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S6 ICT | Network Security and Encryption Practice Paper 2 (Asymmetric Encryption and Transport Security)
S6 ICT | Network Security and Encryption Practice Paper 2 (Asymmetric Encryption and Transport Security)
Ten items from p and q through n, phi(n) and d, then signature direction, TLS session keys, default-deny firewall rules and certificate chains.
Attempt first, mark afterwards Allow about thirty-five minutes; write the RSA formula before substituting, and check modular arithmetic with python or a calculator.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 items |
| Suggested time | About 35 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S6 ICT | Network Security and Encryption |
| Level | S6 (Secondary 6) ICT |
02 Questions (10 items)
- FoundationRSA uses the primes p = 61 and q = 53. What is the modulus n?
- FoundationContinuing, phi(n) = (p-1)(q-1) is what, and with e = 17 what is the private exponent d?
- FoundationEncrypt the plaintext m = 65 with the public key (e = 17, n = 3233). What is the ciphertext c = m^e mod n?
- FoundationUsing the private exponent d = 2753 on that ciphertext, what plaintext comes back?
- IntermediateFor a digital signature, which key signs and which key verifies?
- IntermediateWhen a browser visits an HTTPS site, how is the symmetric session key normally obtained?
- IntermediateA firewall rule allows only TCP 443 and denies everything else. Scanning ports 22, 80, 443, how many are denied?
- ChallengeIn the certificate chain, who issues a website certificate, and why does the browser trust it?
- ChallengeUsing a VPN on public Wi-Fi mainly mitigates which risk?
- Error redoError drill: a student claims a padlock and HTTPS mean the site is trustworthy. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 3233 | n = p x q = 61 x 53 = 3233, and n is part of the public key. |
| 2 | phi(n) = 3120 and d = 2753 | (61-1)(53-1) = 3120; d is the modular inverse of e modulo phi(n), and 17 x 2753 mod 3120 = 1. |
| 3 | 2790 | 65 to the power 17, reduced modulo 3233, gives 2790; real systems use n of 2 048 bits or more and this is only a demonstration. |
| 4 | 65 | c to the power d modulo n returns 65, which is why the private key must stay secret: leaking it hands over decryption. |
| 5 | The sender signs with the private key and the receiver verifies with the sender public key | The direction is the reverse of encryption: a signature proves who sent it and that the content is unchanged, and anyone with the public key can verify. |
| 6 | It is agreed securely through asymmetric encryption or a key exchange, and symmetric encryption then carries the data | Asymmetric is slow but solves key distribution, symmetric is fast but hard to distribute; TLS combines both for their strengths. |
| 7 | 2 denied (22, 80) | Of the three ports only 443 is allowed and the other 2 fall under the default deny, so the website still works while management ports stay closed. |
| 8 | A certificate authority (CA) issues it, and the browser trusts it through the built-in root certificate store | Trust starts from pre-installed roots and is verified link by link down to the site certificate; one bad link raises a warning. |
| 9 | Eavesdropping or man-in-the-middle snooping on unencrypted traffic by others on the same network | A VPN builds an encrypted tunnel so the local network sees only ciphertext, though it does not stop you from logging into a phishing site. |
| 10 | HTTPS only secures the transport, and certificates can be issued to fraudulent sites, so the content still needs judgement | The padlock means the connection matches the domain on the certificate, not that the operator is honest; phishing sites use HTTPS too. |
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.
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