Encrypting a message with a Caesar shift

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Date:

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Quick Questions

Shade in the A or B circle for each question.

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

A X

B 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

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

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

A FDW

B ZXQ

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

Match the Pairs

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

Left column		Right column
1Key 3: A -> D		AA whole message encrypted with a shift of 3, with every letter moved forward 3 places and the space left alone.
2HI MUM -> KL PXP		BWith a key of 3, the plaintext letter A is encrypted as the letter D, three places forward.
3Write the alphabet, then write a second alphabet shifted by the key underneath, and swap each letter for the one below it.		CThe space was removed. Spaces stay unchanged, so the answer should be JR KRPH.
4The key number tells you how many places to move each letter forward along the alphabet.		DThe letter was moved backward by one place. To encrypt, move forward 3 places, so A becomes D.
5Key 3: A -> Z		EKey 3: A -> D, B -> E, C -> F
6GO HOME -> JRKRPH		FABCDEFGHIJKLMNOPQRSTUVWXYZ -> DEFGHIJKLMNOPQRSTUVWXYZABC

Definitions

Caesar shift encryption

Fill in the Blanks

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

Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: _EFGHIJKLMNOPQRSTUVWXYZABC

Key 3: A becomes D, B becomes _

Plain: A CAT Cipher: D _DW

newPosition = position _ key

Plain: CAT Key: 3 Cipher: ___

Spot the Error

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

Question 1

A Shift letters forward by key

B Key = 3

C Shift letters back by key

Question 2

A Key = 3

B A -> D

C B -> D

D C -> F

Question 3

A Key = 3, so A becomes C

B B becomes E

C C becomes F

Question 4

A Plaintext: GO HOME

B Key = 3

C Ciphertext: JRKRPH

Question 5

A ABCDEFGHIJKLNOPQRSTUVWXYZ

B DEFGHIJKLMNOPQRSTUVWXYZABC

C Match top letter to bottom

Answer Key

Encrypting a message with a Caesar shift

Section A: Quick Questions

B — D

A — Forward along the alphabet by the key number

B — They stay as spaces

A — FDW

A — D

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

Plain:  ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC

Key 3: A becomes D, B becomes E

Plain:  A CAT
Cipher: D FDW

newPosition = position + key

Plain:  CAT
Key:    3
Cipher: FDW

Section E: Spot the Error

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'.

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'.

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

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

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.