← ACARA Digital Technologies Years 7–8

Using a cipher wheel or strip

A tool with two alphabet rings. Turn the inner ring to the key shift so each plaintext letter lines up with its ciphertext letter on the outer ring. ROT13 is a shift of 13.

Part of: ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 54, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 105

Understanding Using a cipher wheel or strip in ACARA Digital Technologies Years 7–8

What it does: A key of 5 means the inner A sits beside the letter 5 places along, which is F. Setting the key is the first step in using the wheel. With a key of 1, C lines up with D, A with B and T with U. Reading each letter across the rings gives the ciphertext DBU. Count 5 places on from C: D, E, F, G, H. So C becomes H. ROT13 is a shift of 13, which is halfway round the 26-letter alphabet. 13 + 13 = 26, a full turn of the alphabet. So the second shift undoes the first.

Common mistakes: To encrypt, you move forward along the ring by the key shift. With a key of 3, A becomes D, so the line should be plain + key. ROT13 is a shift of 13, so the key must be 13. A shift of 12 would turn A into M, not N.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 674–695) and word search (Modules 635–695).

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Learning

Duo

Your key is a shift of 3. The plaintext is on the inner ring and the ciphertext is on the outer ring. How do you set the wheel?

Turn the inner ring so inner A lines up with outer DTurn the inner ring so inner A lines up with outer C

Matches

Key 3: A -> C

↓

A key of 3 moves A three places to D, not C. C is only a shift of 2, so the inner ring was turned the wrong amount.

Applying

Meanings

Term

cipher wheel

Meaning

A tool with two alphabet rings. Turn the inner ring to the key shift so each plaintext letter lines up with its ciphertext letter on the outer ring. ROT13 is a shift of 13.

Practice

Key: 13 (ROT13)

Testing

Gaps

key =

Check Answer

Spots

2SET plain TO 'A'

3SET cipher TO plain - key

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Quick Questions

1. Your key is a shift of 3. The plaintext is on the inner ring and the ciphertext is on the outer ring. How do you set the wheel?

A. Turn the inner ring so inner A lines up with outer D ✓

B. Turn the inner ring so inner A lines up with outer C

A shift of 3 means counting 3 letters on from A: B, C, D. So inner A sits under outer D.

2. The wheel is set to a shift of 3. You find plaintext B on the inner ring. What ciphertext letter is on the outer ring?

A. E ✓

B. D

With a shift of 3, B moves 3 places on to C, D, E. The ciphertext letter is E.

3. Your wheel is set to a shift of 5. Which ciphertext letter lines up with plaintext C?

A. G

B. H ✓

Count 5 places on from C: D, E, F, G, H. So C becomes H.

4. How far do you turn the inner ring to set up ROT13?

A. 26 places

B. 13 places ✓

ROT13 is a shift of 13, which is halfway round the 26-letter alphabet.

5. The wheel is set to ROT13. What does plaintext A become?

A. N ✓

B. M

Count 13 places on from A and you land on N. Inner A lines up with outer N.

Match the Pairs

Key 3: A -> D → Turn the inner ring so plaintext A lines up with D on the outer ring. Every letter moves 3 places, so HI becomes KL.

ROT13: A -> N → A shift of 13 turns A into N and N back into A. Using the wheel twice returns the original message.

Setting the key: turn the inner ring until A lines up with the letter that matches the shift → Key 5: A lines up with F

Reading plaintext to ciphertext: find each plaintext letter on the inner ring and read the outer ring letter beside it → Key 1: CAT -> DBU

Key 3: A -> C → A key of 3 moves A three places to D, not C. C is only a shift of 2, so the inner ring was turned the wrong amount.

ROT13: A -> M → ROT13 is a shift of 13, so A lines up with N. M is a shift of 12, which is one place short.

Key Term

cipher wheel

A tool with two alphabet rings. Turn the inner ring to the key shift so each plaintext letter lines up with its ciphertext letter on the outer ring. ROT13 is a shift of 13.

Fill in the Blanks

1. # Turn the inner ring so A meets D key = 3

Hint: Type the key shift: how many letters D is after A on the ring.

2. # ROT13 turns the ring half the alphabet key = 13

Hint: Type the key shift that ROT13 always uses (half of 26 letters).

3. index = 2 key = 3 shifted = (index + key) % 26

Hint: Type the operator that moves a plaintext letter forward by the key.

4. once = rot13("HI") back = rot13(once) print(back) # HI

Hint: Type the function name that turns the ciphertext back into plaintext.

5. ring = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" key = 13 turned = ring[key:] + ring[:key]

Hint: Type the variable that holds the shift used to set the inner ring.

Spot the Error

Question 1

A SET key TO 3

B SET plain TO 'A'

C SET cipher TO plain - key

D PRINT cipher

Line C — To encrypt, you move forward along the ring by the key shift. With a key of 3, A becomes D, so the line should be plain + key.

Question 2

A # ROT13 on the wheel

B key = 12

C plain = 'A'

D cipher = 'N'

Line B — ROT13 is a shift of 13, so the key must be 13. A shift of 12 would turn A into M, not N.

Question 3

A Turn inner ring to key 5

B Plain A lines up with E

C Plain B lines up with G

Line B — With a key shift of 5, A lines up with F (A, B, C, D, E, F is 5 steps). It should say F, which also matches B lining up with G.

Question 4

A text = 'HELLO'

B once = rot13(text)

C back = rot12(once)

D print(back)

Line C — ROT13 is its own reverse because 13 + 13 = 26. Applying rot13 a second time gets HELLO back, but rot12 would not.

Question 5

A key = 0

B plain = 'A'

C cipher = 'B'

Line A — A key of 0 means no shift, so A would stay A. To get B the key must be 1.

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