Symmetric Encryption: Caesar Cipher
One of the simplest methods of encryption is the Caesar Cipher. It is also known as a rotation cipher. The Caesar Cipher demonstrates the principles of symmetric encryption, but should not be used for serious cryptography.
Learning Objectives
You should be able to:
- Describe the algorithm and key in the Caesar Cipher
- Describe a rotation in the Caesar Cipher
- Brute force decrypt a message encrypted with the Caesar Cipher
Video Walkthrough
Use this video to follow along with the steps in this lab.
The Caesar Ciphers
Julius Caesar used this particular cipher for military communications. This was not the first attempt at encrypting data, but Julius Caesar made this particular cipher popular. As far as encryption goes, it is not strong. At all. Fortunately for Julius Caesar, many of his enemies could not read, so a basic encryption scheme worked well enough for his purposes.
The Caesar Cipher is also called a rotation cipher because the alphabet is "rotated" a certain number of characters. The number of rotations is the key. The following table shows an alphabet with zero rotations:
Alphabet with Zero Rotation
| Cleartext | A | B | C | D | E | F | ... |
|---|---|---|---|---|---|---|---|
| Ciphertext | A | B | C | D | E | F | ... |
Alphabet with One Rotation
| Cleartext | A | B | C | D | E | F | ... |
|---|---|---|---|---|---|---|---|
| Ciphertext | Z | A | B | C | D | E | ... |
Alphabet with Two Rotations
| Cleartext | A | B | C | D | E | F | ... |
|---|---|---|---|---|---|---|---|
| Ciphertext | Y | Z | A | B | C | D | ... |
Caesar himself used an alphabet with 3 rotations.
Alphabet with Three Rotations
| Cleartext | A | B | C | D | E | F | ... |
|---|---|---|---|---|---|---|---|
| Ciphertext | X | Y | Z | A | B | C | ... |
Using the table above, the word FACE would be encrypted as CXZB.
Challenge: Encrypt the word CAFE using a rotation of 3.
Cyber Chef
The CyberChef is an open-source application that lets you build "recipes" of many cybersecurity algorithms. The ROT-13 algorithm implements the Caesar Cipher. The default rotation is 13.
- Go to https://gchq.github.io/CyberChef/.
- In the Operations filter, type rot.
- Click and drag the ROT13 operation into the recipe area.
- Type the word CAFE in the Input. The encrypted text will appear in the Output area.
- Every time you change the input or the operation, CyberChef will "re-bake" the recipe to produce new output.
- Change the Amount in the ROT13 algorithm. Change it to -3. Does that match what you had encrypted earlier?
Brute Force Decrypting the Caesar Cipher
- There are 26 letters in the English alphabet, so there are only 26 possible unique rotations. (A rotation of 27 is the same thing as a rotation of 1.) In a brute force attack, every possible key is tried.
- Imagine that you find the following encrypted text.
Nby Wuymul Wcjbyl cm lyuffs hin mywoly. Sio mbiofx hin omy cn ni jlinywn xunu.
- Paste that text into the Input in the CyberChef.
- Increase or decrease the Amount of the ROT13 rotation until you find something that reads like English.
- How hard was it to decrypt the message?
Caesar Cipher Summary
The Caesar Cipher is not strong. But, it demonstrates how symmetric algorithms work. Symmetric algorithms have algorithms (e.g., shifting letters of the alphabet) and keys (the number of letters to shift). The Caesar Cipher's algorithm is not complex, and the key is easy to brute force. Modern encryption algorithms have very sophisticated algorithms and require much better keys.
Reflection
- Was Julius Caesar wrong to use such a simple encryption algorithm?
- Will people in two thousand years laugh at our current computer systems?
Key Terms
- Caesar Cipher: A type of substitution cipher where each letter in the plaintext is shifted a fixed number of places down or up the alphabet. For example, with a shift of 3, 'A' would be encrypted to 'D', 'B' to 'E', and so on. It is named after Julius Caesar, who reportedly used it in his private correspondence.
- Rotation Cipher: An encryption cipher in which the letters of the plaintext are rotated by a fixed number of positions in the alphabet. The term "rotation" emphasizes the circular nature of the shift, where after 'Z', the alphabet wraps around to 'A'.