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Applied Cryptography

Which of the following is the correct combination of what the TLS 1.3 cipher suite TLS_AES_128_GCM_SHA256 specifies?

  1. AThe AEAD used to encrypt data (AES-128-GCM) and the hash function used for key derivation (HKDF) and other purposes (SHA-256)
  2. BThe key exchange scheme (AES-128) and the signature scheme of the server certificate (SHA-256)
  3. CAES-128-GCM for data encryption and HMAC-SHA256 attached to each record for tamper detection
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Answer: A. The AEAD used to encrypt data (AES-128-GCM) and the hash function used for key derivation (HKDF) and other purposes (SHA-256)

A TLS 1.2 cipher suite such as TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 specified the key exchange (ECDHE), authentication (RSA), cipher (AES-128-GCM) and hash function all together. In TLS 1.3 the key exchange group (such as X25519) and the signature scheme are negotiated in separate extensions, and the cipher suite represents only the pair of an AEAD and the hash function for HKDF. Since the AEAD also detects tampering with the data, no HMAC is attached to each record, and SHA-256 is used for hashing the handshake messages and for key derivation.

  • A. Correct. A TLS 1.3 cipher suite represents only the pair of an AEAD and a hash function.
  • B. AES is an encryption scheme. In TLS 1.3 the key exchange group and the signature scheme are negotiated separately from the cipher suite.
  • C. TLS 1.3 uses AEAD only, so no separate MAC is attached to records. SHA-256 is used for key derivation and similar purposes.
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More Applied Cryptography questions

  1. Which of the following is a correct caution about 0-RTT in TLS 1.3 (sending "early data" together with the first message, using a pre-shared key obtained in an earlier connection)?
  2. Which of the following correctly describes mutual TLS (mTLS)?
  3. 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?
  4. Which of the following correctly pairs the post-quantum cryptography standards published by the US NIST in August 2024 with their types?
  5. Which of the following correctly describes how a key encapsulation mechanism (KEM) such as ML-KEM is used?

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