Transport Encryption, Key Agreement and Forward Secrecy

Lesson plan

Level

Beginner

Time

About 55 minutes; the steps below add up to 56

Learners work out the three things Alice and Bob need for a private call over a tapped line, and whether Friday’s stolen code book decodes Monday’s call, before TLS’s three jobs and forward secrecy are named. Predictions then precede the order of the jobs, which side proves its identity, agreeing a secret in public and whether to reuse codes; checks follow HTTPS, encryption without authentication, the paint picture, the two arrangements and logged session keys. The habit to build: separate who you are talking to, how the secret was agreed and whether it still exists.

Objectives

• Explain transport encryption and the TLS versions in use
• Name the three jobs of a TLS connection
• Follow a simplified TLS 1.3 handshake and what certificate authentication establishes
• Explain Diffie-Hellman key agreement and ephemeral keys
• Explain forward secrecy, what it depends on and what it does not do

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 PKI and Certificates lesson first.
• Three pots of paint, or coloured pens, make the key-agreement picture memorable.
• The lesson takes about 56 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 transport encryption and the TLS versions in use
   • Name the three jobs of a TLS connection
   • Follow a simplified TLS 1.3 handshake and what certificate authentication establishes
   • Explain Diffie-Hellman key agreement and ephemeral keys
   • Explain forward secrecy, what it depends on and what it does not do

   Purpose: Learners work out the three things Alice and Bob need for a private call over a tapped line, and whether Friday’s stolen code book decodes Monday’s call, before TLS’s three jobs and forward secrecy are named. Predictions then precede the order of the jobs, which side proves its identity, agreeing a secret in public and whether to reuse codes; checks follow HTTPS, encryption without authentication, the paint picture, the two arrangements and logged session keys. The habit to build: separate who you are talking to, how the secret was agreed and whether it still exists.

2. 1. A tapped line Think 5 min

   Purpose: Raise the three jobs and forward secrecy through a tapped telephone line.

   Ask

   • What three things must they manage?
   • Can Mallory decode Monday’s call?

   Responses to expect

   • Authentication is the job most often missed.
   • On screen in turns: the opening question, its answer; then the question on Friday’s theft, and its answer.

   Discussion pause: Ask how Alice would know the voice is Bob’s.

   In the lesson window

   • Think it through: Alice wants a private telephone conversation with Bob over a line that Mallory may be tapping. What three things must Alice and Bob manage?
   • Think it through: Mallory records Monday’s coded call. On Friday she steals Bob’s code book. Can she decode Monday’s call?

3. 2. Transport encryption Explain 4 min

   Purpose: Define transport encryption, TLS, HTTPS and the versions in use.

   Ask

   • Ask what HTTPS is.

   Responses to expect

   • HTTP inside TLS.
   • On screen in turns: the opening; then TLS and HTTPS; the check question on what HTTPS is, and its answer; SSL and old TLS versions are obsolete; where the whole web request is followed.

   Misconceptions to challenge

   • A learner may believe: “TLS and SSL are the same.” Respond: SSL is obsolete; TLS 1.2 and 1.3 are in use.

   In the lesson window

   • Check your understanding: What is HTTPS?

4. 3. Three jobs in one connection Explain 6 min

   Purpose: Name authentication, key agreement and symmetric encryption, with the telephone call.

   Ask

   • What does encryption without authentication give?

   Responses to expect

   • A private chat with an impostor.
   • On screen in turns: the opening; then a private telephone conversation, starting with authentication; key agreement; symmetric encryption; where the analogy stops; the check question on encryption without authentication, and its answer; each job needs the others.

   In the lesson window

   • Check your understanding: What does encryption without authentication give you?

5. 4. The handshake, step by step Predict 6 min

   Purpose: Follow the simplified TLS 1.3 handshake.

   Ask

   • In what order do the jobs happen?

   Responses to expect

   • Some put authentication first.
   • On screen in turns: the opening; then the question on ordering the jobs, and its answer; a TLS 1.3 handshake, one step at a time; the diagram’s stages one at a time.

   In the lesson window

   • Think it through: Put the three jobs in the order they happen in a TLS 1.3 connection: speak in the code, check it is really the bank, agree a code.

6. 5. Certificate authentication in HTTPS Predict 4 min

   Purpose: Say what server authentication establishes, and what it does not.

   Ask

   • Which side proves its identity?

   Responses to expect

   • The server, normally.
   • On screen in turns: the opening; then what server authentication answers; the question on who proves their identity, and its answer; the visitor proves who they are afterwards.

   In the lesson window

   • Think it through: In an ordinary HTTPS visit, which side proves its identity with a certificate?

7. 6. Key agreement: making a secret in public Predict 8 min

   Purpose: Explain Diffie-Hellman with the paint picture, and ECDHE.

   Ask

   • Can a secret be agreed in public?
   • Why can Mallory not make the colour?

   Responses to expect

   • Disbelief is normal; the paint helps.
   • On screen in turns: the opening; then the question on a secret made in public, and its answer; Diffie-Hellman; mixing paint, starting with the public colour; the private colours; the mixtures sent; adding their own colour; the same colour, and what Mallory has; where the analogy stops; ECDHE; the check question on why Mallory cannot mix it, and its answer; key transport and key agreement.

   Misconceptions to challenge

   • A learner may believe: “Key agreement sends the key across the network.” Respond: Each side computes it; it is never sent.

   Discussion pause: Reveal the paint steps one at a time; before the last, ask what Mallory holds and why she cannot make the final colour.

   If the explanation does not land: Do the paint mixing with three pens on the board.

   In the lesson window

   • Think it through: Can two people who share nothing agree a secret while an eavesdropper hears every message? How might that even be possible?
   • Check your understanding: In the paint picture, why can Mallory not make the final colour?

8. 7. Ephemeral keys Predict 4 min

   Purpose: Define ephemeral and long-term keys.

   Ask

   • One code for all calls, or one per call?

   Responses to expect

   • One per call.
   • On screen in turns: the opening question, its answer; then an ephemeral key; a long-term key; a new key pair for every connection; a new code for each call.

   In the lesson window

   • Think it through: Should Alice and Bob keep one secret code for all their calls, or make a new one for each?

9. 8. Forward secrecy Predict 9 min

   Purpose: Explain forward secrecy, what it depends on and what it does not do.

   Ask

   • Which arrangement protects Monday?
   • What does logging session keys do?

   Responses to expect

   • Many think it protects future traffic too.
   • Diagram: reveal the design without forward secrecy first; ask what Friday’s theft decrypts before revealing the ephemeral design.
   • On screen in turns: the opening; then forward secrecy; the telephone call again, starting with one code book; a new code for every call; the identity card; where the analogy stops; the check question on which arrangement, and its answer; Monday’s recording, Friday’s theft, starting without forward secrecy; with forward secrecy; the secret no longer exists; record now, decrypt later; what forward secrecy depends on, starting with ephemeral exchange; keys actually discarded; resumption: a practical weakening; the check question on logged session keys, and its answer; what forward secrecy does not do, starting with future connections; data at either end; an endpoint already compromised.

   Misconceptions to challenge

   • A learner may believe: “Forward secrecy protects future connections.” Respond: Stolen keys still allow impersonation of new connections.

   To go further

   • Why does session resumption weaken forward secrecy if the ticket key is kept for a long time?

   In the lesson window

   • Check your understanding: In the telephone analogy, which arrangement protects Monday’s call after the Friday theft?
   • Check your understanding: A server logs its session keys for debugging. What does that do to forward secrecy?

10. 9. Security+ terminology Explain 1 min

    Purpose: Link the lesson to the Security+ wording.

    Ask

    • Which objective terms appear here?

    Responses to expect

    • Transport/communication and key exchange.
    • On screen in turns: the opening; then what the objectives do not name.

11. 10. 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

    • The stolen key tests forward secrecy; TLS 1.0 tests versions; the phishing site tests authentication.
    • On screen in turns: the opening; then the question on a stolen certificate key, and its answer; the question on TLS 1.0, and its answer; the question on a phishing padlock, and its answer.

    In the lesson window

    • Apply it: A bank’s certificate private key is stolen. Its connections used ECDHE with TLS 1.3. What is still at risk?
    • Apply it: A supplier’s server still offers TLS 1.0. What do you advise?
    • Apply it: A user reaches a phishing site that has a valid certificate. What did server authentication establish?

12. 11. Check your understanding Check 3 min

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

    Ask

    • Answer individually, then compare before revealing.

    Responses to expect

    • The forward-secrecy questions link to that step.
    • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

13. 12. What to remember Consolidate 2 min

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

    Ask

    • Without looking: the three jobs of a TLS connection.

    Responses to expect

    • Authenticate, agree keys, encrypt.
    • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

14. 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

    • Protect what travels.
    • Check, agree, encrypt.
    • A secret made in public.
    • Fresh codes, destroyed afterwards.
    • Past calls stay safe; future ones need a new key.

Consolidation

• Protect what travels.
• Check, agree, encrypt.
• A secret made in public.
• Fresh codes, destroyed afterwards.
• Past calls stay safe; future ones need a new key.