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S4 ICT: Network Security and Encryption (Paper 2 (intermediate))
S4 ICT: Network Security and Encryption (Paper 2 (intermediate))
Paper 2 (intermediate): 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 Caesar cipher shifts three places forward; what does HELLO become?
- FoundationA Caesar cipher has 26 keys and a machine tries one key per second; how many seconds at worst?
- FoundationTen users communicate pairwise with symmetric encryption; how many keys are needed in total?
- IntermediateThe same ten users switch to asymmetric encryption with one key pair each; how many keys in total?
- IntermediateRSA uses primes p = 3 and q = 11; what is the modulus n?
- IntermediateWith p = 3 and q = 11, what is the totient (p − 1)(q − 1)?
- ChallengeWith e = 3 and totient 20, find d such that 3d is 1 modulo 20.
- ChallengeEncrypt m = 4 with e = 3 and n = 33: what is 4 cubed modulo 33?
- ChallengeDecrypt 31 with d = 7: does 31 to the power 7 modulo 33 return 4?
- Error redoA student signs a document by encrypting its hash with the receiver public key. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | KHOOR | H, E, L, L and O each move three places to K, H, O, O and R. |
| 2 | 26 seconds | Trying every shift one by one takes at most 26 seconds. |
| 3 | 45 keys | 10 times 9 divided by 2 gives 45, counting each pair once. |
| 4 | 20 keys | Each user holds a public and a private key, so 10 times 2 gives 20. |
| 5 | 33 | n equals p times q, that is 3 times 11, which is 33. |
| 6 | 20 | Two times ten gives 20. |
| 7 | 7 | Three times seven is 21, which leaves 1 modulo 20, so d is 7. |
| 8 | 31 | Four cubed is 64, and 64 minus 33 leaves 31. |
| 9 | Yes, it gives 4 | 31 to the seventh power is 4 modulo 33, so the exponents undo each other. |
| 10 | A signature uses the sender’s own private key, so anyone with the sender’s public key can verify it | Encrypting with the receiver public key cannot prove who sent the message, since everyone may hold it. |
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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