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Applied Cryptography

Which of the following correctly describes a change from TLS 1.2 in TLS 1.3 (RFC 8446)?

  1. AFor data encryption, only cipher suites using CBC mode, with security improved over TLS 1.2, are now used
  2. BKey exchange that sends key material with RSA encryption was removed, and data encryption is now authenticated encryption (AEAD) only
  3. CServer certificates are no longer needed, and the server is no longer authenticated
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Answer: B. Key exchange that sends key material with RSA encryption was removed, and data encryption is now authenticated encryption (AEAD) only

TLS 1.3 removed RSA key transport and static DH, which lack forward secrecy; key exchange is now (EC)DHE, a pre-shared key (PSK), or a combination of the two. Data encryption is also limited to AEAD such as AES-GCM and ChaCha20-Poly1305, and the schemes combining CBC mode with a MAC, which had been behind attacks such as padding oracle attacks, are gone. In addition, the handshake completes in one round trip (1-RTT), and much of the handshake, including the server certificate, is encrypted. The server is still authenticated with a certificate and a signature.

  • A. It is the other way round. Cipher suites using CBC mode were removed, leaving AEAD only.
  • B. Correct. It was streamlined to key exchange with forward secrecy and AEAD only.
  • C. In TLS 1.3 the server is still authenticated with a certificate and a signature. The certificate is sent encrypted within the handshake.
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More Applied Cryptography questions

  1. Which of the following is the correct combination of what the TLS 1.3 cipher suite TLS_AES_128_GCM_SHA256 specifies?
  2. 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)?
  3. Which of the following correctly describes mutual TLS (mTLS)?
  4. 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?
  5. Which of the following correctly pairs the post-quantum cryptography standards published by the US NIST in August 2024 with their types?

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