Decoding Secrets: Exploring the Caesar Cipher

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• Computer Science
• Further Education (Key Stage 5)

Decoding Secrets: Exploring the Caesar Cipher

Learning Objectives

At the end of the lesson, you will understand the Caesar Cipher and create your own to encrypt messages.

What do you already know about secret codes and ciphers?

Introduction to Cryptography

Cryptography is the art of writing or solving codes. It secures communication and protects information.

What is the Caesar Cipher?

Take 5 minutes to research this (not using AI), and prepare to feedback in a class discussion.

What is the Caesar Cipher?

The Caesar Cipher is a substitution (or shift) cipher where each letter in the plaintext is shifted a certain number of places down the alphabet. This new alphabet is called the ciphertext.

How the Caesar Cipher Works

Choose a shift value, replace each letter with the letter that is 'n' positions down the alphabet.

In this example, E is shifted backwards 3 times

Let's Create Our Own Cipher

Choose a shift value. Write out the alphabet, then the shifted alphabet below it. This is your cipher.

E = B + 3

Therefore

Ciphertext = Plaintext + 3

D = (A+3)

F = (C+3)

C = (Z+3)

Deciphering Messages

Reverse the process: shift the letters back by the same number. Practice with a simple message.

Deciphering Another's Message

Now that you know what Caesar's cipher is, what are the techniques we can use to decrypt them?

Try decrypting this secret message:

P ht Zdpttpun

Deciphering Another's Message

There are two things that make Deciphering easier. Repeating characters and length.

• Zdpttpun

• P ht Zdpttpun

P is likely to be 'A' or 'I,' and ht needs to be a two letter word that follows afterwards. If you presume P = I, then the most likely word to come after that is 'Am.' So substitute for am.

I am Zdimmiun. What do we think this says?

Group Activity Instructions

Form groups of 2-3. Create a secret message using your cipher with the following rules.

• Simple sentence (Between 5 - 10 words)

• Plain English (No Welsh, or Welsh names)

• Must use a consistent shifting value (+3, -6, 9 etc)

• Use spaces where appropriate

• DO NOT use ChatGPT or any other AI tool. They do not work.

Positive

3) For every code you crack

2) For every group that fails to crack your code

Negative

1) For every none English word or place

2) For sentences that don't make sense

4) If your shifting value changes

Email your code and secret message to me when done

How easy was that?

Question: Would you use Caesar's Cipher, or another shifting cipher to protect your communications or work?

Take 10 minutes to research the weaknesses and strengths of this Cipher, and prepare to defend your answer.

Create your own

You will now create your own Cipher, but it must follow these rules.

• No more than 3 operations (Example below)

  • Operation 1: Flip the text

  • Operation 2: Use a shifting value of +2

  • Operation 3: Flip only the first half of the Ciphertext

• You must be able to reverse it

• Simple English (No Welsh)

• 5 Word sentence

Group Activity Instructions

Create your own Cipher, following these rules:

• No more than 3 operations (Example below)

  • Operation 1: Flip the text

  • Operation 2: Use a shifting value of +2

  • Operation 3: Flip only the first half of the Ciphertext

• You must be able to reverse it

• 5 Word sentence

Positive

6) For every code you crack

3) For every group that fails to crack your code

Negative

1) For every none English word or place

2) For sentences that don't make sense

8) If you can't decode your secret message

Email your code, secret message and 3 operations

Write down 3 things you learned in this lesson.

Write down 2 things you want to know more about.

Ask 1 question about something you haven't quite understood yet.