The shape of the lesson
By the end of the lesson, learners can
- Why encrypt?: Data travelling across networks passes through many devices and can be intercepted by a hacker.
- Symmetric: one key: The lab starts with a Caesar cipher, a very simple symmetric method: shift every letter along the alphabet by the key.
- Asymmetric: two keys: Switch the lab to asymmetric mode.
- Purpose: intercepted data is meaningless without the key; it does not stop interception.
- Plaintext to ciphertext: using an algorithm and a key.
- Symmetric: one key encrypts and decrypts; the key must be shared and could be intercepted.
- Asymmetric: encrypt with the receiver's public key; decrypt with the receiver's private key, which is never sent.
One picture every idea returns to
Encryption scrambles plaintext into ciphertext using an algorithm and a key, so that if data is intercepted it is meaningless to anyone without the key. Encryption does not stop interception. In symmetric encryption the same key encrypts and decrypts, so the key has to be shared, and it could be intercepted too. In asymmetric encryption there is a matching pair of keys: data encrypted with the receiver's public key can only be decrypted with the receiver's private key, which is never sent.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Meaningless without the key.. Anything sent across a network can be intercepted. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Why encrypt? | Data travelling across networks passes through many devices and can be intercepted by a hacker. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Symmetric: one key | The lab starts with a Caesar cipher, a very simple symmetric method: shift every letter along the alphabet by the key. Project tab 6 of the lab (Symmetric and asymmetric encryption) and let a student drive it while the class predicts. Model the worked example Caesar shift on the board, one line at a time. Your turn: The ciphertext MJQQT was made with a Caesar shift of 5. Decrypt it. | Pairs |
| 21 to 30 min | Teach and check: Asymmetric: two keys | Switch the lab to asymmetric mode. Project tab 6 of the lab (Public and private keys) and let a student drive it while the class predicts. Model the worked example Compare on the board, one line at a time. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Asymmetric: two keys, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Encryption does not stop data being intercepted or stolen. | It makes intercepted data meaningless (unreadable) without the key. Write exactly that. |
| Real symmetric encryption uses very long keys, not a shift of 3. | The Caesar cipher only shows the idea: the same key locks and unlocks. |
| The sender uses the receiver's public key, not its own. | The answer "the message is encrypted with a private key and decrypted with a public key" is the wrong way round for sending secret data. |
Symmetric and asymmetric encryption: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Explain the purpose of encrypting data before it is transmitted.
State the meaning of the terms plaintext and ciphertext.
A Caesar cipher with a key of 4 is used. (a) Encrypt the word SECRET. (b) State which type of encryption a Caesar cipher is.
Describe how data is encrypted and decrypted using symmetric encryption, and give one drawback.
Priya wants to send her bank a confidential message using asymmetric encryption. Describe how this is done.
A hacker intercepts both the public key and the encrypted message in question 5. Explain why the message is still secure.
Answer key and marking notes
- If the data is intercepted it cannot be understood / is meaningless.
- Because it cannot be decrypted without the key, so the data stays confidential (encryption does not stop interception).
- Plaintext: the original data before encryption.
- Ciphertext: the data after it has been encrypted.
- (a) WIGVIX.
- (b) Symmetric: the same key (shift of 4) is used to encrypt and decrypt.
- The plaintext is encrypted using an algorithm and a key to produce ciphertext.
- The receiver decrypts the ciphertext using the same key.
- Drawback: the key has to be sent to the receiver, and if it is intercepted the data can be decrypted.
- The bank has a public key and a matching private key.
- The bank's public key is sent to / made available to Priya.
- Priya encrypts the message using the bank's public key.
- The bank decrypts the ciphertext using its private key, which is never shared.
- The public key can only be used to encrypt the message, not to decrypt it.
- Only the matching private key can decrypt it, and that key is kept by the bank and never transmitted.
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the purpose of encrypting data before it is transmitted?
- AIf the data is intercepted, it is meaningless without the key
- BThe data cannot be intercepted while it is being sent
- CErrors caused by interference can be detected and corrected
- DThe data becomes smaller, so it is transmitted more quickly
A Caesar cipher shifts each letter 3 places forward in the alphabet, wrapping round from Z to A. What is ZEBRA encrypted to?
- A
CHEUD - B
WBYOX - C
AFCSB - D
CHEUA
Which statement describes symmetric encryption?
- AThe same key is used to encrypt and to decrypt the data
- BA public key encrypts the data and a private key decrypts it
- CThe data is encrypted twice, once by each computer
- DNo key is needed, because the algorithm is kept secret
Ana sends Ben a confidential file using asymmetric encryption. Ana encrypts it with Ben's public key. Which key is used to decrypt it?
- ABen's private key
- BBen's public key
- CAna's private key
- DAna's public key
A message was encrypted with a Caesar cipher that shifts each letter 4 places forward. The ciphertext is HEXE. What is the plaintext?
- A
DATA - B
LIBI - C
EBUB - D
CZSZ
A company sends its staff a symmetric key by email, then uses it to encrypt all their messages. A hacker intercepts that email. Why is asymmetric encryption a better choice here?
- AThe key that is sent cannot decrypt; the private key is never sent
- BAsymmetric encryption makes it impossible for the hacker to intercept any of the messages
- CAsymmetric encryption uses the same key at both ends, so no key ever needs to be sent
- DThe hacker would need to intercept the payload and the trailer of each packet
Quiz key
Encryption scrambles plaintext into ciphertext so that anyone who intercepts it cannot understand it without the key. It does not stop interception itself. Detecting transmission errors is the job of parity checks, checksums, echo checks and ARQ. Encryption does not compress data, so it does not make it smaller.
Z → C (wrapping round), E → H, B → E, R → U, A → D, giving CHEUD. WBYOX shifts each letter 3 places backwards, which is decryption. AFCSB shifts by only 1 place. CHEUA forgets to shift the final A.
In symmetric encryption one key does both jobs, so the sender and receiver must both have it. A public and private key pair is asymmetric encryption. Encrypting twice is not what symmetric means. Encryption always uses a key with the algorithm; secrecy of the key, not the algorithm, protects the data.
Ben's public and private keys are a matching pair, so a file encrypted with Ben's public key can only be decrypted with Ben's private key, which only Ben holds. The public key cannot decrypt what it encrypted. Ana's keys are a different pair and cannot decrypt data encrypted with Ben's public key.
Decryption shifts each letter 4 places back: H → D, E → A, X → T, E → A, giving DATA. LIBI shifts 4 places forward again, which encrypts a second time. EBUB shifts back only 3 places and CZSZ shifts back 5 places, so both use the wrong key.
With symmetric encryption the one key that decrypts has to be shared, so intercepting it lets the hacker read everything. With asymmetric encryption only the public key is shared; it can encrypt but not decrypt, and the private key that decrypts is never transmitted. Encryption never prevents interception. Using the same key at both ends describes symmetric, not asymmetric, encryption. Packet structure has nothing to do with whether the data can be decrypted.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.