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Applied Cryptography

Which of the following correctly describes mutual TLS (mTLS)?

  1. AA scheme that applies TLS encryption twice over to double the strength of the encryption
  2. BA scheme that uses both of the protocols TLS 1.2 and TLS 1.3 at the same time within one connection
  3. CA scheme in which not only the server but also the client presents a certificate, so each side authenticates the other
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Answer: C. A scheme in which not only the server but also the client presents a certificate, so each side authenticates the other

In ordinary TLS the client only checks the server's certificate, and the client is authenticated by mechanisms on top of TLS such as passwords or tokens. In mutual TLS the server also asks the client for a certificate, and the client sends its certificate and a signature made with the corresponding private key. It is often used for authentication between machines with no human operating them, such as communication between microservices, connections between IoT devices and servers, and networks based on the zero-trust approach.

  • A. It is not a scheme that layers encryption. "Mutual" means that authentication goes in both directions.
  • B. One connection uses one version of TLS, decided in the handshake.
  • C. Correct. The client is also authenticated with a certificate and a private key.
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More Applied Cryptography questions

  1. Searching for the key of an n-bit symmetric-key cipher with Grover's algorithm on a quantum computer is said to take about 2^(n/2) computations. Using this simple guideline, which of the following is closest to the security of AES-128 against quantum computers?
  2. Which of the following correctly pairs the post-quantum cryptography standards published by the US NIST in August 2024 with their types?
  3. Which of the following correctly describes how a key encapsulation mechanism (KEM) such as ML-KEM is used?
  4. 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?
  5. 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?

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