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S4 ICT: Network Security and Encryption (Paper 3 (challenge))
S4 ICT: Network Security and Encryption (Paper 3 (challenge))
Paper 3 (challenge): ten network security and encryption questions with answers and one-line notes.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 ICT: Network Security and Encryption |
| Level | S4 |
02 Questions (10 items)
- FoundationA six-letter password uses only the 26 lowercase letters; what is the key space 26 to the power 6?
- FoundationWhat is that key space in bits of entropy, to one decimal place?
- FoundationA system has 2 to the power 40 keys and an attacker tries a billion keys per second; what is the average time in seconds?
- IntermediateAn RSA key is 4096 bits long; how many bytes is that?
- IntermediateTwelve users need 66 symmetric keys but only 24 asymmetric keys; how many keys are saved?
- IntermediateMD5 produces 128 bits and SHA-256 produces 256 bits; by how many bytes do they differ?
- ChallengeIn a digital signature the sender hashes the message; which key encrypts that hash?
- ChallengeIn a TLS session, what do asymmetric and symmetric encryption each do?
- ChallengeA certificate runs from 1 January 2026 to 1 January 2027; how many days remain on 15 June 2026?
- Error redoA student says an AES-encrypted message must reveal whether anyone tampered with it. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 308915776 | Multiplying 26 by itself six times gives 308915776. |
| 2 | 28.2 bits | The base-two logarithm of 308915776 is about 28.2 bits. |
| 3 | About 549.8 seconds | Two to the fortieth is about 1.1 trillion keys, half of them on average divided by a billion gives about 549.8 seconds. |
| 4 | 512 bytes | 4096 divided by 8 gives 512 bytes. |
| 5 | 42 keys | 66 minus 24 leaves 42, and the gap widens as users are added. |
| 6 | 16 bytes | The difference of 128 bits is 16 bytes, so SHA-256 output is twice as long. |
| 7 | The sender own private key | The receiver opens it with the sender public key, which proves origin and integrity. |
| 8 | Asymmetric encryption exchanges the session key, and symmetric encryption protects the data afterwards | Symmetric ciphers are fast enough for bulk data, while asymmetric ones solve key distribution. |
| 9 | 200 days | From 15 June to the end of the year is 199 days and the first of January adds one more. |
| 10 | Encryption only hides content; detecting changes needs a hash or a message authentication code | Confidentiality and integrity are separate goals, handled by encryption and hashing respectively. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Encryption serves three aims: hiding content, proving origin and detecting tampering, and each aim uses a different tool. A Caesar cipher has only 26 keys and falls to brute force or frequency analysis, while modern systems exchange a session key with asymmetric encryption and then send data with fast symmetric encryption, using a hash plus a private key for digital signatures. Count keys, bits and bytes carefully before calculating.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.
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