Page 1: Cryptography

Unit 4, Lab 2, Page 1

BH: It would be nice to look at some serious ciphers, e.g., Playfair.

In this lab, you will learn about security risks on the Internet and how data can be encoded to protect it.

On this page, you will experiment with different ways of encoding and decoding a message and learn a simple method of symmetric encryption.

1. Write a secret message to send to a partner. Invent some way of encoding the message. Then give the encoded message to your partner.
2. Your partner will give you an encoded message. When you get it, try to decode it. What strategies are you using to decode it?
3. With another pair of students, compare the methods you used to encode your messages. Was anyone able to decode their message? How?

: Encryption and Decryption

IOC-2.B.5

• Encryption is the process of encoding data to prevent unauthorized access.
• Decryption is the process of decoding the data.

You might have used a substitution cipher to encode your message, substituting each letter of the alphabet with some other letter. You could substitute letters in any order, like this:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

PQVFUBZOTHWYELIXRNAMGDSCKJ

That’s called a simple substitution cipher.

An easier way is to just shift the letters in order. For example, this is a shift of 3:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

DEFGHIJKLMNOPQRSTUVWXYZABC

That’s a special kind of simple substitution cipher called a Caesar Cipher (named for Julius Caesar) or a shift cipher.

: Symmetric Encryption

IOC-2.B.5

Substitution ciphers are examples of symmetric encryption because they use the same key for both encryption and decryption.

There are more complicated methods of symmetric encryption, but they always use the same key at both ends. How has symmetric encryption been used?

Up through the 1970s, symmetric encryption was all people had, but they used much more complicated algorithms than substitution ciphers. These days, simple substitution ciphers are used as puzzles in magazines, but breaking the German Enigma cipher in World War II, which you’ll learn about in Unit 5, took a building full of mathematicians and computing equipment.

4. The person who receives your message has to know how to decode it. Describe an algorithm for decoding a message that was created with a Caesar (shift) cipher.

5. If you intercepted a message that was created with a simple substitution cipher, how might you decode it without knowing the key?
6. What are some ways the sender and receiver could make the encryption more secure than a simple substitution cipher?

1. A simple substitution cipher is relatively easy to break because some letters are used more frequently than others.

   • Read about Al-Kindi, the Arabic philosopher and mathematician who first described the frequency analysis method of code-breaking, in “Breaking Substitution Ciphers” (Blown to Bits pages 166-169).
   • 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)
2. Read “Secret Keys and One-Time Pads” (Blown to Bits pages 169-173) to learn about Vigenère ciphers and methods used during World War 2 and the Cold War.