CS: Cryptography: Ciphers and Encryption Worksheet

Question 1

Use a Caesar cipher with a shift of 3 to encrypt the message CAT. Shift each letter 3 places forward in the alphabet.

Accepted Answer

The encrypted message is FDW. C shifts to F, A shifts to D, and T shifts to W.

Question 2

A message was encrypted with a Caesar cipher using a shift of 5. The ciphertext is MJQQT. Decrypt the message.

Accepted Answer

The decrypted message is HELLO. Each ciphertext letter is shifted 5 places backward in the alphabet.

Question 3

Using A = 0 through Z = 25, encrypt the letter R with a Caesar shift of 9. Show the modular arithmetic.

Accepted Answer

R is 17. Adding the shift gives 17 + 9 = 26. Since 26 mod 26 = 0, the encrypted letter is A.

Question 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.

Accepted Answer

A simple substitution cipher is not very secure because it preserves letter patterns and letter frequencies. An attacker can use frequency analysis to guess common letters, words, and repeated patterns.

Question 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?

Accepted Answer

An attacker might guess that X represents E. This is reasonable because frequency analysis compares common ciphertext symbols with common letters in the language.

Question 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.

Accepted Answer

H is 7 and N is 13, so 7 + 13 = 20, which is U. I is 8 and O is 14, so 8 + 14 = 22, which is W. The ciphertext is UW.

Question 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.

Accepted Answer

U is 20 and N is 13, so 20 - 13 = 7, which is H. W is 22 and O is 14, so 22 - 14 = 8, which is I. The plaintext is HI.

Question 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.

Accepted Answer

The key must be truly random and at least as long as the message. The key must also be used only once and kept secret from attackers.

Question 9

Encrypt the 4-bit plaintext 1010 using XOR with the 4-bit key 1100. Show the result bit by bit.

Accepted Answer

The ciphertext is 0110. The XOR steps are 1 XOR 1 = 0, 0 XOR 1 = 1, 1 XOR 0 = 1, and 0 XOR 0 = 0.

Question 10

Explain the difference between symmetric encryption and asymmetric encryption. Include what kind of keys are used in each system.

Accepted Answer

Symmetric encryption uses the same secret key for encryption and decryption. Asymmetric encryption uses a public key and a private key, where one key can encrypt and the matching key can decrypt.

Question 11

A website uses HTTPS. Explain how encryption helps protect a user's password when it is sent from the browser to the website.

Accepted Answer

Encryption changes the password into unreadable ciphertext while it travels across the network. Even if someone intercepts the data, they should not be able to read the password without the correct key.

Question 12

A classmate says, "Hashing is the same as encryption because both hide data." Explain why this statement is not correct.

Accepted Answer

Hashing is not the same as encryption. Encryption is designed to be reversible with the correct key, while a cryptographic hash is designed to be one-way and produce a fixed-size digest that cannot practically be turned back into the original data.

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CS: Cryptography: Ciphers and Encryption

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