Encryption
Data Transmission
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 |
Common exam question
Differences between symmetric and asymmetric encryption
Question: Describe the differences, or give two, between symmetric and asymmetric encryption, sometimes with a similarity asked first (2–4 marks).
Asked in 2 of the 17 papers. The 4-mark version credits each fact: symmetric encryption uses one shared key, known to both sender and receiver, that both encrypts and decrypts; asymmetric uses two different keys, a public key to encrypt and a private key to decrypt, and anyone may know the public key but only the recipient knows the private key. The 2-mark version writes every difference as a contrast, so give each as a pair; it also credits that symmetric involves sending the key to the other party and asymmetric does not, and that symmetric is less secure.
For a similarity (1 mark), give one of: both use a key, both scramble the data so it is meaningless, both turn plaintext into ciphertext. The trap is swapping the keys: the public key encrypts, the private key decrypts.
The next two sections look at each family in turn.