Twofish

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. Twofish is a symmetric block cipher.

A False: it uses a public key and a private key

B True: one shared key encrypts and decrypts fixed-size blocks

2. How big is the block that Twofish encrypts at a time?

A 128 bits

B 64 bits

3. What is the largest key size Twofish supports?

A 256 bits

B 56 bits

4. Which statement about Twofish's history is correct?

A It was a finalist in the competition that chose AES

B It was the winner of the competition and became AES

5. Which cipher is Twofish the successor of?

A RSA

B Blowfish

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1block_size = 128		Aplain = decrypt(encrypt(plain, key), key)
2max_key_bits = 256		Baes_finalists = ["Twofish", "Rijndael"]
3Twofish was one of the finalists in the competition to choose AES, but it did not win		CTwofish uses 128-bit blocks, not 64-bit. The 64-bit block size belongs to Blowfish, its predecessor.
4Twofish is symmetric, so the same key encrypts and decrypts the data		DTwofish is symmetric, so it uses one shared secret key, not a public key. Public keys belong to asymmetric ciphers like RSA.
5twofish_block_size = 64		ETwofish encrypts data in 128-bit blocks, double the 64-bit block size of Blowfish
6encrypt(data, public_key) # Twofish		FTwofish accepts keys of up to 256 bits, so a longer key means a bigger keyspace for an attacker to search

Definitions

Twofish

Fill in the Blanks

Complete each line of code by filling in the missing parts.

blockBits = ___

maxKeyBits = ___

IF keyBits __ 256 THEN OUTPUT "Valid Twofish key size" END IF

cipher = "Twofish" ciphertext = _______(plaintext, key)

cipher = "Twofish" isAesFinalist = ____

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A cipher = 'Twofish'

B block_bits = 64

C max_key_bits = 256

D print(cipher, block_bits)

Question 2

A cipher = 'Twofish'

B max_bits = 512

C key_bits = 192

D print(key_bits <= max_bits)

Question 3

A cipher = 'Twofish'

B type = 'symmetric'

C predecessor = 'AES'

D print(cipher, predecessor)

Question 4

A cipher = 'Twofish'

B aes_finalist = False

C if aes_finalist:

D print('AES contest')

Question 5

A shared_key = 'k3y'

B message = 'hello'

C tf = Twofish(shared_key)

D secret = tf.decrypt(message)

Answer Key

Twofish

Section A: Quick Questions

B — True: one shared key encrypts and decrypts fixed-size blocks

A — 128 bits

A — 256 bits

A — It was a finalist in the competition that chose AES

B — Blowfish

Section B: Match the Pairs

block_size = 128 → Twofish encrypts data in 128-bit blocks, double the 64-bit block size of Blowfish

max_key_bits = 256 → Twofish accepts keys of up to 256 bits, so a longer key means a bigger keyspace for an attacker to search

Twofish was one of the finalists in the competition to choose AES, but it did not win → aes_finalists = ["Twofish", "Rijndael"]

Twofish is symmetric, so the same key encrypts and decrypts the data → plain = decrypt(encrypt(plain, key), key)

twofish_block_size = 64 → Twofish uses 128-bit blocks, not 64-bit. The 64-bit block size belongs to Blowfish, its predecessor.

encrypt(data, public_key) # Twofish → Twofish is symmetric, so it uses one shared secret key, not a public key. Public keys belong to asymmetric ciphers like RSA.

Section C: Definitions

Twofish — A symmetric block cipher and Blowfish's successor. It encrypts 128-bit blocks with keys up to 256 bits and was a finalist in the competition that chose AES.

Section D: Fill in the Blanks

blockBits = 128

maxKeyBits = 256

IF keyBits <= 256 THEN
    OUTPUT "Valid Twofish key size"
END IF

cipher = "Twofish"
ciphertext = encrypt(plaintext, key)

cipher = "Twofish"
isAesFinalist = True

Section E: Spot the Error

B — Twofish encrypts 128-bit blocks. 64-bit blocks belong to older ciphers such as Blowfish and DES. The line should be block_bits = 128.

B — Twofish keys can be up to 256 bits, not 512. The line should be max_bits = 256.

C — Twofish is the successor of Blowfish, so Blowfish is its predecessor. AES was a rival finalist and not Twofish's predecessor. The line should be predecessor = 'Blowfish'.

B — Twofish was a finalist in the competition that chose AES, so the value should be True. It was not the winner, but it was a finalist.

D — The plaintext message is being turned into ciphertext, so it must be encrypted. The line should be secret = tf.encrypt(message).