DSE ICT | Network Security and Encryption Lesson-Prep Guide (Tutor Edition)

DSE ICT | Network Security and Encryption Lesson-Prep Guide (Tutor Edition) | Keys, Hashing and Phishing

2026-11-02

Tutor lesson-prep guides · Secondary ICT

Students say encryption just means turning text into gibberish, but freeze when asked why HTTPS uses two kinds of encryption. This lesson links four terms with one lock-and-key story, then shows why passwords need salt.

Objectives — Students can: (1) tell symmetric from asymmetric encryption and give the role of each in HTTPS; (2) compute password space and cracking time from character set and length; (3) state the three properties of a hash; (4) say what a firewall, an IDS and a VPN each stop.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Indices	What is 2 to the power 10	about 1,024
Binary	How many values in 8 bits	256
Data in transit	What passes between browser and server	Requests and responses, readable on the wire

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Threat map	Interception, impersonation, tampering, denial	Students sort four cases
10-24 min	Symmetric	One key, fast, hard to distribute	Get 2.18e+14 combinations
24-38 min	Asymmetric	Public key encrypts, private key decrypts	Explain why HTTPS uses both
38-48 min	Hashing	One-way, fixed length, avalanche	Write 64 hex characters
48-56 min	Defences	Firewall, IDS, VPN, salt	One line each
56-60 min	Exit check	Three questions marked on the spot	Independent work

03 Teaching script (read it out)

Scene 1: Symmetric encryption: one key, fast, but how do you ship the key

A 62-character set (upper case, lower case, digits) gives 62^8 = 2.183e+14 eight-character passwords. At one billion guesses per second that is 2.183e+05 seconds, about 61 hours, roughly two and a half days.
Symmetric ciphers such as AES use one key for both directions: fastest, best for bulk data, but the key must reach the other side safely. That is the key distribution problem.

Ask the student: going from 8 to 10 characters multiplies the space by how much (62 squared = 3,844).

Scene 2: Asymmetric encryption: public lock, private key

The receiver publishes a public key; anyone can encrypt with it, only the private key decrypts. Signing runs the other way: sign with the private key, verify with the public key.
Key length matters enormously: a 128-bit key space is 2^72, about 4.72e+21, times a 56-bit one.
HTTPS in practice: the handshake uses asymmetric encryption to agree a one-off symmetric session key, then bulk data travels under that symmetric key. Asymmetric solves distribution, symmetric solves speed.

Warn: public keys are meant to be public; leaking the private key is the fatal error.

Scene 3: Hashing and defences: one-way, not encryption

A hash turns any input into a fixed-length output: SHA-256 is always 64 hexadecimal characters, one changed character changes the whole digest (avalanche), and the input cannot be recovered.
Passwords are stored with a salt: the same password with different random values gives different digests, which defeats rainbow tables.
A firewall blocks ports and addresses by rule; an IDS only detects and alerts; a VPN builds an encrypted tunnel against interception; phishing is not stopped by technology, only by checking domains and links.

Ask the student: why a hash cannot be used as encryption (it is one-way, nothing to reverse).

04 Question types and worked steps

Type	Example	Steps	Answer
Classify	Symmetric versus asymmetric use	Count the keys	HTTPS uses both
Calculation	Password space	Character set to the power of length	2.18e+14
Calculation	Cracking time	Space divided by guesses per second	about 61 hours
Application	Four defences	One sentence each	Map each to a threat

Sanity check (teach this): Password space grows exponentially with length, not linearly, and every calculation must state the assumed guess rate. A hash output is fixed length, so an answer claiming the digest grows with the input means the property was missed.

05 Independent practice (3 questions)

Question	Answer	What to watch
Space for 8 characters from a 62-character set	2.183e+14	Use powers, not multiplication
Hours to exhaust it at one billion per second	about 61 hours	Divide, then divide by 3600
Why HTTPS uses two ciphers	Asymmetric agrees the key, symmetric moves the data	One for distribution, one for speed

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating a hash as encryption	Says a hash can be decrypted	“A hash is a one-way street, which is why passwords are reset, not recovered.”
Thinking double length doubles the space	Answers 62 times 8	“It is 62 to the power 8; every extra character multiplies by 62.”
IDS described as a firewall	Says an IDS blocks traffic	“An IDS only alerts; blocking is the firewall or an IPS.”
Swapping the two keys	Encrypts with the private key to send data	“Public encrypts, private decrypts; reverse it and everyone can read.”

07 Homework (paper, section and question numbers)

Secondary ICT chapter index (includes this chapter) — Independent practice questions 1 to 3 on this page
DSE lesson-prep overview and course map — All subject chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Space for 62^8	2.183e+14	Three correct means move on
Hex length of a SHA-256 digest	64	One wrong means re-teach fixed length
Division of labour in HTTPS	Asymmetric agrees the key, symmetric carries data	Two wrong means replay act two

09 Tutor FAQ

Must students memorise AES and RSA key sizes?

No. They must know symmetric is faster and asymmetric solves key distribution.

Is a salt a key?

No: a salt is a public random value that makes identical passwords hash differently.

Can software stop phishing completely?

No. Tools reduce it; users still check the domain and the certificate.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.