From Scientific American:

Introduction
If you need to send a secret message to a friend, how could you prevent other people from reading it? One way is to encrypt the message—that is, use a secret code that only you and your friend know. Try this activity to learn how to create your own “Caesar cipher,” a popular type of code that is easy to learn.

Background
Cryptography is the study of writing or solving secret codes that are used for secure communication. Historically, codes have been used by politicians, spies and countries at war to prevent their enemies from knowing what they’re up to. Many of the earliest codes, or “ciphers,” such as the one you will create in this project were easy to create by hand. Now cryptography is essential in computer science for keeping everything from e-mails to bank account information secure.

The Caesar cipher, named after Roman Emperor Julius Caesar is one of the earliest and most widely known ciphers. It is a simple form of a “substitution cipher” where you replace each letter of the alphabet with another letter by shifting the whole alphabet a certain number of letters (wrapping around to the beginning once you reach the end). For example, this would be your key and code if you shift each letter by three spaces:

Plain:     ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher:   XYZABCDEFGHIJKLMNOPQRSTUVW

So, when you write your message, the letter A gets replaced with X, B gets replaced with Y and so on. For example, the word “HELLO” reads:

Plain:    HELLO
Cipher:  EBIIL

In order to decode your message, you need to share the “key” (the number 3) with your friend. After that you can send messages that are written in cipher so other people can’t read them!

Materials

Preparation

Procedure

Observations and results
Once you and your friend both understand how to use a Caesar cipher it should be relatively …

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