Practice encrypting and decrypting messages, analyzing simple ciphers, and explaining core ideas in modern cryptography.
Read each problem carefully. Show your work in the space provided. Use A = 0, B = 1, C = 2, and so on when a problem asks for modular arithmetic with letters.
Practice with classic ciphers, keys, encryption, and security ideas
CS - Grade 9-12
- 1
Use a Caesar cipher with a shift of 3 to encrypt the message CAT. Shift each letter 3 places forward in the alphabet.
- 2
A message was encrypted with a Caesar cipher using a shift of 5. The ciphertext is MJQQT. Decrypt the message.
- 3
Using A = 0 through Z = 25, encrypt the letter R with a Caesar shift of 9. Show the modular arithmetic.
- 4
A substitution cipher maps plaintext letters to ciphertext letters using a secret shuffled alphabet. Explain one reason a simple substitution cipher is usually not secure against a determined attacker.
- 5
In English text, the letter E often appears more than any other letter. In a long ciphertext made with a simple substitution cipher, the symbol X appears most often. What might an attacker reasonably guess, and why?
- 6
Use the Vigenere cipher to encrypt the plaintext HI using the key NO. Use A = 0, B = 1, and so on. Add the plaintext value and key value modulo 26 for each letter.
- 7
Use the Vigenere cipher to decrypt the ciphertext UW using the key NO. Use A = 0, B = 1, and so on. Subtract the key value from the ciphertext value modulo 26.
- 8
A one-time pad can provide perfect secrecy when used correctly. List two conditions that must be true for a one-time pad to be secure.
- 9
Encrypt the 4-bit plaintext 1010 using XOR with the 4-bit key 1100. Show the result bit by bit.
- 10
Explain the difference between symmetric encryption and asymmetric encryption. Include what kind of keys are used in each system.
- 11
A website uses HTTPS. Explain how encryption helps protect a user's password when it is sent from the browser to the website.
- 12
A classmate says, "Hashing is the same as encryption because both hide data." Explain why this statement is not correct.