← ACARA Digital Technologies Years 7–8

Encrypting a message with a Caesar shift

Move each plaintext letter forward along the alphabet by the key number. With a key of 3, A becomes D. Write the new letter and keep spaces unchanged.

Part of: ACARA Digital Technologies Years 7–8 — Networks and Cyber Threats — Module 51, ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 102, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 10, WA Computer Science ATAR Year 11 — Cyber Security — Module 66

Understanding Encrypting a message with a Caesar shift in ACARA Digital Technologies Years 7–8

What it does: The shifted alphabet starts at D, so it is a shift of 3. Each plaintext letter is swapped for the letter below it. With a key of 3, every letter moves forward 3 places, so A, B and C become D, E and F. To encrypt, you shift forward by the key. Moving backward is how you decrypt. Only letters are shifted. Spaces are kept unchanged. C becomes F, A becomes D and T becomes W. ZXQ comes from moving backward, which is the wrong direction. For a key of 3, the shifted row starts at D. So A lines up with D, B with E, and so on.

Common mistakes: Caesar encryption moves each plaintext letter forward along the alphabet by the key. Moving back is decryption. It should say 'Shift letters forward by key'. With a key of 3, B moves forward three places: C, D, E. So B becomes E, not D. The line should be 'B -> E'.

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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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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Using a Caesar shift with a key of 3, what does the plaintext letter A become?

Matches

Key 3: A -> Z

↓

The letter was moved backward by one place. To encrypt, move forward 3 places, so A becomes D.

Applying

Meanings

Term

Caesar shift encryption

Meaning

Move each plaintext letter forward along the alphabet by the key number. With a key of 3, A becomes D. Write the new letter and keep spaces unchanged.

Practice

Key = 3

Testing

Gaps

Cipher: EFGHIJKLMNOPQRSTUVWXYZABC

Check Answer

Spots

2Key = 3

3Shift letters back by key

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Quick Questions

1. Using a Caesar shift with a key of 3, what does the plaintext letter A become?

A. X

B. D ✓

Encrypting moves each letter forward by the key. A moves forward 3 places: B, C, D. So A becomes D.

2. Which way do you move each plaintext letter when you ENCRYPT with a Caesar shift?

A. Forward along the alphabet by the key number ✓

B. Backward along the alphabet by the key number

To encrypt, you shift forward by the key. Moving backward is how you decrypt.

3. A message has spaces between its words. What happens to the spaces when you encrypt it?

A. They turn into letters

B. They stay as spaces ✓

Only letters are shifted. Spaces are kept unchanged.

4. Encrypt the word CAT with a key of 3. Which is the ciphertext?

A. FDW ✓

B. ZXQ

C becomes F, A becomes D and T becomes W. ZXQ comes from moving backward, which is the wrong direction.

5. You write the alphabet on a top row, then a shifted alphabet underneath for a key of 3. Which letter sits under A?

A. D ✓

B. C

For a key of 3, the shifted row starts at D. So A lines up with D, B with E, and so on.

Match the Pairs

Key 3: A -> D → With a key of 3, the plaintext letter A is encrypted as the letter D, three places forward.

HI MUM -> KL PXP → A whole message encrypted with a shift of 3, with every letter moved forward 3 places and the space left alone.

Write the alphabet, then write a second alphabet shifted by the key underneath, and swap each letter for the one below it. → ABCDEFGHIJKLMNOPQRSTUVWXYZ -> DEFGHIJKLMNOPQRSTUVWXYZABC

The key number tells you how many places to move each letter forward along the alphabet. → Key 3: A -> D, B -> E, C -> F

Key 3: A -> Z → The letter was moved backward by one place. To encrypt, move forward 3 places, so A becomes D.

GO HOME -> JRKRPH → The space was removed. Spaces stay unchanged, so the answer should be JR KRPH.

Key Term

Caesar shift encryption

Move each plaintext letter forward along the alphabet by the key number. With a key of 3, A becomes D. Write the new letter and keep spaces unchanged.

Fill in the Blanks

1. Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC

Hint: Type the letter that sits under A when the alphabet is shifted by the key of 3.

2. Key 3: A becomes D, B becomes E

Hint: Type the letter that is 3 places after B in the alphabet.

3. Plain: A CAT Cipher: D FDW

Hint: Shift this letter forward by the key. The space stays unchanged.

4. newPosition = position + key

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

5. Plain: CAT Key: 3 Cipher: FDW

Hint: Type the ciphertext made by shifting each plaintext letter forward by the key.

Spot the Error

Question 1

A Shift letters forward by key

B Key = 3

C Shift letters back by key

Line C — Caesar encryption moves each plaintext letter forward along the alphabet by the key. Moving back is decryption. It should say 'Shift letters forward by key'.

Question 2

A Key = 3

B A -> D

C B -> D

D C -> F

Line C — With a key of 3, B moves forward three places: C, D, E. So B becomes E, not D. The line should be 'B -> E'.

Question 3

A Key = 3, so A becomes C

B B becomes E

C C becomes F

Line A — A key of 3 moves A forward three places: B, C, D. So A becomes D, not C.

Question 4

A Plaintext: GO HOME

B Key = 3

C Ciphertext: JRKRPH

Line C — Spaces stay unchanged in a Caesar cipher, so the space between the words must stay. It should be 'Ciphertext: JR KRPH'.

Question 5

A ABCDEFGHIJKLNOPQRSTUVWXYZ

B DEFGHIJKLMNOPQRSTUVWXYZABC

C Match top letter to bottom

Line A — The top row must be the full alphabet, but the letter M is missing. Without it the two rows won't line up. It should be ABCDEFGHIJKLMNOPQRSTUVWXYZ.

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