Code breakers
=============

.. rubric:: (due: Thursday, November 2, 8:00 AM)


ASCII codes
-----------

Each character on a computer keyboard is assigned an `ASCII code <http://www.theasciicode.com.ar>`_, which
is an integer in the range 0-127. The ASCII code of a character can be
obtained using the ``ord()`` function:


.. code:: python

    for c in "This is MTH 337":
        print('{:>3} {}'.format(ord(c), "'"+c+"'"))

.. container:: output

      \  84 'T'
      104 'h'
      105 'i'
      115 's'
      \  32 ' '
      105 'i'
      115 's'
      \  32 ' '
      \  77 'M'
      \  84 'T'
      \  72 'H'
      \  32 ' '
      \  51 '3'
      \  51 '3'
      \  55 '7'



Conversely, the function ``chr()`` converts ASCII codes into characters:

.. code:: python

    char_list = []
    for n in [104, 101, 108, 108, 111]:
        char_list.append(chr(n))
        txt = ''.join(char_list)
    print(txt)


.. container:: output

    hello


Text encryption
---------------

In order to securely send a confidential message one usually needs to
encrypt it in some way to conceal its content. Here we consider the following
encryption scheme:

-  One selects a secret key, which is sequence of characters. This key is used
   to both encrypt and decrypt the message.
-  Characters of the secret key and characters of the message are converted
   into ASCII codes. In this way the key is transformed into a
   sequence of integers :math:`(k_1, k_2, \dots, k_r)`, and the message becomes
   another sequence of integers :math:`(m_1, m_2, \dots, m_s)`. If :math:`r < s`
   then the secret key sequence is extended by repeating it as many times as
   necessary until it matches the length of the message.
-  Let :math:`c_i` be the reminder from the division of
   :math:`m_i+k_i` by 128. The sequence of numbers
   :math:`(c_1, c_2, \dots, c_s)` is the encrypted message.

For example, if the message is 'Top secret!' and the secret key is 'buffalo'
then the encrypted message is:

.. math:: \tt{54,100,86,6,84,81,82,84,90,90,7}



In order to decrypt the message we work backwards: for each number :math:`c_i`
we compute the reminder from the division of :math:`c_i-k_i` by 128. This
number is equal to :math:`m_i`, so converting it into a character
we get the :math:`i`-th letter of the message.

Project
-------

Each file listed below contains text (a fragment of a book in English)
encrypted using the method described above. The secret key used to encrypt
the text is a word, different for each file. A dictionary of over 60,000 English
words is available `here <https://raw.githubusercontent.com/en-wl/wordlist/master/alt12dicts/5desk.txt>`__,
and all words used as secret keys are listed in this dictionary. Write
code that decrypts the file assigned to you.


**Note.**  This is a programming project. Your project report does not need
include narrative, beyond comments explaining how your code works. The project will
be graded according to the following rubrics:

* Code that successfully decrypts the text file: 70%
* Report organization and code documentation: 30%

-  :download:`ahlawat.txt<ahlawat.txt>`
-  :download:`bennett.txt<bennett.txt>`
-  :download:`budniewski.txt<budniewski.txt>`
-  :download:`chen.txt<chen.txt>`
-  :download:`coates.txt<coates.txt>`
-  :download:`denz.txt<denz.txt>`
-  :download:`diaz.txt<diaz.txt>`
-  :download:`durrence.txt<durrence.txt>`
-  :download:`eichhorn.txt<eichhorn.txt>`
-  :download:`ekstrum.txt<ekstrum.txt>`
-  :download:`fijas.txt<fijas.txt>`
-  :download:`hurley.txt<hurley.txt>`
-  :download:`kaplan.txt<kaplan.txt>`
-  :download:`mccarthy.txt<mccarthy.txt>`
-  :download:`medrano-abzun.txt<medrano-abzun.txt>`
-  :download:`mompoint.txt<mompoint.txt>`
-  :download:`nance.txt<nance.txt>`
-  :download:`oleary.txt<oleary.txt>`
-  :download:`reese.txt<reese.txt>`
-  :download:`shah.txt<shah.txt>`
-  :download:`skolnick.txt<skolnick.txt>`
-  :download:`walley.txt<walley.txt>`
-  :download:`walter.txt<walter.txt>`
-  :download:`wang.txt<wang.txt>`
-  :download:`welte.txt<welte.txt>`
-  :download:`young.txt<young.txt>`