Encryption
Data Transmission · 4 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 4% of your exam marks.
Symmetric vs asymmetric encryption questions are growing as cybersecurity becomes more prominent.
Modern encryption splits into two big families, distinguished by how many keys are used.
| Symmetric encryption | Asymmetric encryption | |
|---|---|---|
| Number of keys | One | Two (a public key + a private key) |
| Key used to encrypt | The shared secret key | The recipient's public key |
| Key used to decrypt | The same shared key | The recipient's private key |
| Key shared with everyone? | No, must be kept secret | The public key can be shared freely; the private key must stay secret |
| Speed | Fast | Slower (more complex maths) |
| Main challenge | Sharing the key securely with the recipient | None: the public key can be sent openly |
| Examples | AES, DES, Wi-Fi (WPA2) | RSA, ECC, HTTPS handshake |
The next two sections look at each family in turn.
Differences between symmetric and asymmetric encryption
What comes up: a 2- or 4-mark question asking you to describe or give differences (and sometimes a similarity) between symmetric and asymmetric encryption.
Write (for differences): (1) In symmetric encryption, the same single shared key is used to both encrypt and decrypt the data; in asymmetric encryption, a different key is used for each direction: a public key to encrypt and a separate private key to decrypt. (2) In asymmetric encryption the public key can be freely shared with anyone, whereas the private key is kept secret by the owner; in symmetric encryption the single key must be kept secret and shared securely with the other party.
Write (for a similarity): Both symmetric and asymmetric encryption transform plaintext into ciphertext, and both require a key to decrypt the data.
Watch out: Do not mix up which key does what in asymmetric encryption. The public key encrypts; the private key decrypts — not the other way around.