Blowfish
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. Blowfish uses one shared key to both encrypt and decrypt data.
2. Which describes the key length of Blowfish?
3. How big is each block that Blowfish encrypts at a time?
4. Which type of cipher is Blowfish?
5. A developer wants a cipher they can use in their app without paying a licence fee. Why could Blowfish suit them?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1cipher = Blowfish(key) | AChecks the key is at most 56 bytes (448 bits), the longest key Blowfish allows | |
| 2len(key) <= 56 | BSets up a Blowfish cipher with one secret key, so the same key encrypts and decrypts the data | |
| 3Blowfish encrypts data in fixed 64-bit blocks, which is 8 bytes at a time | CBlowfish uses 64-bit blocks, not 128-bit blocks. 128-bit blocks belong to AES. | |
| 4Blowfish is a block cipher that is not patented and is free for anyone to use because it is in the public domain | DA Blowfish key can be at most 448 bits (56 bytes), so an 800-bit key is too long. | |
| 5BLOCK_SIZE = 128 # Blowfish bits | ElicenceFee = 0 | |
| 6key = 'a'*100 # 800-bit Blowfish key | FBLOCK_SIZE = 8 |
C
Definitions
Blowfish
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
blockBits ← __
2.
maxKeyBits ← ___
3.
blockBits ← 64
OUTPUT _________
4.
isPublicDomain ← ____
5.
IF keyBits __ 448 THEN
OUTPUT "Valid Blowfish key"
END IF
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
type = "asymmetric"
B
cipher = "Blowfish"
C
blockBits = 64
Question 2
A
keyBytes = 16
B
keyBits = keyBytes * 8
C
maxBits = 448
D
valid = keyBits == maxBits
Question 3
A
blockBits = 128
B
blockBytes = blockBits // 8
C
print(blockBytes)
Question 4
A
cipher = "Blowfish"
B
maxKeyBits = 64
C
minKeyBits = 32
Question 5
A
cipher = "Blowfish"
B
keyLength = "fixed"
C
print(cipher, keyLength)
Answer Key
Blowfish
Section A: Quick Questions
Q1
A — True
Q2
B — Variable length, up to 448 bits
Q3
A — 64 bits
Q4
B — Symmetric block cipher
Q5
A — It is in the public domain, so anyone can use it for free
Section B: Match the Pairs
Q1
cipher = Blowfish(key) → Sets up a Blowfish cipher with one secret key, so the same key encrypts and decrypts the data
Q2
len(key) <= 56 → Checks the key is at most 56 bytes (448 bits), the longest key Blowfish allows
Q3
Blowfish encrypts data in fixed 64-bit blocks, which is 8 bytes at a time → BLOCK_SIZE = 8
Q4
Blowfish is a block cipher that is not patented and is free for anyone to use because it is in the public domain → licenceFee = 0
Q5
BLOCK_SIZE = 128 # Blowfish bits → Blowfish uses 64-bit blocks, not 128-bit blocks. 128-bit blocks belong to AES.
Q6
key = 'a'*100 # 800-bit Blowfish key → A Blowfish key can be at most 448 bits (56 bytes), so an 800-bit key is too long.
Section C: Definitions
Q1
Blowfish — A fast symmetric block cipher with a variable-length key (up to 448 bits) and 64-bit blocks. It is free for anyone to use, as it is in the public domain.Section D: Fill in the Blanks
Q1
blockBits ← 64Q2
maxKeyBits ← 448Q3
blockBits ← 64
OUTPUT blockBitsQ4
isPublicDomain ← TRUEQ5
IF keyBits <= 448 THEN
OUTPUT "Valid Blowfish key"
END IFSection E: Spot the Error
Q1
A — Blowfish is a symmetric cipher because one shared key both encrypts and decrypts. Line 1 should be type = "symmetric".
Q2
D — Blowfish has a variable-length key, so any key up to 448 bits is allowed. The check should be keyBits <= maxBits, not an exact match.
Q3
A — Blowfish works on 64-bit blocks, not 128-bit blocks (AES uses 128). Line 1 should be blockBits = 64.
Q4
B — The maximum Blowfish key length is 448 bits. 64 is the block size, not the key limit. Line 2 should be maxKeyBits = 448.
Q5
B — Blowfish allows a variable-length key, up to 448 bits. Line 2 should be keyLength = "variable".