Symmetric Encryption and What a Cipher Promises · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

One shared key locks and unlocks

Computing · Networks & Security · ages 20-21
Name ______________________   Date ____________

  1. With a Caesar shift of 3, what does A become?

    • C
    • E
    • D

  2. What makes an encryption technique symmetric?

    • The same key locks and unlocks the data
    • Two different keys split the work
    • No key is needed at all

  3. A cipher stays safe mainly by keeping its method a secret.

    Circle one:   True   False

  4. Which job is NOT covered by a plain symmetric cipher alone?

    • Hiding message contents from eavesdroppers
    • Proving who sent the message
    • Scrambling with a shared key

  5. Someone watches your encrypted traffic. What can they still learn?

    • That you sent a message, to whom, and when
    • The contents of each message
    • Your key, directly from the locked text

  6. Why must a key stream never be reused across two messages?

    • Reuse makes messages too short
    • Reuse deletes the key forever
    • Reuse leaks information between the messages

  7. A friend says encryption means nobody knows you talked at all. What is the flaw?

    • Encryption also hides the sender for free
    • Contents hide, but the sending facts stay visible to watchers
    • Encrypted messages travel on private roads

  8. Why did a word-keyed Vigenere resist readers longer than a single Caesar shift?

    • It shifted letters by different amounts down the message
    • It used no key at all
    • It made messages much shorter
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Answer key

For grown-ups. Fold this page away before handing over the rest.

One shared key locks and unlocks W1-mt_egJQEiFWZA-s1

  1. D · Shift A forward by 3 letters: A to B to C to D.
  2. The same key locks and unlocks the data · Sender and receiver use the very same key for scrambling and unscrambling.
  3. False · The method may be public; only the key must stay secret.
  4. Proving who sent the message · Sender proof and tamper detection need other tools beyond the cipher.
  5. That you sent a message, to whom, and when · Encryption hides contents, not the surrounding facts of sending.
  6. Reuse leaks information between the messages · Comparing two messages under one stream exposes their contents to analysis.
  7. Contents hide, but the sending facts stay visible to watchers · The locked box hides what is inside, not the trip it takes through the network.
  8. It shifted letters by different amounts down the message · Varying shifts per position hide the single-shift pattern Caesar shows.
Worksheet · LightMySky