Cryptography Foundations: Ciphers, Keys and Encoding

Lesson plan

Level

Beginner

Time

About 55 minutes; the steps below add up to 56

Learners work out what keeps a locker padlock shut, and whether a public swap table keeps a message secret, before plaintext, cipher and key are named. Predictions then precede a published-scheme scramble, DOG with a shift of three, how many Caesar shifts there are, listener against changer, secret methods and one more bit; checks follow the padlock, a Base64 token, a leaked key and a 65-bit key. The habit to build: ask where the secret is, and whether anything secret is involved at all.

Objectives

• Define plaintext, ciphertext, cipher and key, and tell cryptography, cryptanalysis and cryptology apart
• Explain why encoding is not encryption
• Encrypt and break a Caesar cipher, and explain what it teaches
• Explain Kerckhoffs’s principle
• Explain how key length sets the keyspace

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 CIA Triad lesson first.
• A real combination padlock is a good prop for the opening.
• 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

   • Define plaintext, ciphertext, cipher and key, and tell cryptography, cryptanalysis and cryptology apart
   • Explain why encoding is not encryption
   • Encrypt and break a Caesar cipher, and explain what it teaches
   • Explain Kerckhoffs’s principle
   • Explain how key length sets the keyspace

   Purpose: Learners work out what keeps a locker padlock shut, and whether a public swap table keeps a message secret, before plaintext, cipher and key are named. Predictions then precede a published-scheme scramble, DOG with a shift of three, how many Caesar shifts there are, listener against changer, secret methods and one more bit; checks follow the padlock, a Base64 token, a leaked key and a 65-bit key. The habit to build: ask where the secret is, and whether anything secret is involved at all.

2. 1. A locker padlock Think 5 min

   Purpose: Let learners find that the secret is the combination, not the lock.

   Ask

   • What keeps the locker shut?
   • Is the swap-table message secret?

   Responses to expect

   • “The lock” is a common first answer.
   • On screen in turns: the opening question, its answer; then the question on a table in every book, and its answer.

   Discussion pause: Hold up a padlock and ask what would let a classmate open it.

   In the lesson window

   • Think it through: Everyone can buy the same model of combination padlock for a school locker, and everyone knows how it works. What actually keeps your locker shut?
   • Think it through: You write a message in which every letter is swapped for a number, using a table printed in the back of every exercise book. Is the message secret?

3. 2. The idea Explain 6 min

   Purpose: Name plaintext, cipher, ciphertext and key with the padlock, and the three similar words.

   Ask

   • Which part is the key?

   Responses to expect

   • The combination: the only secret part.
   • On screen in turns: the opening; then plaintext, cipher, key and ciphertext; the combination padlock; why it matters: confidentiality; the check question on which part is the key, and its answer; three words that sound alike, starting with cryptography; cryptanalysis; cryptology; two halves of one job.

   In the lesson window

   • Check your understanding: In the padlock comparison, which part is the key?

4. 3. Encoding is not encryption Predict 6 min

   Purpose: Separate encoding from encryption with Base64.

   Ask

   • Is the scrambled-looking text protected?
   • What does Base64 give a token?

   Responses to expect

   • Many think Base64 is encryption.
   • On screen in turns: the opening; then the question on a scrambled-looking message, and its answer; encoding; encryption; Base64 looks secret and is not; the check question on a token in Base64, and its answer; encoding and encryption, side by side.

   Misconceptions to challenge

   • A learner may believe: “Base64 encrypts data.” Respond: It encodes; there is no key.

   In the lesson window

   • Think it through: A program turns “Meet at noon” into TWVldCBhdCBub29u, using a published scheme and no secret. Is the message now protected?
   • Check your understanding: A developer stores an access token “protected” with Base64. What protection does that give?

5. 4. A cipher you can break yourself Predict 3 min

   Purpose: Encrypt a word with the Caesar cipher by hand, using a shift as the key.

   Ask

   • What does “dog” become?

   Responses to expect

   • On screen in turns: the opening; then the question on shifting “dog”, and its answer; a shift of 3; encrypting CAT, starting with C; A; T; the ciphertext, and decrypting it.

   If the explanation does not land: Have pairs encrypt a short word for each other with a secret shift, then break each other’s.

   In the lesson window

   • Think it through: If every letter moves three places along the alphabet, what does “dog” become? And how would the receiver turn it back?

6. 5. Try it: the Caesar cipher Experiment 6 min

   Purpose: Let learners encrypt with the panel, then break a message by trying every shift, and draw the two lessons; Enigma as history.

   Ask

   • How many shifts?

   Responses to expect

   • Learners enjoy the brute-force panel.
   • Panel: the panel is on screen when the step opens, so the instructor view’s jump to this step lands on it.
   • On screen in turns: the opening; then a substitution cipher; the question on how many shifts to try, and its answer; a small keyspace is fatal; patterns survive; Enigma, starting with the machine; what its breaking teaches.

   In the lesson window

   • Think it through: How many different shifts could an attacker try against a Caesar message, and how long would it take?

7. 6. Alice, Bob and Mallory Predict 4 min

   Purpose: Introduce the conventional cast and why a changer is worse than a listener.

   Ask

   • Which attacker needs more protection against?

   Responses to expect

   • The changer: it needs change detection as well.
   • On screen in turns: the opening; then the cast, starting with Alice and Bob; Mallory; the question on listening or changing, and its answer; Eve, the eavesdropper.

   In the lesson window

   • Think it through: One attacker can only listen; another can also change messages. Which needs more protection against, and what extra protection?

8. 7. Kerckhoffs’s principle Predict 7 min

   Purpose: Explain why security should rest on the key, not a secret method.

   Ask

   • Is a secret method safer?
   • Which leak is easier to recover from?

   Responses to expect

   • “Secret is safer” is intuitive and wrong.
   • On screen in turns: the opening question, its answer; then Kerckhoffs’s principle; security rests on the key; three reasons, starting with testing; methods leak; keys are replaceable; security through obscurity; the check question on what leaked, and its answer; “our algorithm is proprietary”.

   Misconceptions to challenge

   • A learner may believe: “A secret algorithm is more secure.” Respond: Published, tested algorithms are trusted; keys can be replaced.

   In the lesson window

   • Think it through: Is a cipher safer if the way it works is kept secret?
   • Check your understanding: In one case a key leaks; in another, a secret cipher design leaks. Which is easier to recover from?

9. 8. Key length, keyspace and brute force Predict 7 min

   Purpose: Show that each extra bit doubles the keyspace.

   Ask

   • What does one more bit do?
   • How much stronger is 65 bits than 64?

   Responses to expect

   • “Plus one” is the usual guess.
   • On screen in turns: the opening; then the question on one more bit, and its answer; the keyspace, starting with a 1-bit key; a 2-bit key; a 3-bit key; an n-bit key; every extra bit doubles it; why 128 bits is not twice 64; the check question on a 65-bit key, and its answer; compare key lengths only within one kind.

   Discussion pause: Reveal the keyspace one bit at a time; ask how many keys there are before each line appears.

   To go further

   • Why can a long key still be weak if it was generated from predictable values?

   In the lesson window

   • Think it through: Adding one bit to a key: what happens to the number of keys an attacker must try?
   • Check your understanding: Against brute force, how many times stronger is a 65-bit key than a 64-bit key?

10. 9. Security+ terminology Explain 1 min

    Purpose: Link the lesson to the Security+ wording.

    Ask

    • Which sub-topics does this lesson supply?

    Responses to expect

    • Algorithms and key length.
    • 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 supplier tests Kerckhoffs; the configuration file tests encoding; the café tests the cast.
    • On screen in turns: the opening; then the question on “military-grade secret encryption”, and its answer; the question on a Base64 password, and its answer; the question on Eve or Mallory, and its answer.

    In the lesson window

    • Apply it: A supplier says its messaging app uses “military-grade secret encryption we cannot disclose”. What should you ask?
    • Apply it: A configuration file holds a password written in Base64. A colleague says it is encrypted. What do you say?
    • Apply it: An attacker on café Wi-Fi alters the amount in a bank transfer as it travels. Is that Eve or Mallory?

12. 11. 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 encoding question links to its own 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: what does every extra bit do?

    Responses to expect

    • Doubles the keyspace.
    • 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

    • Public method, secret key.
    • No key, no secrecy.
    • Small keyspaces and patterns are fatal.
    • Rest security on the key.
    • Each bit doubles the work.

Consolidation

• Public method, secret key.
• No key, no secrecy.
• Small keyspaces and patterns are fatal.
• Rest security on the key.
• Each bit doubles the work.