Lab 7: Caesar Cipher

Web-CAT: Submit Python programs to this automated grading platform.

Task Outline

The main idea behind the Caesar Cipher is to shift each letter in a secret message by a fixed number of positions. If this shifting behaviour goes further than the end of the alphabet, then it wraps around to the beginning, and continues from there.
The security of this crypto-system relies on having only the sender and the recipient know the secret key, which is the number of places by which the letters have been shifted.

Encryption shift

Python uses a numerical ASCII value to represent each text character in the alphabet. For example, the ASCII value of the letter "a" is 97, and the ASCII value of the letter "z" is 122.
In order to convert between the two formats, you need to use the following Python functions:
- ord(txt) This returns the numerical ASCII code corresponding to the text character txt.
- chr(num) This returns the text character corresponding to the numerical ASCII code num.
You will need to make an adjustment to the numerical output of ord(txt), such that "a" corresponds to 0, "b" corresponds to 1, etc.
Unencrypted text is generally called plaintext, and encrypted text is generally known as ciphertext. The quantity by which the letters have been shifted is called a key.
In general, the Caesar Cipher encrypts messages by rotating each letter by key positions. More formally, p is the ASCII value of a letter in the plaintext, and key is the amount by which each letter is shifted, then the ASCII value of the corresponding letter in the ciphertext c, is computed by the following equation:

$c = (p + \mbox{key})\, \mbox{mod}\, 26$

You may assume that all of the characters in the plaintext messages are in lowercase.
There will be no punctuation present in any of the plaintext messages,with the exception of the space character. You should design your program so that any spaces in the plaintext message are transferred into the encrypted ciphertext.

Description	File Size	File Name
`Python` Source Code for Caesar Cipher	1.5KB	lab07.zip

Contents of lab07.zip:

Lab07CaesarCipher/
├── caesarcipher.py
└── testcaesarcipher.py

Write a Python program in the file caesarcipher.py that encrypts a message using the caesar cipher.
You will write your solution in a function called encrypt(message, key) right below the place where it says: YOUR CODE HERE
When the function call encrypt("hello", 1) is executed, the output of the program should be: ifmmp

Run the file testcaesarcipher.py to execute the PyUnit test bench. PyUnit indicates a successful test with an OK output, and an unsuccessful test with a FAIL output.