Breaking a Caesar cipher by trying every shift

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. How many different shifts could an attacker need to try to break a Caesar cipher?

A Millions

B 25

2. Which method breaks a Caesar cipher without knowing the key?

A Try every shift from 1 to 25 until the text is readable

B Guess one shift and give up if it fails

3. The attacker decrypts ciphertext with shift 7 and gets 'XQZ PLW BTR'. What should they do?

A Try the next shift, because the text is not readable

B Stop, because shift 7 must be the key

4. How does an attacker know they have found the right shift?

A The result turns into readable words

B The result turns into a longer message

5. Why is having only 25 possible keys a weakness?

A The message becomes too long to send

B An attacker can check every key quickly

Match the Pairs

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

Left column		Right column
1for shift in range(1, 26):		ABrute force means trying every shift, not guessing one. The real key could be any number from 1 to 25.
2if "THE" in guess:		BLoops through every possible Caesar shift from 1 to 25 so each one can be tested
3A Caesar cipher has only 25 possible keys, so an attacker can test them all		Ctotal_keys = 25
4Brute force means trying every shift and printing each result to look for readable text		Dprint(shift, decrypt(text, shift))
5for shift in range(1, 25):		Erange(1, 25) stops at 24, so shift 25 is never tested and the correct key could be missed.
6Only try shift 3, because that is the common one.		FChecks whether a decrypted guess contains a real word, which suggests the shift was correct

Definitions

brute force

Fill in the Blanks

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

for shift in range(1, __): print(shift)

___ shift in range(1, 26): print(shift)

for shift in range(1, 26): print(shift, decrypt("KHOOR", _____))

plain = decrypt(text, shift) if "the" __ plain: print("Found it!", shift)

keys = list(range(1, 26)) print(___(keys))

Spot the Error

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

Question 1

A text = "KHOOR"

B for k in range(1, 25):

C print(k, shift(text, -k))

Question 2

A text = "KHOOR"

B for k in range(1, 26):

C plain = decrypt(text, k)

D print(k, text)

Question 3

A shifts = list(range(1, 26))

B count = len(shifts) + 1

C print(count, "keys to try")

Question 4

A for k in range(1, 26):

B guess = decrypt(text, k)

C if "THE" in text:

D print("Found key", k)

Question 5

A total_keys = 26

B tried = 10

C left = total_keys - tried

D print(left, "shifts left")

Answer Key

Breaking a Caesar cipher by trying every shift

Section A: Quick Questions

B — 25

A — Try every shift from 1 to 25 until the text is readable

A — Try the next shift, because the text is not readable

A — The result turns into readable words

B — An attacker can check every key quickly

Section B: Match the Pairs

for shift in range(1, 26): → Loops through every possible Caesar shift from 1 to 25 so each one can be tested

if "THE" in guess: → Checks whether a decrypted guess contains a real word, which suggests the shift was correct

A Caesar cipher has only 25 possible keys, so an attacker can test them all → total_keys = 25

Brute force means trying every shift and printing each result to look for readable text → print(shift, decrypt(text, shift))

for shift in range(1, 25): → range(1, 25) stops at 24, so shift 25 is never tested and the correct key could be missed.

Only try shift 3, because that is the common one. → Brute force means trying every shift, not guessing one. The real key could be any number from 1 to 25.

Section C: Definitions

brute force — Trying every possible key, such as all 25 Caesar shifts, until readable text appears. A cipher with so few keys is weak because an attacker can check them all quickly.

Section D: Fill in the Blanks

for shift in range(1, 26):
    print(shift)

for shift in range(1, 26):
    print(shift)

for shift in range(1, 26):
    print(shift, decrypt("KHOOR", shift))

plain = decrypt(text, shift)
if "the" in plain:
    print("Found it!", shift)

keys = list(range(1, 26))
print(len(keys))

Section E: Spot the Error

B — range(1, 25) stops at 24, so shift 25 is never tested. To try every Caesar key the loop must be range(1, 26).

D — This prints the original ciphertext every time, so the attacker never sees the decrypted attempt. It should print k and plain.

B — A Caesar cipher has only 25 keys, and len(shifts) is already 25. Adding 1 gives 26, which is wrong. It should be count = len(shifts).

C — This checks the original ciphertext, which never changes, so it can never spot readable text. It must check the decrypted guess: if "THE" in guess.

A — A Caesar cipher has 25 useful keys. A shift of 26 returns the original letters, so it is not a real key. It should be total_keys = 25.