Caesar Cipher Download

Introduction

The Caesar cipher is named after Julius Caesar, who used it to encrypt his secret communications. It’s a type of substitution cipher where every letter is shifted a fixed number of positions down the alphabet to produce the encrypted result.

Your challenge is to implement the Caesar cipher in Python! Fill in the code so that we can both encode and decode text. All code you need to edit is located in caesar.py.

You do not need to read or understand the contents of cipher.py.

Examples

For example, with a right shift of 3, the letter ‘a’ is replaced with ‘d’, ‘m’ is replaced with ‘p’, and so on.

If we encrypted the following message with a right shift of 4:

“Julius Caesar”

It would come out as:

“Nypmyw Geiwev”

Which looks like gibberish to the untrained eye. If you want to try some examples out yourself, here’s an online Caesar encoder/decoder.

Additional Details

• It’s recommended that you fill in the functions from top to bottom:
  • shift_letter
  • caesar_encode
  • caesar_decode
• Test each function as you finish it! It’s easier to fix bugs if you easily pinpoint their location.
• There is a hint in the docstring for shift_letter if you need help figuring it out.
• Assume that your inputs will not require any wrapping. This means that you will not get an input of ‘y’ with a right shift of 2 (going to ‘a’). As a bonus challenge, you may implement this behavior.

Testing

Run the doctests to check your work by doing python3 -m doctest caesar.py.

Once you’ve passed all the tests, you can either call the functions yourself from the interpreter or run cipher.py.