Sending a secure message with asymmetric encryption

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Quick Questions

Shade in the A or B circle for each question.

1. Priya wants to send Sam a secure message using asymmetric encryption. Which key does Priya use to encrypt it?

A Sam's public key

B Sam's private key

2. Which step comes first in an asymmetric exchange?

A The receiver shares their public key with the sender

B The sender encrypts the message before getting any key

3. Sam receives Priya's encrypted message. Which key does Sam use to decrypt it?

A Sam's private key

B Priya's public key

4. Does a secret key have to be shared between Priya and Sam before they can send secure messages?

A Yes, they must both share the same secret key first

B No, only the public key is shared and it is not secret

5. A hacker intercepts the message Priya sends to Sam. What can the hacker see?

A The plaintext message, because the public key was also intercepted

B Only ciphertext that cannot be read without the private key

Match the Pairs

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

Left column		Right column
1encrypt(message, receiver_public_key)		AThe receiver turns the ciphertext back into plaintext using their own private key, which is never shared
2decrypt(ciphertext, my_private_key)		BSam sent his private key over the network. The private key must stay secret and only the public key is shared.
3Worked example: Priya wants to send Sam a secret message, so Sam shares his public key and Priya uses it to encrypt		Cciphertext = encrypt("Meet at 5", sam_public_key)
4An interceptor can see the public key and the ciphertext, but cannot read the message without the private key		Dintercepted = [sam_public_key, ciphertext]
5ciphertext = encrypt("Meet at 5", priya_public_key)		EPriya encrypted with her own public key. The sender must use the receiver's public key (Sam's), or Sam cannot decrypt it.
6send(sam_private_key)		FThe sender turns the plaintext into ciphertext using the receiver's public key

Definitions

Encrypt with public key, decrypt with private key

Fill in the Blanks

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

ciphertext ← ENCRYPT(message, __________)

message ← DECRYPT(ciphertext, ___________)

Amy: ciphertext ← _______(msg, public_key)

Eve sees the public key and the __________

1 Receiver shares the public key 2 Sender encrypts with the public key 3 Receiver decrypts with the _______ key

Spot the Error

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

Question 1

A Ben makes a key pair

B Ben shares his public key

C Ava writes the plaintext

D Ava encrypts with private key

Question 2

A Zoe makes a key pair

B Sam wants to message Zoe

C Zoe sends Sam her private key

D Sam encrypts with that key

Question 3

A Mia creates a key pair

B Mia posts the private key

C Mia shares her public key

D Only Mia can decrypt messages

Question 4

A Leo sends ciphertext to Isla

B Eve intercepts the plaintext

C Eve cannot read the message

Question 5

A Isla shares her public key

B Leo encrypts the message

C Leo sends the ciphertext

D Isla decrypts with public key

Answer Key

Sending a secure message with asymmetric encryption

Section A: Quick Questions

A — Sam's public key

A — The receiver shares their public key with the sender

A — Sam's private key

B — No, only the public key is shared and it is not secret

B — Only ciphertext that cannot be read without the private key

Section B: Match the Pairs

encrypt(message, receiver_public_key) → The sender turns the plaintext into ciphertext using the receiver's public key

decrypt(ciphertext, my_private_key) → The receiver turns the ciphertext back into plaintext using their own private key, which is never shared

Worked example: Priya wants to send Sam a secret message, so Sam shares his public key and Priya uses it to encrypt → ciphertext = encrypt("Meet at 5", sam_public_key)

An interceptor can see the public key and the ciphertext, but cannot read the message without the private key → intercepted = [sam_public_key, ciphertext]

ciphertext = encrypt("Meet at 5", priya_public_key) → Priya encrypted with her own public key. The sender must use the receiver's public key (Sam's), or Sam cannot decrypt it.

send(sam_private_key) → Sam sent his private key over the network. The private key must stay secret and only the public key is shared.

Section C: Definitions

Encrypt with public key, decrypt with private key — The receiver shares their public key, the sender encrypts with it, and only the receiver's private key can decrypt it. No secret key is shared, so an interceptor sees only ciphertext.

Section D: Fill in the Blanks

ciphertext ← ENCRYPT(message, public_key)

message ← DECRYPT(ciphertext, private_key)

Amy: ciphertext ← ENCRYPT(msg, public_key)

Eve sees the public key and the ciphertext

1 Receiver shares the public key
2 Sender encrypts with the public key
3 Receiver decrypts with the private key

Section E: Spot the Error

D — The sender must encrypt with the receiver's public key, not a private key. The private key stays with its owner. The line should be 'Ava encrypts with public key'.

C — Zoe must share her public key, not her private key. The private key is never shared because it is the only key that can decrypt. The line should be 'Zoe shares her public key'.

B — The private key must be kept secret by its owner. If it is posted publicly, anyone could decrypt Mia's messages. The line should be 'Mia keeps the private key safe'.

B — The message was encrypted before sending, so an interceptor only captures ciphertext, not plaintext. The line should be 'Eve intercepts the ciphertext'.

D — The receiver decrypts with their private key. The public key is only used to encrypt. The line should be 'Isla decrypts with private key'.