Symmetric, Asymmetric and Hybrid Encryption

Lesson plan

Level

Beginner

Time

About 55 minutes; the steps below add up to 55

Learners try to get a strongbox key to a friend they have never met, then see how an open padlock avoids the problem, before symmetric and asymmetric encryption are named. Predictions then precede which of two keys to give away, multiplying against factoring, which family is faster and how to combine them for a 40 GB backup; checks follow an emailed key, who can open Bob’s padlock, RSA against AES and what Bob’s private key decrypts. The habit to build: for every operation, say which key is used and whose it is.

Objectives

• Explain symmetric encryption, AES and the key-distribution problem
• Explain asymmetric encryption and the roles of the public and private keys
• Describe what RSA and ECC rest on, and why key lengths are not comparable across kinds
• Compare symmetric and asymmetric encryption
• Explain how hybrid encryption uses both

Before the lesson

• Open this page on your own screen and use “Open the lesson window” to open the lesson in a second window. Share only that second window.
• Learners need the Cryptography Foundations lesson first.
• A padlock and a box make the analogy concrete; keep the key in your pocket.
• The lesson takes about 55 minutes.
• In presentation mode every step is shown in turns: Next reveals one moment, part or answer at a time, and the instructor view names what is on screen and what comes next.

Steps

1. Introduction Opening

   On screen: the lesson’s title, module and number; Next adds its objectives and key terms, as the lesson page lists them; Next again begins the first step.

   Objectives on screen

   • Explain symmetric encryption, AES and the key-distribution problem
   • Explain asymmetric encryption and the roles of the public and private keys
   • Describe what RSA and ECC rest on, and why key lengths are not comparable across kinds
   • Compare symmetric and asymmetric encryption
   • Explain how hybrid encryption uses both

   Purpose: Learners try to get a strongbox key to a friend they have never met, then see how an open padlock avoids the problem, before symmetric and asymmetric encryption are named. Predictions then precede which of two keys to give away, multiplying against factoring, which family is faster and how to combine them for a 40 GB backup; checks follow an emailed key, who can open Bob’s padlock, RSA against AES and what Bob’s private key decrypts. The habit to build: for every operation, say which key is used and whose it is.

2. 1. One strongbox, two cities Think 5 min

   Purpose: Raise the key-distribution problem, and the padlock way round it.

   Ask

   • How could you get the key there safely?
   • What would an open padlock change?

   Responses to expect

   • “Post it” and “meet” come first; neither scales.
   • On screen in turns: the opening question, its answer; then the question on an open padlock, and its answer.

   Discussion pause: Let learners propose ways to send the key before revealing.

   In the lesson window

   • Think it through: You and a friend want to pass a locked strongbox back and forth, and each of you needs a copy of its key. You live in different cities and have never met. How could you get the key to your friend safely?
   • Think it through: What if your friend could send you an open padlock that only they can unlock?

3. 2. Symmetric encryption: one shared key Explain 8 min

   Purpose: Explain symmetric encryption, AES and the distribution and scale problems.

   Ask

   • What is the risk of emailing the key?

   Responses to expect

   • Learners underrate the scale problem.
   • On screen in turns: the opening; then Alice and Bob share one key; a strongbox with one key; AES, starting with block size; key lengths; speed; the key-distribution problem; the check question on emailing the key, and its answer; the problem of scale.

   In the lesson window

   • Check your understanding: Alice emails the shared key to Bob. What is the risk?

4. 3. Asymmetric encryption: a key pair Predict 8 min

   Purpose: Explain the key pair through Bob’s padlocks, and where the analogy stops.

   Ask

   • Which key would you give away?
   • Who can open the box?

   Responses to expect

   • Some think Alice can reopen it.
   • Diagram: reveal the symmetric row first, then ask which keys the asymmetric row will use before it appears.
   • On screen in turns: the opening question, its answer; then a key pair, starting with the public key; the private key; what one does, only the other undoes; one cannot be worked out from the other; the key pair; Bob’s open padlocks; the check question on who can open the box, and its answer; the two families side by side, starting with symmetric; asymmetric.

   Misconceptions to challenge

   • A learner may believe: “The public key decrypts messages sent to its owner.” Respond: The private key decrypts them.

   In the lesson window

   • Think it through: Suppose there were two keys: one that can only lock and one that can only unlock. Which would you give away freely, and which would you guard?
   • Check your understanding: Alice locks the box with one of Bob’s open padlocks. Who can open it now?

5. 4. RSA and prime numbers Predict 9 min

   Purpose: Explain what RSA and ECC rest on, and why key lengths do not compare across kinds.

   Ask

   • Multiply or factor: which is easier?
   • RSA 2048 or AES 256?

   Responses to expect

   • Longer is assumed stronger.
   • On screen in turns: the opening; then the question on multiplying or factoring, and its answer; easy forwards, hard backwards; primes hundreds of digits long; RSA key sizes and what 2048-bit means; elliptic curve cryptography; key lengths are not comparable; strength in symmetric bits; the check question on RSA or AES, and its answer; longer, and weaker.

   Misconceptions to challenge

   • A learner may believe: “A longer key is always stronger.” Respond: Only for algorithms of the same kind.

   To go further

   • Why would a large quantum computer threaten RSA and ECC but not AES in the same way?

   In the lesson window

   • Think it through: Which is easier: multiplying 61 by 53, or finding the two primes that multiply to make 3,233?
   • Check your understanding: Using the table, which is stronger: a 2048-bit RSA key or a 256-bit AES key?

6. 5. Comparing the two Predict 4 min

   Purpose: Compare the families on keys, speed and difficulty.

   Ask

   • Which is faster, and what is each one’s difficulty?

   Responses to expect

   • Speed is usually guessed correctly.
   • On screen in turns: the opening question, its answer; then the two compared, with the asymmetric column held back; the asymmetric column; public key and private key, side by side.

   Discussion pause: The asymmetric column is held back: ask for each row before revealing it.

   In the lesson window

   • Think it through: Before reading the table: which do you expect to be faster, symmetric or asymmetric encryption, and what is each one’s main difficulty?

7. 6. Hybrid encryption: using both together Predict 9 min

   Purpose: Combine the two with the school backup step-through.

   Ask

   • How would you send 40 GB?
   • What does Bob’s private key decrypt?

   Responses to expect

   • Many think the backup itself is encrypted with RSA.
   • On screen in turns: the opening question, its answer; then the hybrid rule; sending a large backup, step by step; the diagram’s stages one at a time; the check question on what Bob decrypts, and its answer; how encryption is normally done.

   If the explanation does not land: Put a small key in an envelope locked with Bob’s padlock, and the backup in a strongbox opened by that key.

   In the lesson window

   • Think it through: Symmetric encryption is fast but cannot share its own key; asymmetric shares keys easily but is slow. How could you use both to send a 40 GB backup?
   • Check your understanding: In the step-through, what does Bob’s private key decrypt?

8. 7. Security+ terminology Explain 1 min

   Purpose: Link the lesson to the Security+ wording.

   Ask

   • Which objective terms appear here?

   Responses to expect

   • Public and private key, symmetric, asymmetric, key exchange.
   • On screen in turns: the opening; then what the objectives do not name.

9. 8. Apply it in a new situation Apply 4 min

   Purpose: Set up independent application, ideally on a later day.

   Ask

   • Who will try these now, and who later in the week?

   Responses to expect

   • Ten staff tests scale; the file tests which key; Mallory’s padlocks test trust in public keys.
   • On screen in turns: the opening; then the question on ten staff, and its answer; the question on a confidential file, and its answer; the question on Mallory’s padlocks, and its answer.

   In the lesson window

   • Apply it: Ten staff each need a private channel with every other one, using symmetric keys only. Roughly how many keys is that?
   • Apply it: Alice wants to send Bob a confidential file using asymmetric encryption. Whose key, and which one, does she encrypt with?
   • Apply it: Mallory leaves her own open padlocks in Bob’s basket, labelled “Bob”. What problem does that show?

10. 9. Check your understanding Check 5 min

    Purpose: Check the lesson against the approved revision-bank questions.

    Ask

    • Answer individually, then compare before revealing.

    Responses to expect

    • The which-key question links to the key-pair step.
    • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

11. 10. What to remember Consolidate 2 min

    Purpose: Summarise the five points, the key terms and the misunderstandings.

    Ask

    • Without looking: in hybrid encryption, what encrypts the data?

    Responses to expect

    • A symmetric key.
    • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

12. Lesson recap Closing

    On screen: the lesson’s objectives again; Next adds the closing question, which asks the class to explain each one in their own words without looking back. It says nothing about what anyone has learnt: use the answers to decide what to revisit.

    Points to listen for

    • One shared key: fast, but hard to share.
    • A key pair: lock with public, open with private.
    • Factoring and curves; lengths not comparable.
    • Each family lacks what the other has.
    • Hybrid: symmetric for data, public key for the key.

Consolidation

• One shared key: fast, but hard to share.
• A key pair: lock with public, open with private.
• Factoring and curves; lengths not comparable.
• Each family lacks what the other has.
• Hybrid: symmetric for data, public key for the key.