The big idea: Encryption turns readable data into something meaningless without the right key.
The data still travels across the network where others can see it — but what they see is useless to them.
The awkward question: If both ends need the same key, how does the key get there safely?
Sending it across the same network anyone can watch defeats the whole point. Everything below is the answer to that.
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One key or two, what a certificate adds, and what HTTPS actually does. Step through all four.
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Symmetric
- One key locks and unlocks
- Fast, and efficient on large amounts of data
- Both ends must already have that key
- Getting it to the other end safely is the hard part
- Everyone you share with needs a key, so the number grows quickly
Asymmetric
- Two matched keys — a public one and a private one
- Anything locked with the public key opens only with the private one
- The public key can be given to anyone; the private one never leaves
- Nothing secret has to travel
- Much slower, so it is not used for bulk data
Why both are used together: Asymmetric solves the key problem but is slow. Symmetric is fast but needs a shared key.
So asymmetric is used once, to agree a symmetric key safely — and everything after that is symmetric. HTTPS does exactly this.
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A public key alone proves nothing: Anyone can publish a public key and claim it belongs to a bank.
A digital certificate is that public key together with an identity, signed by a certificate authority the browser already trusts.
What the certificate contains
What that buys
Key management
Why management is the weak point
How this is tested — you must describe the PROCESS — which key does what, in what order. It comes up two ways:
Paper 1 Section A
- Describe encryption or certificates, 3-5 marks
- Distinguish symmetric from asymmetric
- Explain the role of a certificate authority
Paper 1 Section B — case study
- Explain how the case study secures its connections
- Discuss the risks in how they manage keys
The classic trap: Saying the private key encrypts and the public key decrypts. For confidentiality it is the other way round: anyone can lock with the public key, and only the holder of the private key can open it.
Describe what happens when a browser connects to a bank's website using HTTPS.
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