Generating a one-time pad key in Python
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. A one-time pad key for the message "HELLO" must be how long?
2. Which line builds a one-time pad key with one random number (0 to 25) per character?
3. Which import is needed before using random.randint() to make a key?
4. What range of numbers does random.randint(0, 25) produce?
5. After using a one-time pad key to encrypt a message, what should you do with that key?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1key = [random.randint(0, 25) for c in message] | ALoads Python's random module so you can call random.randint() to pick random key values | |
| 2import random | Bkey = [random.randint(0, 25) for c in message] | |
| 3A one-time pad key must be exactly as long as the message, so build it with one random value per character | CThe same random number is repeated for every character, so the key is really one value copied many times and is easy to crack. | |
| 4For stronger security, the secrets module can pick each random key value instead of random | DA one-time pad key must never be reused. Using it for a second message lets attackers compare the two messages and break the encryption. | |
| 5key = [random.randint(0, 25)] * len(message) | Ekey = [secrets.randbelow(26) for c in message] | |
| 6key = [random.randint(0, 25) for c in message] # reuse the same key for the next message | FBuilds a key list with one random number from 0 to 25 for every character in the message |
C
Definitions
key = [random.randint(0, 25) for c in message]
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
import ______
msg = "HELLO"
key = []
2.
key = [random._______(0, 25) for c in msg]
3.
key = [random.randint(0, __) for c in msg]
4.
key = []
for c in ___:
key.append(random.randint(0, 25))
5.
key = [_______.randbelow(26) for c in msg]
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
from random import randint
B
key = ''
C
for c in 'HELLO':
D
key.append(randint(0, 25))
Question 2
A
message = 'ATTACK'
B
import random
C
key = []
D
for c in message:
E
n = secrets.randbelow(26)
F
key.append(n)
Question 3
A
import random
B
message = 'HELLO'
C
key = []
D
for c in key:
E
n = random.randint(0, 25)
F
key.append(n)
Question 4
A
from random import randint
B
m = 'ATTACK'
C
k = [randint(0,25)] * len(m)
D
print(len(k) == len(m))
Question 5
A
from random import randint
B
def make_key():
C
key = []
D
for c in message:
E
n = randint(0, 25)
F
key.append(n)
Answer Key
Generating a one-time pad key in Python
Section A: Quick Questions
Q1
A — 5 values, one for each character
Q2
A — key = [random.randint(0, 25) for c in message]
Q3
A — import random
Q4
B — Any whole number from 0 to 25, including both ends
Q5
B — Never reuse it for another message
Section B: Match the Pairs
Q1
key = [random.randint(0, 25) for c in message] → Builds a key list with one random number from 0 to 25 for every character in the message
Q2
import random → Loads Python's random module so you can call random.randint() to pick random key values
Q3
A one-time pad key must be exactly as long as the message, so build it with one random value per character → key = [random.randint(0, 25) for c in message]
Q4
For stronger security, the secrets module can pick each random key value instead of random → key = [secrets.randbelow(26) for c in message]
Q5
key = [random.randint(0, 25)] * len(message) → The same random number is repeated for every character, so the key is really one value copied many times and is easy to crack.
Q6
key = [random.randint(0, 25) for c in message]
# reuse the same key for the next message → A one-time pad key must never be reused. Using it for a second message lets attackers compare the two messages and break the encryption.
Section C: Definitions
Q1
key = [random.randint(0, 25) for c in message] — Builds a one-time pad key by picking one random number from 0 to 25 for each character in the message, so the key is as long as the message. Never reuse a key.Section D: Fill in the Blanks
Q1
import random
msg = "HELLO"
key = []Q2
key = [random.randint(0, 25) for c in msg]Q3
key = [random.randint(0, 25) for c in msg]Q4
key = []
for c in msg:
key.append(random.randint(0, 25))Q5
key = [secrets.randbelow(26) for c in msg]Section E: Spot the Error
Q1
B — The key must start as an empty list so that .append() can add one random number per character. A string has no append method, so it should be key = [].
Q2
B — The code calls secrets.randbelow(26), so it must import the secrets module. Importing random leaves secrets undefined and causes a NameError. It should be import secrets.
Q3
D — The loop must run once for each character of the message. key is still empty, so the loop never runs and no key is built. It should be for c in message:.
Q4
C — This picks one random number and repeats it for every position, so the key is not random per character. The comprehension should pick a new number for each character: k = [randint(0,25) for c in m].
Q5
B — The function needs the message as a parameter so the key can be as long as the message. Without it, message is undefined inside the loop. It should be def make_key(message):.