Computer Science · Grade 9

Active learning ideas

Introduction to Cryptography

Cryptography is inherently about hands-on application, making active learning essential. When students actively encode and decode messages, they grapple directly with the concepts of keys, algorithms, and security, moving beyond abstract definitions to concrete understanding.

• Ontario Curriculum Expectations
• CS.HS.CY.2
• CS.HS.S.9

• 30–45 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Escape Room
• 45 min · Pairs

Format Name: Caesar Cipher Challenge

Students work in pairs to create messages using a Caesar cipher with a randomly assigned shift value. They then exchange their encrypted messages and attempt to decrypt them, discussing the process and potential vulnerabilities.

Differentiate between symmetric and asymmetric encryption methods.

Facilitation Tip

For the Caesar Cipher Challenge, circulate to ensure pairs are correctly applying their randomly assigned shift value and not defaulting to common shifts like 3.

• Remember
• Apply
• Analyze
• Relationship Skills
• Self-Management

Activity 02

• Escape Room
• 30 min · Small Groups

Format Name: Symmetric vs. Asymmetric Encryption Sort

Provide students with cards describing various encryption scenarios and key characteristics. In small groups, they sort these cards into categories of symmetric and asymmetric encryption, justifying their choices.

Analyze how encryption protects data confidentiality during transmission and storage.

Facilitation Tip

During the Symmetric vs. Asymmetric Encryption Sort, observe how groups categorize the scenarios, prompting discussion if they struggle to differentiate based on key usage.

• Remember
• Apply
• Analyze
• Relationship Skills
• Self-Management

Activity 03

• Escape Room
• 40 min · Individual

Format Name: Public Key Cryptography Simulation

Using a simplified online tool or a paper-based simulation, students experience sending a message encrypted with a public key and decrypting it with a private key, illustrating the core concept of asymmetric encryption.

Design a simple substitution cipher and explain its limitations.

Facilitation Tip

In the Public Key Cryptography Simulation, encourage students to articulate why the public key can be shared but the private key must remain secret, connecting it to the simulation's outcome.

• Remember
• Apply
• Analyze
• Relationship Skills
• Self-Management

A few notes on teaching this unit

Experienced teachers approach cryptography by grounding abstract concepts in tangible activities, like message encoding. Avoid simply lecturing on algorithms; instead, focus on the 'why' behind encryption – data security and privacy – and use simulations to make complex ideas accessible. Emphasize that cryptography is a dynamic field, not a static set of rules.

Students will demonstrate understanding by successfully encrypting and decrypting messages using different methods and by articulating the core differences between symmetric and asymmetric encryption. They will show an ability to apply cryptographic principles to simple scenarios.

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Watch Out for These Misconceptions

• During the Caesar Cipher Challenge, watch for students assuming all simple substitution ciphers are equally secure.

  Guide students to discuss how easily their Caesar cipher could be broken if the shift value were known, prompting them to consider factors beyond just substitution that contribute to security.

• During the Symmetric vs. Asymmetric Encryption Sort, students might struggle to differentiate between methods, assuming all encryption is the same.

  Redirect students by having them physically group the cards based on whether a single key or a key pair is described, then discuss the implications for key distribution and security.

• During the Public Key Cryptography Simulation, students may think that once a message is encrypted, it's permanently unreadable.

  Prompt students to consider what would happen if they lost their private key or if someone else intercepted it, relating this back to the reversibility of the encryption process and the importance of key management.

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Methods used in this brief

• Escape Room