AP Cybersecurity
AP Cybersecurity / All lessons / Unit 5: Securing Applications and Data
5Unit 5: Securing Applications and Data
AP Cybersecurity topic 5.4

Asymmetric Cryptography

Asymmetric (public key) cryptography uses a key pair. Anyone can encrypt with your public key, but only your private key can decrypt. Signing works the other way: you sign with your private key and others verify with your public key. HTTPS uses this to agree on a symmetric key and to prove a site's identity with certificates.

Asymmetric Cryptography vocabulary

  • Public key: A key you share freely so others can encrypt to you or verify your signature.
  • Private key: A key only you hold, used to decrypt and sign.
  • Digital signature: Proof a message came from the key owner and was not changed.
  • Certificate: A file, vouched for by a trusted authority, that links a public key to an identity.

AP-style practice questions

Try each one, then open the answer. To save your progress, take this quiz in the interactive course.

Skill: Mitigate Risk

1. Priya wants to send Marco a file that only Marco can decrypt. Which key should Priya use to encrypt it?

  1. (A) Priya's private key
  2. (B) Priya's public key
  3. (C) Marco's public key
  4. (D) Marco's private key
Show answer

(C) Marco's public key Anything encrypted with Marco's public key can only be decrypted with Marco's private key.

Skill: Mitigate Risk

2. Marco wants everyone to be able to verify that an announcement truly came from him. Which key does he use to sign it?

  1. (A) Marco's private key
  2. (B) Marco's public key
  3. (C) A shared symmetric key
  4. (D) Priya's public key
Show answer

(A) Marco's private key Signing uses the sender's private key, and anyone can verify with the matching public key.

Skill: Analyze Risk

3. Which of the following best explains why a 4096-bit key is more secure than a 1024-bit key for the same algorithm?

  1. (A) Longer keys create vastly more possible keys, making brute-force attacks impractical
  2. (B) Longer keys encrypt data faster
  3. (C) Longer keys do not need to be kept private
  4. (D) Longer keys make files smaller
Show answer

(A) Longer keys create vastly more possible keys, making brute-force attacks impractical Each added bit doubles the key space.

Skill: Analyze Risk

4. An adversary obtains Marco's public key. Which of the following can the adversary now do?

  1. (A) Read messages encrypted to Marco
  2. (B) Sign messages as Marco
  3. (C) Encrypt a message that only Marco can read
  4. (D) Change Marco's private key
Show answer

(C) Encrypt a message that only Marco can read Public keys are meant to be shared. They allow encrypting to the owner, not decrypting or signing.

Open 5.4 in the interactive course