The Caesar Cipher

The key for the Caesar Cipher will be a number from 1 to 26. Unless you know the key (that is, know the number used to encrypt the message), you won’t be able to decrypt the secret code.

The Caesar Cipher was one of the earliest ciphers ever invented. In this cipher, you encrypt a message by taking each letter in the message (in cryptography, these letters are called symbols because they can be letters, numbers, or any other sign) and replacing it with a “shifted” letter. If you shift the letter A by one space, you get the letter B. If you shift the letter A by two spaces, you get the letter C. Figure 14-1 is a picture of some letters shifted over by three spaces.

To get each shifted letter, draw out a row of boxes with each letter of the alphabet. Then draw a second row of boxes under it, but start a certain number (this number is the key) of spaces over. After the letters at the end, wrap around back to the start of the boxes. Here is an example with the letters shifted by three spaces:

Invent with Python

Making paper cryptography paper tools

A Virtual Cipher Wheel

Assignments:
1. En-Crypt: Caesar Cipher  – Cryptography

Use the cipherwheel to encrypt a message. In a piece of paper, share the encrypted message with a partner.
In this post, you will type the message, the encrypted message and the one piece of information needed for your partner to decrypt it.

2. De-Crypt: Caesar Cipher –  Cryptography
In this post type your partner’s encrypted message and the one piece of information needed to decrypt it. What is the message?

NOTE: Include your partner’s name

In this post type your partner’s encrypted message and the one piece of information needed to decrypt it. What is the message?

NOTE: Include your partner’s name

3. Your Cipher – Cryptography

Design your own cipher and a device to easily encrypt and decrypt messages. Think ahead and prepare a checklist of materials you might need.

1. Describe your cipher.
2. Draw a diagram for your device.
3. Build your device.
4. Type here decryption instructions.

4. Friend’s Cipher – Cryptography

Find a classmate and exchange ciphers, instructions and encrypted message. After you checked with your classmate that you were able to decrypt the message, comment on the following:
1. Efficiency of the cipher
2. Quality of the instructions

NOTE: If the instructions were not clear or you couldn’t follow them, help your classmate make the right changes.
Include the name of your classmate.

5. Brute Force – Cryptography

If you didn’t have a key to decipher an encrypted message, how would write a program to decrypt it?

Write the pseudocode for you decrypting program using brute force.

Submit the instructions for your encryption device in the corresponding post. Include your name in the instructions.
Print your instructions and attach it to your device. I will “glue/attach” it to a poster. Please hand everything with your name on it.