Home · A-Level Computer Science 9618 · Security · Symmetric and asymmetric encryption, quantum cryptography · Practice
A2 · Practice questions · Symmetric and asymmetric encryption, quantum cryptography

Which key, which end?

Six original Cambridge-style questions on how encryption works, key pairs and quantum cryptography, with mark-scheme solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Name the owner of the key.

01
Recall
[3 marks]

Explain the terms plaintext, ciphertext and symmetric key cryptography.

  • Plaintext: the original data before encryption, which can be read and understood. ✓
  • Ciphertext: the data after an encryption algorithm and key have been applied, which cannot be understood without the key. ✓
  • Symmetric key cryptography: the same secret key is used both to encrypt and to decrypt the data. ✓
02
Explain
[4 marks]

A customer sends their bank details to an online shop. Describe how asymmetric encryption keeps the details private during transmission.

  • The shop generates a key pair and publishes its public key; the private key is kept secret by the shop. ✓
  • The customer’s software encrypts the details using the shop’s public key. ✓
  • The ciphertext is sent; if it is intercepted it cannot be understood. ✓
  • Only the shop’s private key can decrypt it, so only the shop can read the details. ✓
03
Explain
[3 marks]

A government department publishes an announcement and wants the public to be confident it is genuine. Explain how a key pair is used.

  • The department encrypts the announcement (or a digest of it) with the department’s private key. ✓
  • Members of the public decrypt it with the department’s public key. ✓
  • Because only the department holds the private key, successful decryption proves the announcement came from the department. ✓
04
Analysis
[3 marks]

Compare symmetric and asymmetric encryption, giving one advantage of each.

  • Symmetric uses one shared secret key; asymmetric uses a linked public and private key pair. ✓
  • Symmetric advantage: much faster, so suitable for large volumes of data. ✓
  • Asymmetric advantage: no secret key needs to be sent, which solves the key distribution problem. ✓
05
Explain
[4 marks]

Describe the purpose of quantum cryptography and how it detects an eavesdropper.

  • Purpose: to distribute an encryption key securely (quantum key distribution). ✓
  • The key is sent as a stream of photons, each polarised to represent a bit. ✓
  • Measuring a photon changes its quantum state, so an eavesdropper introduces errors. ✓
  • Sender and receiver compare part of the key; a high error rate shows eavesdropping and the key is discarded. ✓
06
Analysis
[4 marks]

A company is considering quantum cryptography for links between its offices in two countries. Give two benefits and two drawbacks.

  • Benefit: any interception of the key is detected. ✓
  • Benefit: security depends on physics, so it is not broken by faster or quantum computers; long keys can be changed frequently. ✓
  • Drawback: needs expensive dedicated hardware and a fibre or line-of-sight link. ✓
  • Drawback: limited range because photons degrade over long distances (relays needed between countries), a low key rate, and polarisation can change in transit causing errors. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.