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Applied Cryptography

Which of the following correctly describes how a key encapsulation mechanism (KEM) such as ML-KEM is used?

  1. AThe sender signs the shared secret with its own private key and sends it, and the receiver verifies the signature with the sender's public key and receives the secret
  2. BThe sender creates a shared secret and a ciphertext wrapping it from the receiver's public key, and the receiver extracts the same secret with its private key
  3. CThe sender and receiver hold the same symmetric key in advance and encrypt messages directly with it
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Answer: B. The sender creates a shared secret and a ciphertext wrapping it from the receiver's public key, and the receiver extracts the same secret with its private key

A KEM consists of three operations: key generation, encapsulation and decapsulation. The sender encapsulates with the receiver's public key, obtaining a random shared secret together with a ciphertext that wraps it, and sends only the ciphertext. The receiver decapsulates with its private key to obtain the same shared secret, after which they use symmetric-key encryption with keys based on it. A KEM is a mechanism for sharing keys, so a separate signature such as ML-DSA is used to authenticate the other party.

  • A. This is how digital signatures are used. A KEM is a mechanism for sharing keys and has no signature function.
  • B. Correct. The receiver opens the ciphertext made by encapsulation with its private key.
  • C. This describes sharing a symmetric key in advance. A KEM creates a shared secret using a public key with a party with whom no secret has been shared beforehand.
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More Applied Cryptography questions

  1. Which of the following correctly describes the idea of hybrid key exchange, used during the migration to post-quantum cryptography, that combines X25519 (Diffie–Hellman key exchange using an elliptic curve) with ML-KEM?
  2. One way to think about when to migrate cryptography is "if the number of years x for which data must be kept secret plus the number of years y the migration to new cryptography takes exceeds the number of years z until a quantum computer able to break the cryptography appears, there is already a danger" (Mosca's inequality). With x = 10 years, y = 5 years and z assumed to be 12 years, which of the following is the correct judgment?
  3. Which of the following correctly describes the role of an HSM (hardware security module)?
  4. Let DEK be a data encryption key and KEK a key encryption key. Which of the following correctly describes envelope encryption, as used in cloud storage and elsewhere?
  5. Which of the following is the main purpose of key rotation, replacing a cryptographic key with a new one at regular intervals?

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