] = 0;
  }

  ~Decrypt() { free(plain_text); }

  Mapping find_map(size_t tries, size_t iterations) {
    Mapping map;
    double best_score = INFINITY;

    while (tries--) {
      Mapping m;
      double s;
      std::tie(m, s) = random_map(iterations);

      if (s < best_score) {
        map = m;
        best_score = s;
        printf("New best try, character score is %g:\n",
               best_score / double(len));
        printf("  ");
        for (uint32_t i = 0; i < 26; i++) {
          putchar(toupper(map[i]));
        }
        printf("\n");
        printf("  %s\n", apply_map(m));
      }
    }

    return map;
  }

  char const *apply_map(Mapping const &m) {
    char map[256];
    m.create_map(&map[0]);
    std::transform(&cipher_text[0], &cipher_text[len], &plain_text[0],
                   [map](unsigned char a) { return map[a]; });

    return plain_text;
  }

protected:
  Corpus create_corpus(char const *file) {
    Corpus m;
    std::ifstream corpus{file};

    char buf[128];
    for (auto i = 0u; i < m.order(); i++) {
      buf[i] = ' ';
    }

    while (corpus.good()) {
      corpus.read(&buf[m.order()], sizeof(buf) - m.order());
      auto buflen = m.order() + corpus.gcount();

      for (size_t i = 0; i < (buflen - m.order() - 1u); i++) {
        m.add(&buf[i], m.order() + 1);
      }
    }

    return m;
  }

  double rate(Mapping const &m, int max_order = -1) const {
    const auto order =
        (-1 == max_order) ? corpus.order() : static_cast<size_t>(max_order);
    double score = 0.;
    char map[256];
    m.create_map(&map[0]);
    std::transform(&cipher_text[0], &cipher_text[len], &plain_text[0],
                   [map](unsigned char a) { return map[a]; });
    for (size_t i = 0, valid = 0; i < len; i++) {
      if (plain_text[i]) {
        valid++;
        if (valid == (order + 1)) {
          score += log(corpus.probability(&plain_text[i - valid], valid));
        } else if (valid > (order + 1)) {
          score += log(
              corpus.final_probability(&plain_text[i - order - 1], order + 1));
        }
      } else {
        if (valid && (valid <= order)) {
          score += log(corpus.probability(&plain_text[i - valid], valid));
        }
        valid = 0;
      }
    }

    return -score;
  }

  std::pair<Mapping, double> random_map(size_t iterations) {
    Mapping m;
    double current_score{INFINITY};
    for (uint32_t i = 0; i < 26; i++) {
      m[i] = 'a' + i;
    }

    std::random_device rd;
    std::mt19937 g(rd());
    do {
      std::shuffle(&m.map[0], &m.map[26], g);
      current_score = rate(m);
    } while (std::isinf(current_score));

    std::uniform_int_distribution<size_t> char_index{0, 25};
    std::uniform_real_distribution random_float;

    int order = 0;
    for (uint32_t i = 0; i < iterations; i++) {
      Mapping m2 = m;
      std::swap(m2[char_index(g)], m2[char_index(g)]);

      int new_order = (corpus.order() * (1 + i) + iterations - 1) / iterations;
      if (new_order != order) {
        current_score = rate(m);
        order = new_order;
      }

      auto new_score = rate(m2, order);
      double diff = current_score - new_score;
      if ((diff > 0) || (random_float(g) <
                         std::exp(diff * double(1 + i) / double(iterations)))) {
        current_score = new_score;
        m = m2;
      }
    }

    return std::pair(m, rate(m));
  }
};

int main(int argc, char const **argv) {
  if (argc < 3) {
    printf("Usage: %s corpus ciphertext\n", argv[0]);
    exit(1);
  }
  printf("Cipher text: %s\n", argv[2]);
  Decrypt d(argv[1], argv[2]);
  Mapping map = d.find_map(50, 1000);
  printf("%s\n", d.apply_map(map));

  return 0;
}