{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import string\n",
    "import itertools"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def sanitised(text):\n",
    "    return ''.join(letter.lower() for letter in text if letter in string.ascii_letters)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'thequickbrownfoxjumpedoverthelzydoga'"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "sanitised('The quick brown fox! jumped OVER the l\\a!z%y* dog....a')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def encipher_letter(previous, key, current):\n",
    "    new = (ord(previous) + ord(key) + ord(current) - 3 * ord('a')) % 26\n",
    "    return chr(new + ord('a'))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def decipher_letter(previous, key, current):\n",
    "    new = (ord(current) - ord(key) - ord(previous) + ord('a')) % 26\n",
    "    return chr(new + ord('a'))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'a'"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "encipher_letter('a', 'a', 'a')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'abcdefghijklmnopqrstuvwxyz'"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "''.join(encipher_letter('a', 'a', l) for l in string.ascii_lowercase)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'cdefghijklmnopqrstuvwxyzab'"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "''.join(encipher_letter('b', 'b', l) for l in string.ascii_lowercase)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'abcdefghijklmnopqrstuvwxyz'"
      ]
     },
     "execution_count": 18,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "''.join(decipher_letter('a', 'a', l) for l in string.ascii_lowercase)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'yzabcdefghijklmnopqrstuvwx'"
      ]
     },
     "execution_count": 21,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "''.join(decipher_letter('b', 'b', l) for l in string.ascii_lowercase)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 20,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'b'"
      ]
     },
     "execution_count": 20,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_letter('a', 'a', 'b')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 23,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
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      ]
     },
     "execution_count": 23,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "''.join(encipher_letter(k, p, l) \n",
    "        for l in string.ascii_lowercase \n",
    "        for k in string.ascii_lowercase \n",
    "        for p in string.ascii_lowercase)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[('a', 'a', 'a', 'a'),\n",
       " ('a', 'b', 'a', 'b'),\n",
       " ('a', 'c', 'a', 'c'),\n",
       " ('a', 'd', 'a', 'd'),\n",
       " ('a', 'e', 'a', 'e'),\n",
       " ('a', 'f', 'a', 'f'),\n",
       " ('a', 'g', 'a', 'g'),\n",
       " ('a', 'h', 'a', 'h'),\n",
       " ('a', 'i', 'a', 'i'),\n",
       " ('a', 'j', 'a', 'j')]"
      ]
     },
     "execution_count": 24,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "encrypted = [(k, p, l, encipher_letter(k, p, l)) \n",
    "             for l in string.ascii_lowercase \n",
    "             for k in string.ascii_lowercase\n",
    "             for p in string.ascii_lowercase]\n",
    "encrypted[:10]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 25,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "all(l == decipher_letter(k, p, e) for k, l, p, e in encrypted)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 39,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def encipher_message(plaintext, keystring='a'):\n",
    "    key = itertools.cycle(keystring)\n",
    "    ciphertext = ''\n",
    "    previous = keystring[0]\n",
    "    for letter, keychar in zip(plaintext, key):\n",
    "        cipherletter = encipher_letter(previous, keychar, letter)\n",
    "        ciphertext += cipherletter\n",
    "        previous = cipherletter\n",
    "        # previous = letter\n",
    "    return ciphertext"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 40,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hlnbk'"
      ]
     },
     "execution_count": 40,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "encipher_message('hello', 'aardvark')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 41,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'nhijn'"
      ]
     },
     "execution_count": 41,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "encipher_message('hello', 'q')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 42,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def decipher_message(ciphertext, keystring='a'):\n",
    "    key = itertools.cycle(keystring)\n",
    "    plaintext = ''\n",
    "    previous = keystring[0]\n",
    "    for letter, keychar in zip(ciphertext, key):\n",
    "        plainletter = decipher_letter(previous, keychar, letter)\n",
    "        plaintext += plainletter\n",
    "        previous = letter\n",
    "        # previous = plainletter\n",
    "    return plaintext"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 43,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello'"
      ]
     },
     "execution_count": 43,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_message('hlnbk', 'aardvark')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 44,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello'"
      ]
     },
     "execution_count": 44,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_message('nhijn', 'q')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 45,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hecoj'"
      ]
     },
     "execution_count": 45,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_message('hlnbk')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 46,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hlwhvjxlznbpdrfthvjxlznbpd'"
      ]
     },
     "execution_count": 46,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "encipher_message('helloooooooooooooooooooooo')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 47,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'helloooooooooooooooooooooo'"
      ]
     },
     "execution_count": 47,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_message('hlwhvjxlznbpdrfthvjxlznbpd')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 48,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'fdkknnnnnnnnnnnnnnnnnnnnnn'"
      ]
     },
     "execution_count": 48,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decipher_message('hlwhvjxlznbpdrfthvjxlznbpd', 'b')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
 ],
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