DerbyCon5_BlockCodingPlainTextAttack.ipynb

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     "text": [
      "Deciphered text: count the number of times Bob , Jim , and Alice are in the tale . the FLAG is                \n",
      " Alice Jim Bob and the count of each with no spaces .                \n",
      "\n",
      "Flag: AliceJimBox1255\n"
     ]
    }
   ],
   "source": [
    "#!/usr/bin/env python3\n",
    "\n",
    "known_cipher_text = 'www.pwndu.edu/~gwalton/homework/crypto/Assignment.cipher.txt'\n",
    "known_plain_text = 'www.pwndu.edu/~gwalton/homework/crypto/Assignment.plain.txt'\n",
    "target_cipher_text = 'www.pwndu.edu/~gwalton/homework/crypto/secret.txt'\n",
    "\n",
    "import collections\n",
    "\n",
    "def get_blocks(file_name):\n",
    "    with open(file_name, 'rb') as f:\n",
    "        return list(iter(lambda:f.read(0x10), b''))\n",
    "\n",
    "def group_blocks(blocks):\n",
    "    groups = collections.defaultdict(set)\n",
    "    for i, _cipher_text in enumerate(blocks):\n",
    "        groups[_cipher_text].add(i)\n",
    "    return groups\n",
    "            \n",
    "def invert_dict(d):\n",
    "    return {tuple(sorted(v)): k for k, v in d.items()}\n",
    "\n",
    "def build_code_book(cipher_blocks, plain_blocks):\n",
    "    p_lines = invert_dict(plain_blocks)\n",
    "    return {invert_dict(cipher_blocks)[k]: p_lines[k] for k in p_lines}\n",
    "\n",
    "def decipher(cipher_blocks, code_book):\n",
    "    for block in cipher_blocks:\n",
    "        if block in code_book:\n",
    "            yield code_book[block]\n",
    "\n",
    "def string_from_blocks(blocks):\n",
    "    return ' '.join(_.decode('ascii').rstrip(' ') for _ in blocks)\n",
    "    \n",
    "print(\"Deciphered text: \" + string_from_blocks(decipher(\n",
    "    get_blocks(target_cipher_text),\n",
    "    build_code_book(\n",
    "        group_blocks(get_blocks(known_cipher_text)),\n",
    "        group_blocks(get_blocks(known_plain_text)),\n",
    "    )\n",
    ")))\n",
    "\n",
    "tale = string_from_blocks(get_blocks(known_plain_text)).lower()\n",
    "print(\"Flag: AliceJimBox%s%s%s\" % (\n",
    "    tale.count('alice'),\n",
    "    tale.count('jim'),\n",
    "    tale.count('bob')\n",
    "))\n",
    "\n"
   ]
  }
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