CS414 Fall 2017 Program #6

Goals

• Reading text files
• Substitution cyphers
• Formatted printing
• Dictionaries
• Counting statistics

Before You Begin:

• Create a folder on your computer where your files will be kept. Create a separate python file for each question!
• To keep your work organized, create a different folder for each lab, and for each programming assignment.

When You Are Done:

To turn your work in, go to mycourses.unh.edu, find CS414, and find the assignment. Then click "Submit Assignment" to upload your main file, cryptogram.py, plus any other files you have created.

I am providing you with a very minimal skeleton to get you going: cryptogram.py .

I am also providing you with a file with quotes for the game: quotes.txt .

Cryptograms

Encryption converts readable information (the plaintext) into unreadable information (the cyphertext). There are many encryption algorithms, but the simplest is a substitution cypher. In a substitution cypher, there is a mapping between letters in the plaintext and letters in the cyphertext.

The Caesar cypher is the simplest substitution cypher (Julius Caesar used it). Each letter in the plaintext is replaced with the letter 3 spaces after it in the alphabet:

plaintext letter	cyphertext letter
A	D
B	E
C	F
D	G
...	...
V	Y
W	Z
X	A
Y	B
Z	C

Thus the plaintext BABY WAX is mapped to the cyphertext EDEB ZDA. Notice that in a substitution cypher a given letter in the plaintext (e.g., B) is always replaced the same way (in this case E).

Usually substitution cyphers don't simply shift a letter forward in the alphabet, as the caesar cypher does; the replacement from plain letter to cypher letter is usually more arbitrary.

Cryptanalysis

Substitution cyphers are easy to break, if the cyphertext is long enough, because some letters occur more frequently. All we need is to count the frequency of letters in the cyphertext, and make some educated guesses. This is possible because English uses some letters much more often than others. In fact, the most common letters in English text are E, T, A, O, I, N, in order of decreasing frequency. Also, some letter pairs and triples occur very frequently. For example, if we are given a three-letter word QXW, and we have guessed that T maps to Q and W maps to E, we should then guess that X maps to H, because the triple THE is very frequent in English text.

Your Task

Write a program, called cryptogram.py, which does the following:

• Open a text file with quotes, read all its lines, and choose one line at random. This is the secret plaintext, which the user must discover.

• Randomly generate a substitution cypher, and replace each letter in the plaintext, to produce the cyphertext.

• Help the user to figure out the plaintext.

• Repeat this loop:
  1. Print the cyphertext, using line wrapping if the text is longer than 72 characters.

  2. Print a table of letter frequencies, to help the user make educated guesses.

  3. Ask the user for a letter x to be replaced.

  4. Ask the user for a letter y to swap it with.

  5. In the cyphertext, replace all occurences of x with y.

  6. Also, replace all occurences of y with x.

  7. If the cyphertext is now the same as the plaintext, stop.

Functions you should write

Break your work into parts, using functions. Here are some functions you could include in your program. Notice that many of these functions use a list of single letters, rather than a string. You can't replace individual letters in a string (strings are immutable), but you can replace entries in a list.

• main() : this controls the game, from beginning to end.

• choose_quote() : this should open a file called quotes.txt, and get a list with all its lines, with one line per list entry. Then, it should pick one of the lines, at random, and return it.

• encrypt(letters) : this function is passed a list of letters. It should return a new list, which has the letters replaced using a substitution cypher. Proceed as follows:
  1. create a list containing the letters of the english alphabet: alphabet = list(string.lowercase) .
  2. use random.shuffle() to make a scrambled version of the alphabet.
  3. create a dictionary substitution = dict(), and initialize it such that substitution[letter] is the letter's replacement.
  4. use this dictionary to create the list of letters of the cyphertext.

• print_wrapped(letters) : this function is passed a list of letters, and uses "".join(letters) to assemble them into a single string. Then it breaks the string into words, and prints them out. As it does so, it checks how long the printed line would be, with each word. If the line would be longer than 72 characters, issue a print to start a new line.

• exchange(letters, x, y) : this function is passed a list of letters, and two letters, stored in x and y. It replaces every occurrence of x with y, and vice versa.

• show_frequencies(letters) : this function is passed a list of letters, and prints a table of frequencies, indicating how often each letter occurs. To do this, create a dictionary counts = dict(), and visit each letter. If it's in counts, increment its entry. Finally, print the counts.