Building a Caesar Cipher (a character-shifting cipher)

This lesson has you implement the Caesar cipher, a classic cipher, as practice for string manipulation using character codes. It's written for anyone searching "Python encryption implementation" or "character codes ord chr".

The Caesar cipher is a simple cipher, dating back to ancient Rome, that replaces each letter by shifting it a fixed number of places through the alphabet. Think of it like a childhood game of shifting the alphabet by a few letters and speaking in "code." You implement it by converting a character to a number (its character code) with ord(), doing arithmetic on that number, then converting back to a character with chr().

The sample code uses .isalpha() to check whether a character is a letter, and only shifts the character code for letters. The key detail is the % 26, which keeps the result within the 26-letter range — implementing the "wrap around" behavior where Z rolls back to A. Non-letter characters (spaces, punctuation) are passed through unchanged.

It's simple and easy to break compared to modern cryptography, but it's a nicely approachable introductory topic for practicing the basic operation of doing arithmetic on character codes. What beginners tend to trip on is that uppercase and lowercase letters have different base character codes — handling both requires a bit of extra care.

As a gateway into modern cryptography, it's worth experiencing the underlying mechanism hands-on. The idea of "treating a character as a number and doing math on it" is also the foundation for learning more advanced cryptographic techniques later.