X-Git-Url: https://git.njae.me.uk/?p=computing-unplugged-book.git;a=blobdiff_plain;f=stream-cipher.ipynb;fp=stream-cipher.ipynb;h=213eb834ee1cdf3d4207f0ef4036acb05bc990c3;hp=0000000000000000000000000000000000000000;hb=1cf9930f9655bba6e70d555ae408db8cde8289d5;hpb=41a19b0744e04270472eb78aa8a7a96c8be3659c

diff --git a/stream-cipher.ipynb b/stream-cipher.ipynb
new file mode 100644
index 0000000..213eb83
--- /dev/null
+++ b/stream-cipher.ipynb
@@ -0,0 +1,491 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "import string"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 40,
+   "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": 41,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'thequickbrownfoxjumpedoverthelzydoga'"
+      ]
+     },
+     "execution_count": 41,
+     "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": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def encipher_letter(previous, current):\n",
+    "    new = (ord(previous) + ord(current) - 2 * ord('a')) % 26\n",
+    "    return chr(new + ord('a'))"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 15,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def decipher_letter(previous, current):\n",
+    "    new = (ord(current) - ord(previous) - 2 * ord('a')) % 26\n",
+    "    return chr(new + ord('a'))"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'a'"
+      ]
+     },
+     "execution_count": 4,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "encipher_letter('a', 'a')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'abcdefghijklmnopqrstuvwxyz'"
+      ]
+     },
+     "execution_count": 6,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "''.join(encipher_letter('a', l) for l in string.ascii_lowercase)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'bcdefghijklmnopqrstuvwxyza'"
+      ]
+     },
+     "execution_count": 7,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "''.join(encipher_letter('b', l) for l in string.ascii_lowercase)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 17,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'abcdefghijklmnopqrstuvwxyz'"
+      ]
+     },
+     "execution_count": 17,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "''.join(decipher_letter('a', l) for l in string.ascii_lowercase)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 18,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'b'"
+      ]
+     },
+     "execution_count": 18,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "decipher_letter('a', 'b')"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "e = (p + c) % 26\n",
+    "c = e - p"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 20,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'abcdefghijklmnopqrstuvwxyzbcdefghijklmnopqrstuvwxyzacdefghijklmnopqrstuvwxyzabdefghijklmnopqrstuvwxyzabcefghijklmnopqrstuvwxyzabcdfghijklmnopqrstuvwxyzabcdeghijklmnopqrstuvwxyzabcdefhijklmnopqrstuvwxyzabcdefgijklmnopqrstuvwxyzabcdefghjklmnopqrstuvwxyzabcdefghiklmnopqrstuvwxyzabcdefghijlmnopqrstuvwxyzabcdefghijkmnopqrstuvwxyzabcdefghijklnopqrstuvwxyzabcdefghijklmopqrstuvwxyzabcdefghijklmnpqrstuvwxyzabcdefghijklmnoqrstuvwxyzabcdefghijklmnoprstuvwxyzabcdefghijklmnopqstuvwxyzabcdefghijklmnopqrtuvwxyzabcdefghijklmnopqrsuvwxyzabcdefghijklmnopqrstvwxyzabcdefghijklmnopqrstuwxyzabcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwyzabcdefghijklmnopqrstuvwxzabcdefghijklmnopqrstuvwxy'"
+      ]
+     },
+     "execution_count": 20,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "''.join(encipher_letter(k, l) for l in string.ascii_lowercase for k in string.ascii_lowercase)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 21,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "[('a', 'a', 'a'),\n",
+       " ('b', 'a', 'b'),\n",
+       " ('c', 'a', 'c'),\n",
+       " ('d', 'a', 'd'),\n",
+       " ('e', 'a', 'e'),\n",
+       " ('f', 'a', 'f'),\n",
+       " ('g', 'a', 'g'),\n",
+       " ('h', 'a', 'h'),\n",
+       " ('i', 'a', 'i'),\n",
+       " ('j', 'a', 'j')]"
+      ]
+     },
+     "execution_count": 21,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "encrypted = [(k, l, encipher_letter(k, l)) for l in string.ascii_lowercase for k in string.ascii_lowercase]\n",
+    "encrypted[:10]"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 22,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "True"
+      ]
+     },
+     "execution_count": 22,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "all(l == decipher_letter(k, e) for k, l, e in encrypted)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 55,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def encipher_message(plaintext, key='a'):\n",
+    "    ciphertext = ''\n",
+    "    previous = key\n",
+    "    for letter in plaintext:\n",
+    "        cipherletter = encipher_letter(previous, letter)\n",
+    "        ciphertext += cipherletter\n",
+    "        previous = cipherletter\n",
+    "        # previous = letter\n",
+    "    return ciphertext"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 56,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hlwhv'"
+      ]
+     },
+     "execution_count": 56,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "encipher_message('hello')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 57,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'xbmxl'"
+      ]
+     },
+     "execution_count": 57,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "encipher_message('hello', 'q')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 54,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def decipher_message(ciphertext, key='a'):\n",
+    "    plaintext = ''\n",
+    "    previous = key\n",
+    "    for letter in ciphertext:\n",
+    "        plainletter = decipher_letter(previous, letter)\n",
+    "        plaintext += plainletter\n",
+    "        previous = letter\n",
+    "        # previous = plainletter\n",
+    "    return plaintext"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 58,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hello'"
+      ]
+     },
+     "execution_count": 58,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "decipher_message('hlwhv')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 59,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'xello'"
+      ]
+     },
+     "execution_count": 59,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "decipher_message('xbmxl')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 60,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hello'"
+      ]
+     },
+     "execution_count": 60,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "decipher_message('xbmxl', 'q')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 61,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'hlwhvjxlznbpdrfthvjxlznbpd'"
+      ]
+     },
+     "execution_count": 61,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "encipher_message('helloooooooooooooooooooooo')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 62,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'helloooooooooooooooooooooo'"
+      ]
+     },
+     "execution_count": 62,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "decipher_message('hlwhvjxlznbpdrfthvjxlznbpd')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.4.2"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}