A3/permutation_substitution_code.ipynb

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 40,
   "metadata": {},
   "outputs": [],
   "source": [
    "s = 'qmnjvsa nv wewc flct vprj tj tvvplvl fv xja vqildhc xmlnvc nacyclpa fc gyt vfvw. fv wgqyp, pqq pqcs y wsq rx qmnjvafy cgv tlvhf cw tyl aeuq fv xja tkbv cqnsqs. lhf avawnc cv eas fuqb qvq tc yllrqr xxwa cfy. psdc uqf avrqc gefq pyat trac xwv taa wwd dv eas flcbq. vd trawm vupq quw x decgqcwt, yq yafl vlqs yqklhq! snafq vml lhvqpawr nqg_vfusr_ec_wawy qp fn wgawdgf.'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "punc = ''' _.,!'''\n",
    "text=\"\"\n",
    "# Removing punctuations and space in string\n",
    "for ele in s:\n",
    "    if ele not in punc:\n",
    "        text+=ele\n",
    "         \n",
    "# printing result\n",
    "print(text)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 42,
   "metadata": {},
   "outputs": [],
   "source": [
    "# To permute the text \n",
    "def permutation_cipher(permutation, encrypted_text):\n",
    "    permutated_text = \"\"\n",
    "    encrypted_text = encrypted_text[:len(encrypted_text) - (len(encrypted_text)%len(permutation)) ] #truncating the encrypted_text\n",
    "\n",
    "    for block in range(0, len(encrypted_text), len(permutation)):\n",
    "        for pos in permutation:\n",
    "            permutated_text += encrypted_text[block+(pos-1)]\n",
    "    return permutated_text\n",
    "\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 43,
   "metadata": {},
   "outputs": [],
   "source": [
    "from itertools import permutations\n",
    "\n",
    "# For block size = 4, trying all permutations\n",
    "\n",
    "for i in list(permutations(range(1, 5))): # list of permutations of [1, 2, 3, 4]\n",
    "    x = permutation_cipher(i, text)\n",
    "    password = (x[(text.find('lhvqpawr')):(text.find('lhvqpawr') + len('lhvqpawr'))]) #\"password\"\n",
    "    if password[2] == password[3]:\n",
    "        print(password)\n",
    "        print(i)\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# For block size = 5, trying all permutations\n",
    "\n",
    "for i in list(permutations(range(1, 6))): # permutations of [1, 2, 3, 4, 5]\n",
    "    x = permutation_cipher(i, text)\n",
    "    password = (x[(text.find('lhvqpawr')):(text.find('lhvqpawr') + len('lhvqpawr'))]) #\"password\"\n",
    "    if password[2] == password[3]:\n",
    "        print(password)\n",
    "        print(i)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "possible_permutation = []\n",
    "\n",
    "for i in list(permutations(range(1, 6))): # permutations of [1, 2, 3, 4, 5]\n",
    "    x = permutation_cipher(i, text)\n",
    "    password = (x[(text.find('vmllhvqpawr')):(text.find('vmllhvqpawr') + len('vmllhvqpawr'))]) #\"thepassword\"\n",
    "    if password[5] == password[6] and len(set(x[(text.find('vml')):(text.find('vml') + len('vml'))]).intersection(x[(text.find('lhvqpawr')):(text.find('lhvqpawr') + len('lhvqpawr'))])) == 0:\n",
    "        print(password)\n",
    "        print(i)\n",
    "        possible_permutation.append(i)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "cipher = []\n",
    "\n",
    "for i in possible_permutation:\n",
    "    x = permutation_cipher(i, text)\n",
    "    four_letters = (x[(text.find('qpfn')):(text.find('qpfn') + len('qpfn'))]) # \"go to\" or \"to go\" mapped to \"qp fn\"\n",
    "    if four_letters[1] == four_letters[3] and four_letters[0] != four_letters[2]:\n",
    "        print(four_letters)\n",
    "        print(i)\n",
    "        cipher.append(i)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 47,
   "metadata": {},
   "outputs": [],
   "source": [
    "permutated_text = permutation_cipher(cipher[0], text)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(permutated_text)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "permutated_text[(text.find('vmllhvqpawr')):(text.find('vmllhvqpawr') + len('vmllhvqpawr'))] "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 50,
   "metadata": {},
   "outputs": [],
   "source": [
    "# This is mapped to \"thepassword\"\n",
    "# So we got the initial mapping \n",
    "# a = t\n",
    "# f = h\n",
    "# v = e\n",
    "# h = p\n",
    "# q = a\n",
    "# l = s\n",
    "# r = w\n",
    "# w = o\n",
    "# n = r\n",
    "# p = d"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 51,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Mapping after mono-substitution\n",
    "mapping = {\n",
    "'a' : 't',\n",
    "'b' : 'v',\n",
    "'c' : 'i',\n",
    "'d' : 'u',\n",
    "'e' : 'c',\n",
    "'f' : 'h',\n",
    "'g' : 'g',\n",
    "'h' : 'p',\n",
    "'i' : 'q',\n",
    "'j' : 'b',\n",
    "'k' : 'z',\n",
    "'l' : 's',\n",
    "'m' : 'k',\n",
    "'n' : 'r',\n",
    "'p' : 'd',\n",
    "'q' : 'a',\n",
    "'r' : 'w',\n",
    "'s' : 'f',\n",
    "'t' : 'l',\n",
    "'u' : 'm',\n",
    "'v' : 'e',\n",
    "'w' : 'o',\n",
    "'x' : 'y',\n",
    "'y' : 'n'\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "decrypted_text = ''.join(mapping.get(ch, ch) for ch in permutated_text)\n",
    "\n",
    "print(decrypted_text)"
   ]
  }
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