Cipher Disk In-Class Activity

Hands-on practice with monoalphabetic and polyalphabetic substitution ciphers.

By Drs. Dave Eargle and Anthony Vance

Use the cipher disk at the following site to answer the questions below:

Note: To begin, make sure that the letter A on the inner disk is aligned with the letter A on the outer disk.

Part 1: Monoalphabetic Cipher

Encrypt the plaintext secret

In cryptography, the unencrypted text you can read is called the “plaintext.” The encrypted text is called “ciphertext.”

Using the wheel, encrypt the word secret by rotating the outer wheel three positions counter clockwise so that the letter A on the inner wheel aligns with the letter D on the outer wheel. The inner wheel shows the plaintext, and the outer wheel shows the ciphertext.

Question : What is the resulting word?

Encrypt a word of your choice

Encrypt a short word using the above method, but choose a different key, a number between 1–25 (we can’t use 0, because that will just give us the plaintext). Write the ciphertext on a piece of paper and ask your partner to decrypt it.

Question : What is the maximum number of guesses that your partner has to make to find the plaintext??
Question : Assuming each key is equally likely, on average, how many guesses does your partner have to make to find the plaintext?
Question : Was your partner able to make fewer guesses than your answers to Questions 1 and 2? If so, how was your partner able to do it faster?

Part 2: Polyalphabetic Cipher

You can make this cipher stronger by using more than one key to encrypt the message. For example, encrypt the message:

Secret message: launchattackcharly

Keyword repeated: securitysecurityse

Use the word security as the key, repeating it so that the key is as long as the message (e.g., securitysecu).

Encrypt by rotating the outer wheel so that s in the word security aligns with a on the inner wheel. Now l in the word launch on the inner wheel maps to the letter d on the outer wheel, so d is the ciphertext.

Next, rotate the outer wheel so that e in the word security aligns with a on the inner wheel. Now a in the word launch on the inner wheel maps to e on the outer wheel.

Question : What is the rest of the ciphertext for “LaunchAttack” using the polyalphabetic key “security”? Compare your answer to your partner’s.
Question : Imagine that you encrypted an entire page of text with the keyword “security”. What patterns do you think would help an attacker guess the key?

One-time pad: A key equal in length to the message

What if you used a random polyalphabetic key that is as long as the message.

For example, let’s say our plaintext is:


And the polyalphabetic key is a string of random characters as long as the message:


Question : Discuss with your partner whether an attack in this case is possible. Are there any repeating patterns?