Demonstrate Encryption and Encoding

Cryptography Lab: Encrypting and Decrypting Messages

This lab exercise will introduce students to the concept of cryptography by using a simple Caesar Cipher for encryption and decryption.

Target Audience: CBTC students

Learning Objectives:

• Understand the basic principles of encryption and decryption.
• Apply a Caesar Cipher for encoding and decoding messages.
• Explore the limitations of simple ciphers.

Lab Materials:

Computers with internet access Pen and paper or a text editor Caesar Cipher reference sheet

Caesar Cipher Reference Sheet:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

Lab Procedure:

Introduction (5 minutes):

• Briefly explain the concept of cryptography and its importance in securing information.
• Introduce the Caesar Cipher as a simple example of a substitution cipher.

Caesar Cipher Operation (10 minutes):

• Distribute the Caesar Cipher reference sheet.
• Explain how the cipher works by shifting each letter in the message a certain number of positions (called the key) based on the reference sheet.
• For example, with a key of 3, "hello" becomes "khoor."
• Emphasize that the key needs to be shared between the sender and receiver for successful decryption.

Encryption Activity (20 minutes):

• Divide students into pairs.
• Each pair will choose a secret key (a number between 1-25).
• Provide a short message (or have students write their own) for them to encrypt.
• Students will use the Caesar Cipher reference sheet to encrypt their message using their chosen key.
• Encourage them to write down the encrypted message and their key on a separate piece of paper to simulate secure communication.

Decryption Activity (20 minutes):

• Students will swap their encrypted messages with another pair (acting as the receiver).
• The receiving pair will need to decipher the message using the Caesar Cipher reference sheet and the key provided by the sender.
• Students can discuss the importance of keeping the key confidential to prevent unauthorized decryption.

Wrap-up and Discussion (10 minutes):

• Discuss the ease of cracking a Caesar Cipher with simple brute-force methods (trying all possible keys).
• Briefly mention the existence of more complex and secure ciphers used in modern cryptography.
• (Optional) If using computers with internet access, you can introduce online Caesar Cipher tools for demonstration.

Assessment:

• Observe students' participation during the explanation and activities.
• Collect the encrypted messages and keys from each pair to verify their understanding.
• Facilitate a class discussion about the limitations of the Caesar Cipher and the importance of stronger encryption methods.

Safety Considerations:

• Remind students to keep their chosen keys confidential during the activity.
• Emphasize that this lab is for educational purposes and simple ciphers are not suitable for real-world secure communication.

Ceaser Cipher How to

The Caesar Cipher is a simple substitution cipher where each letter in a message is shifted a certain number of positions down the alphabet. The provided reference sheet helps you visualize this shift to encrypt and decrypt messages.

Here's how it works:

Shift Value (Key): You choose a secret number between 1 and 25. This number represents the number of positions each letter will be shifted in the alphabet. This secret number is your key, and it's crucial to keep it confidential for decryption.

Encryption:

• Write your message down.
• Look at the top row of the reference sheet, which represents the alphabet before encryption (plain text).
• Find each letter of your message in the top row.
• Now, move down that same column by the number of positions specified by your key. For example, if your key is 3, move down 3 positions.
• The letter you land on in the bottom row is the encrypted version of the original letter.
• Repeat this process for each letter in your message.

Decryption:

• You'll need the original message (encrypted text) and the key used for encryption.
• Follow the same steps as encryption, but in reverse.
• Look at the encrypted letter in the bottom row of the reference sheet.
• Move up the same column by the number of positions specified by your key.
• The letter you land on in the top row is the original letter from the message.

Example:

Message: "hello"
Key: 3
Encryption:

H -> K (Move down 3 positions in the "H" column)
E -> H (Move down 3 positions in the "E" column)
L -> O (Move down 3 positions in the "L" column)
L -> O (Move down 3 positions in the "L" column)
Encrypted message: "khoor"

Decryption:

K -> H (Move up 3 positions in the "K" column)
H -> E (Move up 3 positions in the "H" column)
O -> L (Move up 3 positions in the "O" column)
O -> L (Move up 3 positions in the "O" column)
Decrypted message: "hello" (Original message)