Description

Encryption is the process of obscuring information to make it unreadable without special knowledge. For centuries, people have devised schemes to encrypt messages – some better than others – but the advent of the computer and the Internet revolutionized the field. These days, it’s hard not to encounter some sort of encryption, whether you are buying something online or logging into a shared computer system. Encryption lets you share information with other trusted people, without fear of disclosure.

A cipher is an algorithm for performing encryption (and the reverse, decryption). The original information is called plaintext. After it is encrypted, it is called ciphertext. The ciphertext message contains all the information of the plaintext message, but it is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those for whom it is not intended.

A cipher usually depends on a piece of auxiliary information, called a key. The key is incorporated into the encryption process; the same plaintext encrypted with two different keys should have two different ciphertexts. Without the key, it should be difficult to decrypt the resulting ciphertext into readable plaintext.

This assignment will deal with a well-known (though not very secure) encryption method called the Caesar cipher. Some vocabulary to get you started on this problem:

Encryption – the process of obscuring or encoding messages to make them unreadable until they are decrypted
Decryption – making encrypted messages readable again by decoding them
Cipher – algorithm for performing encryption and decryption
Plaintext – the original message
Ciphertext – the encrypted message. Note: a ciphertext still contains all of the original message information, even if it looks like gibberish.
The Caesar Cipher

The idea of the Caesar Cipher is to pick an integer and shift every letter of your message by that integer. In other words, suppose the shift is k . Then, all instances of the i-th letter of the alphabet that appear in the plaintext should become the (i+k)-th letter of the alphabet in the ciphertext. You will need to be careful with the case in which i + k > 26 (the length of the alphabet). Here is what the whole alphabet looks like shifted three spots to the right:

Original: a b c d e f g h i j k l m n o p q r s t u v w x y z
3-shift: d e f g h i j k l m n o p q r s t u v w x y z a b c
Using the above key, we can quickly translate the message “happy” to “kdssb” (note how the 3-shifted alphabet wraps around at the end, so x -> a, y -> b, and z -> c).

Note!! We are using the English alphabet for this problem – that is, the following letters in the following order:

>>> import string
>>> print string.ascii_lowercase abcdefghijklmnopqrstuvwxyz
We will treat uppercase and lowercase letters individually, so that uppercase letters are always mapped to an uppercase letter, and lowercase letters are always mapped to a lowercase letter. If an uppercase letter maps to “A”, then the same lowercase letter should map to “a”. Punctuation and spaces should be retained and not changed. For example, a plaintext message with a comma should have a corresponding ciphertext with a comma in the same position.

| plaintext | shift | ciphertext |
| —————-|———–|——————|
| ‘abcdef’ | 2 | ‘cdefgh’ |
| ‘Hello, World!’ | 5 | ‘Mjqqt, Btwqi!’ |
| ” | any value | ” |

Getting Started

To get started, download the ps6.zip file. Extract it to your working directory. The files inside are:

ps6.py – a file containing three classes that you will have to implement. words.txt – a file containing valid English words (should be in the same folder as your ps6..py file).
story.txt – a file containing an encrypted message that you will have to decode (should be in the same folder as your ps6..py file).
This will be your first experience coding with classes! We will have a Message class with two subclasses PlaintextMessage and CiphertextMessage .

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Problem 1 – Build the Shift Dictionary and Apply Shift
The Message class contains methods that could be used to apply a cipher to a string, either to encrypt or to decrypt a message (since for Caesar codes this is the same action).

In the next two questions, you will fill in the methods of the Message class found in ps6.py according to the specifications in the docstrings. The methods in the Message class already filled in are:

__init__(self, text)
The getter method get_message_text(self)
The getter method get_valid_words(self), notice that this one returns a copy of self.valid_words to prevent someone from mutating the original list.
In this problem, you will fill in two methods:

Fill in the build_shift_dict(self, shift) method of the Message class. Be sure that your dictionary includes both lower and upper case letters, but that the shifted character for a lower case letter and its uppercase version are lower and upper case instances of the same letter. What this means is that if the original letter is “a” and its shifted value is “c”, the letter “A” should shift to the letter “C”.

If you are unfamiliar with the ordering or characters of the English alphabet, we will be following the letter ordering displayed by string.ascii_lowercase and string.ascii_uppercase:

>>> import string
>>> print(string.ascii_lowercase) abcdefghijklmnopqrstuvwxyz >>> print(string.ascii_uppercase)
ABCDEFGHIJKLMNOPQRSTUVWXYZ
A reminder from the introduction page – characters such as the space character, commas, periods, exclamation points, etc will not be encrypted by this cipher – basically, all the characters within string.punctuation, plus the space (‘ ‘) and all numerical characters (0 – 9) found in string.digits.

Paste your implementation of the Message class in the box below.
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Problem 2 – PlaintextMessage
For this problem, the graders will use our implementation of the Message class, so don’t worry if you did not get the previous parts correct.

PlaintextMessage is a subclass of Message and has methods to encode a string using a specified shift value. Our class will always create an encoded version of the message, and will have methods for changing the encoding.

Implement the methods in the class PlaintextMessage according to the specifications in ps6.py. The methods you should fill in are:

__init__(self, text, shift): Use the parent class constructor to make your code more concise.
The getter method get_shift(self)
The getter method get_encrypting_dict(self): This should return a COPY of self.encrypting_dict to prevent someone from mutating the original dictionary. The getter method get_message_text_encrypted(self)
change_shift(self, shift): Think about what other methods you can use to make this easier. It shouldn’t take more than a couple lines of code.
Paste your implementation of the entire PlaintextMessage class in the box below. _________________________________________________________________________________
Problem 3 – CiphertextMessage
For this problem, the graders will use our implementation of the Message and PlaintextMessage classes, so don’t worry if you did not get the previous parts correct.

Given an encrypted message, if you know the shift used to encode the message, decoding it is trivial. If message is the encrypted message, and s is the shift used to encrypt the message, then apply_shift(message, 26-s) gives you the original plaintext message. Do you see why?

The problem, of course, is that you don’t know the shift. But our encryption method only has 26 distinct possible values for the shift! We know English is the main language of these emails, so if we can write a program that tries each shift and maximizes the number of English words in the decoded message, we can decrypt their cipher! A simple indication of whether or not the correct shift has been found is if most of the words obtained after a shift are valid words. Note that this only means that most of the words obtained are actual words. It is possible to have a message that can be decoded by two separate shifts into different sets of words. While there are various strategies for deciding between ambiguous decryptions, for this problem we are only looking for a simple solution.

Fill in the methods in the class CiphertextMessage acording to the specifications in ps6.py. The methods you should fill in are:

__init__(self, text): Use the parent class constructor to make your code more concise.
_________________________________________________________________________________ Problem 4 – Decrypt a Story
For this problem, the graders will use our implementation of the Message, PlaintextMessage, and CiphertextMessage classes, so don’t worry if you did not get the previous parts correct.

Now that you have all the pieces to the puzzle, please use them to decode the file story.txt. The file ps6.py contains a helper function get_story_string() that returns the encrypted version of the story as a string. Create a CiphertextMessage object using the story string and use decrypt_message to return the appropriate shift value and unencrypted story string.
Paste your function decrypt_story() in the box below. _________________________________________________________________________________