Decrypting a message with a Caesar shift

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. A message was encrypted with a Caesar shift of 3. To decrypt it, which way do you move each letter?

A Forward 3 places along the alphabet

B Back 3 places along the alphabet

2. The key is 1. What is the plaintext letter for the ciphertext letter C?

A B

B D

3. The key is 1. What is the plaintext letter for the ciphertext letter A?

A Z

B B

4. Decrypt KHOOR with a key of 3. Which plaintext is correct?

A NKRRU

B HELLO

5. The key is 5. What is the plaintext letter for the ciphertext letter D?

A Y

B I

Match the Pairs

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

Left column		Right column
1K shifted back 3 = H		Aplain = (cipher - key) mod 26
2B shifted back 3 = Y		BKHOOR with key 5 = FCJJM (nonsense)
3To decrypt, subtract the key from each letter's position and wrap around the 26 letters		CWrapping from A back to Z: B goes back to A, then Z, then Y.
4Check that the result is a real message, because a wrong key gives nonsense		DDecrypting: move the ciphertext letter K three places back along the alphabet to get the plaintext letter H.
5KHOOR with key 3 -> NKRRU		EThe shift did not wrap around. Going back 1 from A wraps to Z.
6A shifted back 1 -> no letter, stop		FThe letters were moved forward by 3, which is how you encrypt. To decrypt, move each letter back by 3 to get HELLO.

Definitions

shift back by key

Fill in the Blanks

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

plainNum ← cipherNum - ___

plainNum ← (cipherNum - key) ___ 26

FOR EACH letter IN __________ pos ← position(letter) - key

IF plaintext is a real message THEN OUTPUT plaintext ____ try another key

num = ord(letter) - 65 plain = (num - key) % __

Spot the Error

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

Question 1

A direction = 'forward'

B key = 3

C cipher = 'KHOOR'

D print(decrypt(cipher, key))

Question 2

A def decrypt_letter(c, key):

B n = ord(c) - 65

C n = (n + key) % 26

D return chr(n + 65)

Question 3

A n = (0 - 3) % 25

B print(n)

C print(chr(n + 65))

Question 4

A cipher = 'KHOOR'

B key = 3

C plain = ''

D for c in plain:

E n = (ord(c)-65-key) % 26

F plain += chr(n + 65)

Question 5

A plain = decrypt(cipher, key)

B if plain == cipher:

C print('Key found')

D else:

E print('Try another key')

Answer Key

Decrypting a message with a Caesar shift

Section A: Quick Questions

B — Back 3 places along the alphabet

A — B

A — Z

B — HELLO

A — Y

Section B: Match the Pairs

K shifted back 3 = H → Decrypting: move the ciphertext letter K three places back along the alphabet to get the plaintext letter H.

B shifted back 3 = Y → Wrapping from A back to Z: B goes back to A, then Z, then Y.

To decrypt, subtract the key from each letter's position and wrap around the 26 letters → plain = (cipher - key) mod 26

Check that the result is a real message, because a wrong key gives nonsense → KHOOR with key 5 = FCJJM (nonsense)

KHOOR with key 3 -> NKRRU → The letters were moved forward by 3, which is how you encrypt. To decrypt, move each letter back by 3 to get HELLO.

A shifted back 1 -> no letter, stop → The shift did not wrap around. Going back 1 from A wraps to Z.

Section C: Definitions

shift back by key — To decrypt a Caesar ciphertext, move each letter back along the alphabet by the key number to get the plaintext. A wraps back to Z. Then check the result is a real message.

Section D: Fill in the Blanks

plainNum ← cipherNum - key

plainNum ← (cipherNum - key) MOD 26

FOR EACH letter IN ciphertext
  pos ← position(letter) - key

IF plaintext is a real message THEN
  OUTPUT plaintext
ELSE try another key

num = ord(letter) - 65
plain = (num - key) % 26

Section E: Spot the Error

A — Decrypting a Caesar cipher moves each letter BACK along the alphabet by the key. Moving forward is what encryption does. It should be direction = 'back'.

C — This adds the key, which moves the letter forward and re-encrypts it. To decrypt you subtract the key: n = (n - key) % 26.

A — The alphabet has 26 letters, so wrapping must use % 26. With % 26, shifting A back 3 gives 23, which is X. Using 25 gives the wrong letter.

D — The loop must go through each letter of the ciphertext. plain is empty at this point, so nothing would be decrypted. It should be for c in cipher:.

B — After decrypting you must check that the result is a real message that makes sense. Comparing it with the ciphertext does not test that. It should be if plain makes sense:.