stream-cipher.ipynb

   1 {
   2  "cells": [
   3   {
   4    "cell_type": "code",
   5    "execution_count": 5,
   6    "metadata": {
   7     "collapsed": true
   8    },
   9    "outputs": [],
  10    "source": [
  11     "import string"
  12    ]
  13   },
  14   {
  15    "cell_type": "code",
  16    "execution_count": 40,
  17    "metadata": {
  18     "collapsed": true
  19    },
  20    "outputs": [],
  21    "source": [
  22     "def sanitised(text):\n",
  23     "    return ''.join(letter.lower() for letter in text if letter in string.ascii_letters)"
  24    ]
  25   },
  26   {
  27    "cell_type": "code",
  28    "execution_count": 41,
  29    "metadata": {
  30     "collapsed": false
  31    },
  32    "outputs": [
  33     {
  34      "data": {
  35       "text/plain": [
  36        "'thequickbrownfoxjumpedoverthelzydoga'"
  37       ]
  38      },
  39      "execution_count": 41,
  40      "metadata": {},
  41      "output_type": "execute_result"
  42     }
  43    ],
  44    "source": [
  45     "sanitised('The quick brown fox! jumped OVER the l\\a!z%y* dog....a')"
  46    ]
  47   },
  48   {
  49    "cell_type": "code",
  50    "execution_count": 2,
  51    "metadata": {
  52     "collapsed": false
  53    },
  54    "outputs": [],
  55    "source": [
  56     "def encipher_letter(previous, current):\n",
  57     "    new = (ord(previous) + ord(current) - 2 * ord('a')) % 26\n",
  58     "    return chr(new + ord('a'))"
  59    ]
  60   },
  61   {
  62    "cell_type": "code",
  63    "execution_count": 15,
  64    "metadata": {
  65     "collapsed": true
  66    },
  67    "outputs": [],
  68    "source": [
  69     "def decipher_letter(previous, current):\n",
  70     "    new = (ord(current) - ord(previous) - 2 * ord('a')) % 26\n",
  71     "    return chr(new + ord('a'))"
  72    ]
  73   },
  74   {
  75    "cell_type": "code",
  76    "execution_count": 4,
  77    "metadata": {
  78     "collapsed": false
  79    },
  80    "outputs": [
  81     {
  82      "data": {
  83       "text/plain": [
  84        "'a'"
  85       ]
  86      },
  87      "execution_count": 4,
  88      "metadata": {},
  89      "output_type": "execute_result"
  90     }
  91    ],
  92    "source": [
  93     "encipher_letter('a', 'a')"
  94    ]
  95   },
  96   {
  97    "cell_type": "code",
  98    "execution_count": 6,
  99    "metadata": {
 100     "collapsed": false
 101    },
 102    "outputs": [
 103     {
 104      "data": {
 105       "text/plain": [
 106        "'abcdefghijklmnopqrstuvwxyz'"
 107       ]
 108      },
 109      "execution_count": 6,
 110      "metadata": {},
 111      "output_type": "execute_result"
 112     }
 113    ],
 114    "source": [
 115     "''.join(encipher_letter('a', l) for l in string.ascii_lowercase)"
 116    ]
 117   },
 118   {
 119    "cell_type": "code",
 120    "execution_count": 7,
 121    "metadata": {
 122     "collapsed": false
 123    },
 124    "outputs": [
 125     {
 126      "data": {
 127       "text/plain": [
 128        "'bcdefghijklmnopqrstuvwxyza'"
 129       ]
 130      },
 131      "execution_count": 7,
 132      "metadata": {},
 133      "output_type": "execute_result"
 134     }
 135    ],
 136    "source": [
 137     "''.join(encipher_letter('b', l) for l in string.ascii_lowercase)"
 138    ]
 139   },
 140   {
 141    "cell_type": "code",
 142    "execution_count": 17,
 143    "metadata": {
 144     "collapsed": false
 145    },
 146    "outputs": [
 147     {
 148      "data": {
 149       "text/plain": [
 150        "'abcdefghijklmnopqrstuvwxyz'"
 151       ]
 152      },
 153      "execution_count": 17,
 154      "metadata": {},
 155      "output_type": "execute_result"
 156     }
 157    ],
 158    "source": [
 159     "''.join(decipher_letter('a', l) for l in string.ascii_lowercase)"
 160    ]
 161   },
 162   {
 163    "cell_type": "code",
 164    "execution_count": 18,
 165    "metadata": {
 166     "collapsed": false
 167    },
 168    "outputs": [
 169     {
 170      "data": {
 171       "text/plain": [
 172        "'b'"
 173       ]
 174      },
 175      "execution_count": 18,
 176      "metadata": {},
 177      "output_type": "execute_result"
 178     }
 179    ],
 180    "source": [
 181     "decipher_letter('a', 'b')"
 182    ]
 183   },
 184   {
 185    "cell_type": "markdown",
 186    "metadata": {},
 187    "source": [
 188     "e = (p + c) % 26\n",
 189     "c = e - p"
 190    ]
 191   },
 192   {
 193    "cell_type": "code",
 194    "execution_count": 20,
 195    "metadata": {
 196     "collapsed": false
 197    },
 198    "outputs": [
 199     {
 200      "data": {
 201       "text/plain": [
 202        "'abcdefghijklmnopqrstuvwxyzbcdefghijklmnopqrstuvwxyzacdefghijklmnopqrstuvwxyzabdefghijklmnopqrstuvwxyzabcefghijklmnopqrstuvwxyzabcdfghijklmnopqrstuvwxyzabcdeghijklmnopqrstuvwxyzabcdefhijklmnopqrstuvwxyzabcdefgijklmnopqrstuvwxyzabcdefghjklmnopqrstuvwxyzabcdefghiklmnopqrstuvwxyzabcdefghijlmnopqrstuvwxyzabcdefghijkmnopqrstuvwxyzabcdefghijklnopqrstuvwxyzabcdefghijklmopqrstuvwxyzabcdefghijklmnpqrstuvwxyzabcdefghijklmnoqrstuvwxyzabcdefghijklmnoprstuvwxyzabcdefghijklmnopqstuvwxyzabcdefghijklmnopqrtuvwxyzabcdefghijklmnopqrsuvwxyzabcdefghijklmnopqrstvwxyzabcdefghijklmnopqrstuwxyzabcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwyzabcdefghijklmnopqrstuvwxzabcdefghijklmnopqrstuvwxy'"
 203       ]
 204      },
 205      "execution_count": 20,
 206      "metadata": {},
 207      "output_type": "execute_result"
 208     }
 209    ],
 210    "source": [
 211     "''.join(encipher_letter(k, l) for l in string.ascii_lowercase for k in string.ascii_lowercase)"
 212    ]
 213   },
 214   {
 215    "cell_type": "code",
 216    "execution_count": 21,
 217    "metadata": {
 218     "collapsed": false
 219    },
 220    "outputs": [
 221     {
 222      "data": {
 223       "text/plain": [
 224        "[('a', 'a', 'a'),\n",
 225        " ('b', 'a', 'b'),\n",
 226        " ('c', 'a', 'c'),\n",
 227        " ('d', 'a', 'd'),\n",
 228        " ('e', 'a', 'e'),\n",
 229        " ('f', 'a', 'f'),\n",
 230        " ('g', 'a', 'g'),\n",
 231        " ('h', 'a', 'h'),\n",
 232        " ('i', 'a', 'i'),\n",
 233        " ('j', 'a', 'j')]"
 234       ]
 235      },
 236      "execution_count": 21,
 237      "metadata": {},
 238      "output_type": "execute_result"
 239     }
 240    ],
 241    "source": [
 242     "encrypted = [(k, l, encipher_letter(k, l)) for l in string.ascii_lowercase for k in string.ascii_lowercase]\n",
 243     "encrypted[:10]"
 244    ]
 245   },
 246   {
 247    "cell_type": "code",
 248    "execution_count": 22,
 249    "metadata": {
 250     "collapsed": false
 251    },
 252    "outputs": [
 253     {
 254      "data": {
 255       "text/plain": [
 256        "True"
 257       ]
 258      },
 259      "execution_count": 22,
 260      "metadata": {},
 261      "output_type": "execute_result"
 262     }
 263    ],
 264    "source": [
 265     "all(l == decipher_letter(k, e) for k, l, e in encrypted)"
 266    ]
 267   },
 268   {
 269    "cell_type": "code",
 270    "execution_count": 55,
 271    "metadata": {
 272     "collapsed": true
 273    },
 274    "outputs": [],
 275    "source": [
 276     "def encipher_message(plaintext, key='a'):\n",
 277     "    ciphertext = ''\n",
 278     "    previous = key\n",
 279     "    for letter in plaintext:\n",
 280     "        cipherletter = encipher_letter(previous, letter)\n",
 281     "        ciphertext += cipherletter\n",
 282     "        previous = cipherletter\n",
 283     "        # previous = letter\n",
 284     "    return ciphertext"
 285    ]
 286   },
 287   {
 288    "cell_type": "code",
 289    "execution_count": 56,
 290    "metadata": {
 291     "collapsed": false
 292    },
 293    "outputs": [
 294     {
 295      "data": {
 296       "text/plain": [
 297        "'hlwhv'"
 298       ]
 299      },
 300      "execution_count": 56,
 301      "metadata": {},
 302      "output_type": "execute_result"
 303     }
 304    ],
 305    "source": [
 306     "encipher_message('hello')"
 307    ]
 308   },
 309   {
 310    "cell_type": "code",
 311    "execution_count": 57,
 312    "metadata": {
 313     "collapsed": false
 314    },
 315    "outputs": [
 316     {
 317      "data": {
 318       "text/plain": [
 319        "'xbmxl'"
 320       ]
 321      },
 322      "execution_count": 57,
 323      "metadata": {},
 324      "output_type": "execute_result"
 325     }
 326    ],
 327    "source": [
 328     "encipher_message('hello', 'q')"
 329    ]
 330   },
 331   {
 332    "cell_type": "code",
 333    "execution_count": 54,
 334    "metadata": {
 335     "collapsed": true
 336    },
 337    "outputs": [],
 338    "source": [
 339     "def decipher_message(ciphertext, key='a'):\n",
 340     "    plaintext = ''\n",
 341     "    previous = key\n",
 342     "    for letter in ciphertext:\n",
 343     "        plainletter = decipher_letter(previous, letter)\n",
 344     "        plaintext += plainletter\n",
 345     "        previous = letter\n",
 346     "        # previous = plainletter\n",
 347     "    return plaintext"
 348    ]
 349   },
 350   {
 351    "cell_type": "code",
 352    "execution_count": 58,
 353    "metadata": {
 354     "collapsed": false
 355    },
 356    "outputs": [
 357     {
 358      "data": {
 359       "text/plain": [
 360        "'hello'"
 361       ]
 362      },
 363      "execution_count": 58,
 364      "metadata": {},
 365      "output_type": "execute_result"
 366     }
 367    ],
 368    "source": [
 369     "decipher_message('hlwhv')"
 370    ]
 371   },
 372   {
 373    "cell_type": "code",
 374    "execution_count": 59,
 375    "metadata": {
 376     "collapsed": false
 377    },
 378    "outputs": [
 379     {
 380      "data": {
 381       "text/plain": [
 382        "'xello'"
 383       ]
 384      },
 385      "execution_count": 59,
 386      "metadata": {},
 387      "output_type": "execute_result"
 388     }
 389    ],
 390    "source": [
 391     "decipher_message('xbmxl')"
 392    ]
 393   },
 394   {
 395    "cell_type": "code",
 396    "execution_count": 60,
 397    "metadata": {
 398     "collapsed": false
 399    },
 400    "outputs": [
 401     {
 402      "data": {
 403       "text/plain": [
 404        "'hello'"
 405       ]
 406      },
 407      "execution_count": 60,
 408      "metadata": {},
 409      "output_type": "execute_result"
 410     }
 411    ],
 412    "source": [
 413     "decipher_message('xbmxl', 'q')"
 414    ]
 415   },
 416   {
 417    "cell_type": "code",
 418    "execution_count": 61,
 419    "metadata": {
 420     "collapsed": false
 421    },
 422    "outputs": [
 423     {
 424      "data": {
 425       "text/plain": [
 426        "'hlwhvjxlznbpdrfthvjxlznbpd'"
 427       ]
 428      },
 429      "execution_count": 61,
 430      "metadata": {},
 431      "output_type": "execute_result"
 432     }
 433    ],
 434    "source": [
 435     "encipher_message('helloooooooooooooooooooooo')"
 436    ]
 437   },
 438   {
 439    "cell_type": "code",
 440    "execution_count": 62,
 441    "metadata": {
 442     "collapsed": false
 443    },
 444    "outputs": [
 445     {
 446      "data": {
 447       "text/plain": [
 448        "'helloooooooooooooooooooooo'"
 449       ]
 450      },
 451      "execution_count": 62,
 452      "metadata": {},
 453      "output_type": "execute_result"
 454     }
 455    ],
 456    "source": [
 457     "decipher_message('hlwhvjxlznbpdrfthvjxlznbpd')"
 458    ]
 459   },
 460   {
 461    "cell_type": "code",
 462    "execution_count": null,
 463    "metadata": {
 464     "collapsed": true
 465    },
 466    "outputs": [],
 467    "source": []
 468   }
 469  ],
 470  "metadata": {
 471   "kernelspec": {
 472    "display_name": "Python 3",
 473    "language": "python",
 474    "name": "python3"
 475   },
 476   "language_info": {
 477    "codemirror_mode": {
 478     "name": "ipython",
 479     "version": 3
 480    },
 481    "file_extension": ".py",
 482    "mimetype": "text/x-python",
 483    "name": "python",
 484    "nbconvert_exporter": "python",
 485    "pygments_lexer": "ipython3",
 486    "version": "3.4.2"
 487   }
 488  },
 489  "nbformat": 4,
 490  "nbformat_minor": 0
 491 }