Activity Sheet: Secret Codes
🎯 Objectives
- Understand how to create a strong password that is hard to steal.
- Learn how encryption works to protect information.
- Discover how encryption can be broken (decryption).
⏳ Activity Duration
1 hour 30 minutes
👥 Number of Participants
- Minimum: 2 participants with 2 computers.
- Depends on available computers (participants can work in pairs on a single computer).
🏫 Recommended Age
- Participants must be able to read, write and count (approximately 7 years and older).
🖥️ Required Materials
- One computer per participant.
- The explanatory sheet for the Scratch program under redevelopment in opensource format*.
- The scratch softwar (available online scratch MIT live editor)
🏗️ Activity Flow
1️⃣ Game: Secret Message Exchange
- Scenario: Participants exchange secret messages on paper without letting the facilitator understand them.
- Example: Passing notes in class without the teacher being able to read them.
- Each participant receives a Caesar cipher wheel.
- Pairs are formed, and each duo chooses a shift for their alphabet.
- The pairs separate, and one participant writes an encrypted message for their partner.
- ✅ Success criteria: The message must be a full sentence, correctly written in French.
- The recipient decrypts the message and verifies its accuracy with the sender.
- Groups then exchange their messages with other pairs and try to decrypt a message from another team.
🔍 Decryption Trick
- Identify the most frequently used letter in the message.
- The most common letter in French is “E” (17.26% occurrence).
- By comparing E with the most frequent letter in the encrypted message, participants can determine the shift value.
📊 Letter Frequency in French
| Letter | Frequency (%) | Letter | Frequency (%) |
|———–|—————-|———–|—————-|
| A | 8.40% | N | 7.13% |
| B | 1.06% | O | 5.26% |
| C | 3.03% | P | 3.01% |
| D | 4.18% | Q | 0.99% |
| E | 17.26% | R | 6.55% |
| F | 1.12% | S | 8.08% |
| G | 1.27% | T | 7.07% |
| H | 0.92% | U | 5.74% |
| I | 7.34% | V | 1.32% |
| J | 0.31% | W | 0.04% |
| K | 0.05% | X | 0.45% |
| L | 6.01% | Y | 0.30% |
| M | 2.96% | Z | 0.12% |
🔹 Key Insight:
- The longer the sentence, the easier it is to analyze letter frequency.
- Short words don’t provide enough data for frequency-based decryption.
2️⃣ Construction: Hiding a Secret Code in Math
- Each participant chooses a secret number (e.g., 8345).
- They create math operations that, when solved, result in zero (hiding the secret inside the calculations).
- Participants swap computers and attempt to guess another participant’s secret number.
- They then brainstorm ways to better protect their secret code.
3️⃣ Debriefing: Understanding Security Risks
After the activity, the facilitator leads a discussion on security weaknesses:
🔴 Identified Problems
- Reverse Engineering
- If an attacker can reverse the math operations, they can discover the secret number.
- Frequency Analysis Attack
- The most common letter in the encrypted text can reveal the cipher shift.
- Collision Risk
- A wrong answer might still be validated by the program due to weak encryption.
🔹 What Are the Solutions?
| Category | Ref | Best Practice |
|————-|——–|———————-|
| Passwords | MDP-02 | Use a long and complex password. |
| Passwords | MDP-03 | Use a password that is impossible to guess. |
| Passwords | MDP-06 | Never share your password with anyone. |
| Mobile Security | SM-02 | Encrypt the device’s data. |
| Mobile Security | SM-10 | Do not store confidential information without protection. |
| Ransomware Protection | RAN-08 | Use complex passwords and change them regularly. |
📌 Additional Resources
🔎 Summary
- Encrypt and exchange secret messages using the Caesar cipher.
- Decrypt other messages using letter frequency analysis.
- Create a hidden secret code with math operations.
- Understand cybersecurity threats such as frequency analysis and reverse engineering.
- Discuss security best practices to protect passwords and encrypted data.
This activity teaches the importance of encryption, strong passwords, and data security! 🔐
🎯 Get Involved!
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