Unit 2 Lab 6: Encryption, Page 2

Secret Keys

Caesar Ciphers and Substitution Ciphers

On the previous page (or at some point in your life), you might have used a substitution cipher to encode your message. Perhaps you just shifted each letter of the alphabet by a certain number (like 4):

ABCDEFGHIJKLMNOPQRSTUVWXYZ

XYZABCDEFGHIJKLMNOPQRSTUVW

That's a Caesar cipher (named for Julius Caesar).

Or you may have substituted letters without keeping them in alphabetical order, like this:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

PQVFUBZOTHWYELIXRNAMGDSCKJ

That's a general substitution cipher.

Imagine a pair of wheels, pinned together at their center like this:
The person who receives your message has to know how to decode it. Describe a method for decoding a message that is created with a Caesar cipher. How would you decode a message created with a substitution cipher? What are some ways you could make the substitution cipher a more secure alternative to use?

It turns out that even a general substitution cipher is relatively easy to break because certain letters in our language are used more frequently than others. You might have used this idea to break your partner's code on the previous page.
- You can see an example and read about Al-Kindi, the Arabic philosopher and mathematician who first described frequency analysis, in "Breaking Substitution Ciphers" (Blown to Bits pages 166-169).
- It turns out this method is still useful today, even with better cryptography tools: How a 1200-year-old Hacking Technique Can Already Crack Tomorrow's Encrypted Vaults (Forbes, 9/3/15)

Better methods of encryption

There are many other methods for creating secret messages.

A technical name for "creating a secret message" is encryption. Decoding a secret message is called decryption.

Many of them follow the pattern shown in this image (taken from Blown to Bits page 171).

Diagram of cryptographic scenario from Blown to Bits page 171

This picture shows a way that Alice and Bob might send a message that Eve doesn't understand. What would Eve need in order to decrypt the message?

Read "Secret Keys and One-Time Pads" (Blown to Bits pages 169-173) to learn about Vigenere ciphers and methods used during World War 2 and the Cold War.