Digital Signatures

Lesson plan

Level

Beginner

Time

About 55 minutes; the steps below add up to 55

Learners say what a signature on the headteacher’s electronic order should prove, and which of her keys must act first, before the digital signature is defined. Predictions then precede what to sign instead of the whole document, a changed amount, whether signing hides content, what a verifying signature proves in a dispute and why a phone refuses an update; checks follow what a valid signature shows, which key verifies and a lent smart card. The habit to build: name whose key, and which one, for every signing and every check.

Objectives

• Explain what a digital signature shows, and which key signs and which verifies
• Follow signing and verifying step by step, and what happens when content changes
• Name the properties a signature provides, and the one it does not
• Tell how encryption and signing use key pairs differently
• Explain why non-repudiation is strong evidence, not certainty

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 Encryption and Hashing lessons first.
• Write two columns, “Encrypting” and “Signing”, and fill in whose key does what as the lesson goes.
• 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 what a digital signature shows, and which key signs and which verifies
   • Follow signing and verifying step by step, and what happens when content changes
   • Name the properties a signature provides, and the one it does not
   • Tell how encryption and signing use key pairs differently
   • Explain why non-repudiation is strong evidence, not certainty

   Purpose: Learners say what a signature on the headteacher’s electronic order should prove, and which of her keys must act first, before the digital signature is defined. Predictions then precede what to sign instead of the whole document, a changed amount, whether signing hides content, what a verifying signature proves in a dispute and why a phone refuses an update; checks follow what a valid signature shows, which key verifies and a lent smart card. The habit to build: name whose key, and which one, for every signing and every check.

2. 1. A signed order Think 5 min

   Purpose: Raise what an electronic signature must prove, and which key signs.

   Ask

   • What should it prove?
   • Which key acts first?

   Responses to expect

   • Learners usually reach “her private key” with the encryption reminder.
   • On screen in turns: the opening question, its answer; then the question on which key comes first, and its answer.

   Discussion pause: Ask why the public key could not make a signature only she could produce.

   In the lesson window

   • Think it through: A headteacher must authorise a £10,000 laptop order, and the IT department will act only on her authorisation. On paper, she would sign it. What should a signature on an electronic order be able to prove?
   • Think it through: In encryption, anyone can lock a message with Bob’s public key. For a signature that only the headteacher could make, which key must do the first step?

3. 2. The idea Explain 4 min

   Purpose: Define a digital signature and the direction of the keys.

   Ask

   • What does a valid signature show?

   Responses to expect

   • Some say it shows the content is secret.
   • On screen in turns: the opening; then what a valid signature shows, starting with integrity; the matching private key; the check question on what a valid signature shows, and its answer; notice the direction.

   In the lesson window

   • Check your understanding: What does a valid signature show about a document?

4. 3. Signing and verifying, step by step Predict 9 min

   Purpose: Follow signing and verifying, and a changed amount.

   Ask

   • What could she sign instead?
   • What happens to the changed order?

   Responses to expect

   • The digest step needs the hashing lesson fresh.
   • Diagram: step through the eight steps; before each verifying step, ask what IT must check next.
   • On screen in turns: the opening; then the question on what to sign, and its answer; signing and verifying, step by step; the diagram’s stages one at a time; what happens if the document is changed; the question on a changed amount, and its answer; the check fails; detectable, not prevented.

   Misconceptions to challenge

   • A learner may believe: “A signature stops anyone changing the document.” Respond: It makes the change detectable; it does not prevent it.

   In the lesson window

   • Think it through: Signing a whole document directly would be slow. Using what you know about hashing, what could the headteacher sign instead?
   • Think it through: Someone changes £10,000 to £19,000 after signing and forwards it with the original signature. What happens when IT checks it?

5. 4. What a signature provides Predict 5 min

   Purpose: Name integrity, authentication and non-repudiation, and the absence of confidentiality.

   Ask

   • Does signing keep the contents secret?

   Responses to expect

   • Many assume it does.
   • On screen in turns: the opening question, its answer; then three properties, with what each shows held back; what the signature shows for each; what it does not provide; signed and encrypted.

   Misconceptions to challenge

   • A learner may believe: “A signed document is confidential.” Respond: A signature hides nothing.

   In the lesson window

   • Think it through: Does signing a document keep its contents secret?

6. 5. Encryption and signing use the keys differently Explain 6 min

   Purpose: Contrast how encryption and signing use key pairs.

   Ask

   • Which key does Bob use to check Alice’s signature?

   Responses to expect

   • Bob’s own key is the common slip.
   • On screen in turns: the opening question, its answer; then the point most often confused; encrypting and signing side by side, starting with encrypting; signing; encrypting and signing: do not confuse them; is signing “encrypting the hash”?.

   To go further

   • Why do many systems issue separate certificates for signing and for encryption?

   In the lesson window

   • Check your understanding: Alice signs a message to Bob. Which key does Bob use to check it?

7. 6. Non-repudiation: evidence, not certainty Predict 10 min

   Purpose: Show what the evidence proves in a dispute and what strengthens it.

   Ask

   • What does the verifying signature prove?
   • What does a lent card do?

   Responses to expect

   • “It proves she signed” is the default.
   • On screen in turns: the opening; then the question on “I never approved that”, and its answer; the stored evidence still verifies; what the evidence shows, starting with the document; the matching key; nobody else could have made it; what it cannot show, starting with whose hands were on the key; that the certificate was right; when it was signed; that the signer understood; evidence, not a guarantee; what strengthens the evidence, with the reasons held back; why each helps; the check question on a lent smart card, and its answer; where non-repudiation is taught more widely.

   Misconceptions to challenge

   • A learner may believe: “A valid signature proves who signed.” Respond: It proves which key was used; custody decides the rest.

   Discussion pause: Reveal what the evidence shows, then ask the class what it cannot show before revealing those four points.

   If the explanation does not land: Compare a house key: the door was opened with your key, but who was holding it?

   In the lesson window

   • Think it through: The headteacher says, “I never approved that purchase,” but the signature verifies. What does that prove, and what can it not prove on its own?
   • Check your understanding: The headteacher lent her signing smart card and PIN to a deputy. What does that do to the evidence?

8. 7. Where digital signatures are used Predict 3 min

   Purpose: List common uses, from updates to email.

   Ask

   • Why might a phone refuse an update?

   Responses to expect

   • Not from the publisher, or altered.
   • On screen in turns: the opening question, its answer; then where signatures are used, starting with software updates; certificates; secure web connections; email; documents and contracts.

   In the lesson window

   • Think it through: A phone refuses to install an update because its signature does not check. What might have happened?

9. 8. Security+ terminology Explain 1 min

   Purpose: Link the lesson to the Security+ wording.

   Ask

   • Which objective term does this lesson cover?

   Responses to expect

   • Digital signatures.
   • On screen in turns: the opening; then what the objectives do not spell out.

10. 9. 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 contract tests sign and encrypt; Friday tests timestamps; the colleague tests the loose description.
    • On screen in turns: the opening; then the question on a private, signed contract, and its answer; the question on a key stolen on Friday, and its answer; the question on “encrypting the hash”, and its answer.

    In the lesson window

    • Apply it: A contract must be both private and signed. What is needed?
    • Apply it: A key was reported stolen on Friday. A document signed with it is disputed. What would help most?
    • Apply it: A colleague describes signing as “encrypting the hash with the private key, so the content is hidden”. What is wrong?

11. 10. Check your understanding Check 6 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 comparison step.
    • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

12. 11. What to remember Consolidate 2 min

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

    Ask

    • Without looking: which key signs, and which verifies?

    Responses to expect

    • The signer’s private key signs; the signer’s public key verifies.
    • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

13. 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

    • Private key signs; public key verifies.
    • Sign the digest; any change breaks it.
    • Integrity, authentication, non-repudiation; not secrecy.
    • Recipient’s pair to encrypt; sender’s pair to sign.
    • Evidence, not certainty.

Consolidation

• Private key signs; public key verifies.
• Sign the digest; any change breaks it.
• Integrity, authentication, non-repudiation; not secrecy.
• Recipient’s pair to encrypt; sender’s pair to sign.
• Evidence, not certainty.