Encrypting and decrypting with one shared key

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Quick Questions

Shade in the A or B circle for each question.

1. Which key decrypts a message in symmetric encryption?

A A different key from the one that encrypted it

B The same key that encrypted it

2. Which shows the correct order of the symmetric process?

A Plaintext → key → ciphertext → key → plaintext

B Ciphertext → key → plaintext → key → ciphertext

3. What does the sender do with the plaintext and the key?

A Decrypts the plaintext into ciphertext

B Encrypts the plaintext into ciphertext

4. The receiver uses the wrong key to decrypt some ciphertext. What happens?

A The output is unreadable

B The original plaintext appears anyway

5. In a diagram of symmetric encryption, how many different keys are needed?

A One shared key used at both ends

B Two different keys, one at each end

Match the Pairs

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

Left column		Right column
1plaintext + key → ciphertext		APlaintext → [Key] → Ciphertext → [Key] → Plaintext
2ciphertext + key → plaintext		BThe receiver decrypts the ciphertext with the same shared key to get the original message back
3Symmetric encryption uses the same key to encrypt and to decrypt		CThe sender encrypts the readable message with the shared key to produce unreadable ciphertext
4Diagram of the full process from sender to receiver		DThe keys are different, so decryption with the wrong key gives unreadable output, not the original message
5Sender key = 7, Receiver key = 3, decrypted message is readable		ESender key = 7, Receiver key = 7
6Receiver decrypts: plaintext + key → ciphertext		FThe direction is wrong: decryption turns ciphertext into plaintext, not plaintext into ciphertext

Definitions

symmetric encryption

Fill in the Blanks

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

ciphertext ← _______(plaintext, key)

plaintext ← _______(ciphertext, key)

ciphertext ← ENCRYPT(plaintext, key) plaintext ← DECRYPT(ciphertext, ___)

wrongKey ← "K9" output ← DECRYPT(ciphertext, ________) // output is unreadable

sharedKey ← "SECRET" ciphertext ← ENCRYPT(message, _________) plaintext ← DECRYPT(ciphertext, sharedKey)

Spot the Error

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

Question 1

A plain ← "HELLO"

B key ← 3

C cipher ← ENCRYPT(plain, key)

D result ← DECRYPT(cipher, 5)

E OUTPUT result

Question 2

A key ← 7

B plain ← "MEET AT 9"

C cipher ← DECRYPT(plain, key)

D OUTPUT cipher

Question 3

A key ← 1

B plain ← "CAT"

C ciphertext ← "BZS"

D // shift each letter by key

Question 4

A ciphertext ← "DBU"

B key ← 1

C plaintext ← "ECV"

D OUTPUT plaintext

Question 5

A Step 1: plaintext

B Step 2: decrypt using the key

C Step 3: ciphertext sent

D Step 4: decrypt using the key

E Step 5: plaintext

Answer Key

Encrypting and decrypting with one shared key

Section A: Quick Questions

B — The same key that encrypted it

A — Plaintext → key → ciphertext → key → plaintext

B — Encrypts the plaintext into ciphertext

A — The output is unreadable

A — One shared key used at both ends

Section B: Match the Pairs

plaintext + key → ciphertext → The sender encrypts the readable message with the shared key to produce unreadable ciphertext

ciphertext + key → plaintext → The receiver decrypts the ciphertext with the same shared key to get the original message back

Symmetric encryption uses the same key to encrypt and to decrypt → Sender key = 7, Receiver key = 7

Diagram of the full process from sender to receiver → Plaintext → [Key] → Ciphertext → [Key] → Plaintext

Sender key = 7, Receiver key = 3, decrypted message is readable → The keys are different, so decryption with the wrong key gives unreadable output, not the original message

Receiver decrypts: plaintext + key → ciphertext → The direction is wrong: decryption turns ciphertext into plaintext, not plaintext into ciphertext

Section C: Definitions

symmetric encryption — The sender encrypts plaintext into ciphertext with a key; the receiver decrypts it back to plaintext using the same key. A wrong key gives unreadable output.

Section D: Fill in the Blanks

ciphertext ← ENCRYPT(plaintext, key)

plaintext ← DECRYPT(ciphertext, key)

ciphertext ← ENCRYPT(plaintext, key)
plaintext ← DECRYPT(ciphertext, key)

wrongKey ← "K9"
output ← DECRYPT(ciphertext, wrongKey)
// output is unreadable

sharedKey ← "SECRET"
ciphertext ← ENCRYPT(message, sharedKey)
plaintext ← DECRYPT(ciphertext, sharedKey)

Section E: Spot the Error

D — The receiver must decrypt with the same key the sender used to encrypt. The key is 3, so using 5 gives unreadable output. It should be DECRYPT(cipher, key).

C — The sender turns plaintext into ciphertext by encrypting it, not decrypting it. It should be ENCRYPT(plain, key).

C — Encrypting with key 1 shifts each letter forward by 1, so C, A, T become D, B, U. The broken line shifted backwards. It should be "DBU".

C — Decrypting with the same key 1 shifts each letter back by 1, so D, B, U become C, A, T. The broken line shifted forward. It should be "CAT".

B — Step 2 is the sender turning plaintext into ciphertext, which is encryption. Decryption happens at step 4. It should say encrypt using the key.