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 forward, what does C become?

    • E
    • G
    • F

  2. What makes an encryption scheme symmetric?

    • It uses the same secret key to encrypt and decrypt
    • It uses a public key to encrypt and a private key to decrypt
    • It uses the message's length instead of a secret key

  3. Keeping the algorithm secret is a substitute for using a sound encryption scheme.

    Circle one:   True   False

  4. Which claim is not supplied by confidentiality-only symmetric encryption?

    • Correct decryption uses the shared secret key
    • The message was sent by a particular person
    • Protected contents resist the defined eavesdropper

  5. A network observer cannot decrypt a packet but can see its endpoints and timing. What can they still record?

    • The packet's endpoints and when it was sent
    • The packet's plaintext and the sender's secret key
    • The receiver's password and the sender's plaintext

  6. Two plaintexts are XORed with the same keystream S. What does XORing their ciphertexts reveal?

    • The keystream S without any plaintext knowledge
    • The length of S but no relation between plaintexts
    • The XOR of the two plaintexts because S cancels

  7. A sender encrypts a new shared secret to a public key received without checking whose key it is. What risk remains?

    • Ciphertext length alone lets every observer compute the secret
    • An attacker could substitute their own key and decrypt it
    • The recipient must publish a private key before receiving data

  8. Vigenere uses A for shift 0 and B for shift 1. Encrypt CC with key AB. What is the ciphertext?

    • CD
    • DD
    • CC
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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. F · Three forward steps from C are D, E and F.
  2. It uses the same secret key to encrypt and decrypt · Symmetric encryption uses one shared secret key for both operations.
  3. False · A sound scheme must remain secure even when the attacker knows its algorithm.
  4. The message was sent by a particular person · Confidentiality alone does not identify a sender. Even a shared authentication key does not distinguish all its holders.
  5. The packet's endpoints and when it was sent · Protecting plaintext does not automatically hide the surrounding traffic metadata.
  6. The XOR of the two plaintexts because S cancels · C1 XOR C2 = P1 XOR S XOR P2 XOR S = P1 XOR P2. This exposes a relation, not necessarily both complete messages.
  7. An attacker could substitute their own key and decrypt it · Encryption protects data for the holder of the matching private key. Without authenticating the public key, that holder could be an attacker.
  8. CD · The first C stays C under shift 0. The second C becomes D under shift 1.
Worksheet · LightMySky