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S4 ICT: Network Security and Encryption (Paper 4 (error clinic))
S4 ICT: Network Security and Encryption (Paper 4 (error clinic))
Paper 4 (error clinic): 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)
- FoundationWith a Caesar shift of three, which letter does the ciphertext Z decrypt to?
- FoundationFive users communicate pairwise with symmetric encryption; how many keys in total?
- FoundationAn AES-128 key is 128 bits long; how many bytes is that?
- IntermediateCan the original message be recovered from a SHA-256 digest?
- IntermediateA system has 2 to the power 32 keys and an attacker tries a billion per second; what is the average time?
- IntermediateA four-letter password over 26 letters has what key space?
- ChallengeWhich value in RSA must stay secret?
- ChallengeSix users need 15 symmetric keys but 12 asymmetric keys; what is the difference?
- ChallengeWhat happens when a website certificate passes its expiry date?
- Error redoA student hard-codes a database password in the web source and calls it safe because it is hidden. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | W | Three letters back from Z gives Y, then X, then W. |
| 2 | 10 keys | Five times four divided by two gives 10. |
| 3 | 16 bytes | 128 divided by 8 gives 16 bytes. |
| 4 | No, hashing is one way | A digest is only a fingerprint for comparison, and no reversible version is kept. |
| 5 | About 2.15 seconds | Two to the thirty-second is about 4.29 billion keys, half of which divide by a billion to 2.15 seconds. |
| 6 | 456976 | Multiplying 26 four times gives 456976. |
| 7 | The private exponent d | The modulus and public exponent may be published, but a leaked d hands over the key. |
| 8 | 3 keys | 15 minus 12 leaves 3, and the gap grows with more users. |
| 9 | The browser shows a warning and the connection is no longer treated as secure | The certificate authority signs the validity period, so an expired certificate no longer guarantees anything. |
| 10 | Source code leaks or gets copied into backups, so secrets belong in configuration or a key management service with tight permissions | Secrecy comes from access control and rotation, not from hiding where the file sits. |
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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