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# Who can see your data?

Networks · Keeping it safe

<!-- Speaker note: Students send a PIN past an eavesdropper on café Wi-Fi, first in plain text and then encrypted with a shift key, to explain why data must be encrypted in transit and show that encryption is reversible with the key (AC9TDI8K02 nodes 3 and 4). About 40 minutes (Predict and Try 6 min · Worked example 12 min · Vary 10 min · Practise and Make 12 min). -->

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## Today

**We are learning to** packets pass through Wi-Fi and routers you don't control, and anyone along the way can copy them. So anything private must be encrypted on your device before it leaves: scrambled with a key, so only someone with the key can turn it back. Encryption only works because it can be reversed.

- I can explain why data needs encrypting when it is sent.
- I can encrypt and decrypt a short message with a key.
- I can say what makes a key weak.

<!-- Speaker note: Before class: Try the boards yourself. The kit's chain for node 3: packets leave your device → travel through routers you don't control → anyone along the path can read them → so encryption is necessary. Node 4 (simple encryption) is here as a shift cipher on letters and digits, with the encrypt-decrypt cycle; the Code pathway's Secret messages course is the deeper home of ciphers (Caesar, XOR, cracking, key sharing). For students who have done it, this lesson is the why; for those who haven't, it is a first taste. Defensive framing: the eavesdropper shows why protection matters; no attack techniques are taught. Teacher: Try: ask whose Wi-Fi it is, and who else could be on it. Teacher: Worked example: have students encrypt their initials with key 3. Teacher: Vary: the key 10 task surprises: ask why the digits came through unchanged. -->

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## Why it matters

> You type your PIN on the café's free Wi-Fi. Between you and the bank, who else gets a copy?

<!-- Speaker note: In Packets, routers passed each packet along. But who owns those routers? Your packets go through the Wi-Fi you're on, your internet provider and many other networks. Anyone with access to one of them can copy the packets as they pass. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Predict

**You type your PIN into a website on a café's free Wi-Fi. The website has **no encryption**. Who could read the PIN on its way to the bank?**

- Only people with a password to the bank
- Anyone on the café Wi-Fi, or on any network it passes through
- Nobody: packets are private
- Only the bank

<!-- Speaker note: What happens: Anyone on the path. Packets cross the café's Wi-Fi and other networks you don't control, and plain text can be read by anyone who copies them. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: students who chose "only the bank" hold Sam's misconception; trace the path after the Try. -->

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## Try it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fsystems%2Fnetworks%2Fwho-can-see%2Fstep-2&code=widget%3A+network%0Amode%3A+snoop%0Amessage%3A+PIN+4821%0Aencrypt%3A+off%0Akey%3A+3%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/systems/networks/who-can-see/?full=step-2#step-2)

<!-- Speaker note: Was your prediction right? Now look at the Encrypt button. -->

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## What happened?

* Anyone on the path. Packets cross the café's Wi-Fi and other networks you don't control, and plain text can be read by anyone who copies them.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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## PIN 4821 with key 3

This shift (a Caesar cipher) is easy to break, as the Secret messages course shows. Real encryption uses far stronger maths, but works the same way: a public method and a secret key.

$$\text{PIN 4821}$$

1) $\text{P} \to \text{S},\ \text{I} \to \text{L},\ \text{N} \to \text{Q}$ — Encrypt: each letter moves 3 along.
2) $4821 \to 7154$ — Each digit moves 3 along, wrapping round after 9 (8 → 1).
3) $\text{SLQ 7154}$ — This is what travels, and all Eve can copy.
4) $\text{SLQ 7154} \to \text{PIN 4821}$ — The bank moves everything 3 back: decrypt with the same key.

<!-- Speaker note: Model this live on the board before revealing anything: write each line and say why, pausing to ask the class what comes next. Then click to reveal the lines and compare. Script. Start: write PIN 4821 and say "Your message, before it leaves." Line 1: write P S,I L,N Q and say "Encrypt: each letter moves 3 along." Line 2: write 4821 7154 and say "Each digit moves 3 along, wrapping round after 9 (8 → 1)." Line 3: write SLQ 7154 and say "This is what travels, and all Eve can copy." Line 4: write SLQ 7154 PIN 4821 and say "The bank moves everything 3 back: decrypt with the same key." -->

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## PIN 4821 with key 3

**With key 3, every letter moves 3 along (A → D) and every digit moves 3 along (7 → 0). What does **CAT 12** become?**

- ZXQ 89
- FDW 45
- CAT 12
- DBU 23

* Answer: FDW 45

<!-- Speaker note: C→F, A→D, T→W, 1→4, 2→5. -->

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## Find the mistake

**Sam decides whether it is safe to type a password. Click the line where the reasoning first goes wrong.**

```
 1  I am on the café's free Wi-Fi.
 2  The website has no padlock, so nothing is encrypted.
 3  My message only goes to the bank, so only the bank sees it.
 4  So it is safe to type my password here.
```

* Answer: line 3

<!-- Speaker note: Line 3: the message goes to the bank, but it passes through the café Wi-Fi and other networks first, and anyone there can copy it. Without encryption, they can read it. -->

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## Why reversible?

**Why must encryption be **reversible**?**

- The receiver has to turn it back into the real message with the key
- So it takes less space
- It doesn't have to be
- So that Eve can read it too

* Answer: The receiver has to turn it back into the real message with the key

<!-- Speaker note: Encrypt with the key, decrypt with the key. If it couldn't be reversed, nobody could read the message, including the bank. -->

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## Vary: one setting at a time



- **Encryption on:** 
- **Key 0:** 
- **Key 10:** 
- **Too few keys:** 

[Open this step](https://studio.learningwaypoints.com/tutorials/systems/networks/who-can-see/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Each task changes one setting: encryption on, then key 0, then key 10 (which leaves the digits unchanged), then how many keys there are to try. For students who need more support: An alphabet strip and a 0–9 strip to count along. For early finishers: XOR encryption (Secret messages lesson 5) works on the bits directly. Why does that suit computers better than shifting letters? The hard part, on purpose: The shared-path idea: students picture a private tube to the bank. Trace the packets on the board: whose equipment is each one? When students get stuck: A student says only the receiver sees the message (Sam's mistake). Ask: Which pieces of equipment does the packet pass through? Fix: Every one of them could copy it. / A student shifts the wrong way to decrypt. Ask: Which way did encryption move the letters? Fix: Decrypt by moving back by the same key. -->

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## What needs encrypting

**Which of these most needs encrypting when you send it?**

- The time of a public bus timetable
- A news headline
- Your bank password
- The weather forecast you're reading

* Answer: Your bank password

<!-- Speaker note: Passwords, card numbers and private messages must be encrypted; public information is less of a worry (though encrypting everything also hides what you are reading). -->

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## Decrypt

**A message was encrypted with key 3. You receive **KHOOR**. What did it say?**

- KHOOR
- HELLO
- GDKKN
- NKRRU

* Answer: HELLO

<!-- Speaker note: Move each letter back 3: K→H, H→E, O→L, O→L, R→O. -->

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## The padlock

**What does the padlock in a browser's address bar mean?**

- The site has no adverts
- Data between you and the website is encrypted (HTTPS)
- The website is honest and safe to buy from
- Nobody can see that you visited the site

* Answer: Data between you and the website is encrypted (HTTPS)

<!-- Speaker note: HTTPS encrypts what travels between your browser and that website. -->

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## Review: packets

**Review: a packet is lost on the way. With TCP, what happens?**

- The whole message starts again
- The message arrives with a gap
- The receiver never confirms it, so it is sent again
- The router rebuilds it

* Answer: The receiver never confirms it, so it is sent again

<!-- Speaker note: TCP resends only the packet that was never acknowledged. -->

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## Make: a safe key

Then: the Secret messages course (Code) builds this cipher on micro:bits, cracks it, and shares a key safely.

[Open this step](https://studio.learningwaypoints.com/tutorials/systems/networks/who-can-see/?full=step-6#step-6)

<!-- Speaker note: What this step is for: Choosing a key that scrambles both the letters and the digits reverses the process: students start from what Eve must not see and pick the setting. Hints, in order: Key 0 changes nothing. / Digits wrap round every 10, so keys 10 and 20 leave them unchanged. / Any key from 1 to 25 that isn't 10 or 20 works. -->

<!-- Speaker note: Teacher: ask what a strong key would need (many possibilities). -->

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## Exit ticket

**Why should a password be encrypted before it is sent? Encrypt CAB with key 2.**

* Packets pass through equipment other people control, so anyone on the path could copy plain text. CAB → ECD.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Across the internet (the whole journey, with HTTPS). Code links: Secret messages (Caesar, cracking, XOR, key exchange) is where students build and break ciphers. -->