Cambridge A-Level Computer Science (9618) · lesson 9 of 15 · 30 min
Encryption
TLS/SSL — How HTTPS Works
- Client sends “ClientHello” with supported cipher suites.
- Server responds with its digital certificate (contains public key, signed by a CA).
- Client verifies the certificate against trusted Certificate Authorities.
- Client and server use asymmetric encryption to exchange a session key.
- All subsequent communication uses fast symmetric encryption with the session key.
Firewalls
VPNs
A VPN (Virtual Private Network) creates an encrypted tunnel over a public network. All traffic is encrypted before leaving the device and decrypted at the VPN server. Uses: remote workers accessing corporate networks, privacy on public Wi-Fi, bypassing geo-restrictions.
In asymmetric encryption, which key is used to encrypt data sent to a recipient?
- Recipient’s private key
- Sender’s private key
- Recipient’s public key
- Shared session key
Answer: Recipient’s public key
The recipient’s public key encrypts; only the recipient’s private key can decrypt.
Why does HTTPS switch to symmetric encryption after the handshake?
- Symmetric is more secure
- Symmetric is much faster for bulk data
- Asymmetric keys expire quickly
- Symmetric supports more algorithms
Answer: Symmetric is much faster for bulk data
Asymmetric encryption is computationally expensive; symmetric is fast for large data volumes.
A stateful firewall differs from a packet filter in that it:
- Inspects application-layer content
- Tracks connection state to allow only expected responses
- Blocks all incoming traffic
- Only works with UDP
- Answer: 1
Stateful inspection tracks TCP connection state, allowing only packets that are part of established connections.
Previous lesson: Circuit switching vs packet switching
Back to Cambridge A-Level Computer Science (9618)
Next lesson: The Internet: DNS, HTTP, and web technologies