CPSC120A - Lab 18

Use the command line to create a new directory called lab18 in your labs directory. Make sure all of the .py files that you create for this activity are in that directory.

Caesar Cipher

Cryptography is the science of hiding secrets. One very easy, and surprisingly old, cryptographic protocol is know as the Caesar cipher. It is named after Julius Caesar who would scramble documents containing strategic military information by replacing each character in the document with the character three letters later in the alphabet. In general, the Caesar cipher can shift characters by any amount in the range 0 to 25.

Details

Write a function called caesar_cipher(plaintext, key). The parameter plaintext is a string and key is an integer in the range $[0, 26)$ . The function should return a string with each character shifted forward in the alphabet by the specified amount. If shifting a character puts it outside of the range of lowercase letters, you should wrap around back to the beginning of the alphabet. Including capitalized and non-alphabetic characters in the encrypted text makes it easier to break the encryption and discover the hidden message. The function should remove non-alphabetic characters and convert all upper-case letters to lower-case.

Make sure your program handles all necessary cases gracefully. What additional test cases should you check?

Sample Test Cases

Function Parameters	Expected Output
"scotty", 13	fpbggl
"bouchard", 3	erxfkdug
"The Eagle flies at midnight!", 26	theeaglefliesatmidnight

Hint

Your first step should be to take a character and convert it to an integer in the range $[0, 26)$ . If you have an arbitrary lowercase character ch, you can convert this to an integer by subtracting the ord of the lowercase 'a' character from the ord value of ch.
For some key and character combinations, it is very likely that you will get pushed outside the range of lowercase characters. You can use modular arithmetic to make sure you stay in the range, and accomplish the cycling around described above. Just mod (%) the result of adding the key by 26 to make sure you get an integer still in the necessary range.
After you have performed the key shifting, you can use the chr function to convert your integer back to a character. You will want to add the ord('a') value back to your integer, to make sure you get a lowercase character as opposed to an unprintable one.

Challenge

The function you write above should be able to be used for both encryption and decryption. The only difference for decryption is that you negate the input key value. Add code to your file that prompts the user for a string of text to encrypt, an integer key, and whether they want to encrypt or decrypt the specified string. If the user wants to decrypt a message, you should negate their key, and just use the same function.

Submission

Please show your source code and run your programs for the instructor or lab assistant. Only a programs that have perfect style and flawless functionality will be accepted as complete.