Assignment 12: Character Encoding and Ciphering

due by 9:00 p.m. EST on Tuesday 3/5/24

Preliminaries

In your work on this assignment, make sure to abide by the collaboration policies of the course.

For each problem in this problem set, we will be writing or evaluating some Python code. You are encouraged to use the Spyder IDE which will be discussed/presented in class, but you are welcome to use another IDE if you choose.

If you have questions while working on this assignment, please post them on Piazza! This is the best way to get a quick response from your classmates and the course staff.

If you have questions while working on this assignment, please post them on Piazza! This is the best way to get a quick response from your classmates and the course staff.

Task 1: A Caesar Shift Cipher

100 points; individual-only

Consider the following encoded message: 'iuet kag iqdq tqdq.' What does it mean?

This is a phrase encoded by a scheme called a Caesar shift cipher. A Caesar cipher is a character substitution scheme in which each letter is replaced by another letter. Moreover, it is a scheme in which the substitutions are obtained by “shifting” some number of letters to the right or left.

For example, if we are shifting letters to the right by 3 positions: A becomes D, B becomes E, ..., X becomes A, Y becomes B, and Z becomes C. (i.e., the letters wrap around at the end of the alphabet).

Encoding/Decoding Algorithm

To decipher such a message, you’ll need to know the key to the Cipher. The key describes how many positions to shift. Let’s assume that positive shifts are to the right (i.e., a key of 1 means A becomes B) and that negative shifts are to the left (i.e., a key of -2 means that D becomes B). Note that a key of +3 is equivalent to a key of -23, since in both cases A becomes D, etc. Further, if you encode with a key of +3, you can decode with a key of -3. You only need one function to do both ciphering and deciphering.

Implementation

Write your program as a set of 3 functions, all in the file a12_caesarcipher.py.

1. Write a function called shift_char(c, n) that will shift a single character c (a string variable that is only one character long) by n positions, and return the shifted character. For example:

   >>> shift_char('a',3)  # simple case: no wrap around needed
   'd'
   >>> shift_char('x',3)  # wrap around the end of the alphabet
   'a'
   >>> shift_char('a',-2) # wrap around the beginning of the alphabet
   'y'
   >>> shift_char(' ', 5)  # shifting a non-letter should simply same character
   ' '
   

Notes:

• Start with the easy cases, i.e. a positive shift only without a wrap around.

• Only shift letters. Do not shift non-letter characters. Try: if 'a' <= ch <= 'z'

• Use ord(x) to convert a character to its numeric ASCII code, and chr(n) to convert an ascii code to a character.

• To deal with the wrap around past the end of the alphabet, consider if-else logic. Something like (but not exactly like): if chr(code) > 'z' . If a wrap around has occurred, subtract an appropriate number of characters. Do the same to deal with a wrap-around of the beginning of the alphabet.

• You code must work equally well with both lower case and upper case letters, as well as spaces and non-letter characters (e.g., punctuation marks). Test it!

• Write a function cipher(plaintext, n) to shift all characters in a string of text. Your function should take 2 parameters, the plaintext to shift (a string) and the shift key n (an integer). Set up an accumulator variable (suggested name: ciphertext) in which to accumulate the result. Use a definite loop with the accumulator pattern to go through all characters in plaintext.

For each character, call your shift_char function to obtain the shifted version of that character, which you will concatenate into ciphertext. Return the ciphertext after the loop.

Here are some test cases:

>>> cipher('time', 4)
'xmqi'
>>> cipher('welcome to the machine', 19)
'pxevhfx mh max ftvabgx'

3. Suppose you have some ciphertext (i.e., 'iuet kag iqdq tqdq', or 'prqhb') but you don’t know which decryption key (i.e., the number of characters to shift) to use.

Write a function called codebreaker(ciphertext) that will try all possible Caesar cipher shifts (there are only 26 to try) and print out each possible deciphering, one per line. One of them will stand out as a valid English phrase.

Hints:

• Use a loop to iterate 26 times.

• Inside the loop, call your cipher function from question 2 above, and print out the resulting ciphered version of the string.

Include 3 test cases for your program in the __main__ section, illustrating that it works correctly to decode the ciphertext examples above.

Submitting Your Work

20 points; will be assigned by code review

Log in to GradeScope to submit your work.

Be sure to name your files correctly!

You will submit two files for this assignment: a12_caesarcipher.py.

When you upload the files, the autograder will test your program.

Notes:

• Upload these files to Gradescope before the deadline.

• When you upload, the autograder script will process your file(s).

• You may resubmit as many times as you like before the deadline, and only the grade from the last submission will be counted.