TLS 1.3 Protocol

X25519MLKEM768 is implemented for both roles, as RFC 10024. We offer the hybrid group first as a client and accept it as a server, so a client that offers only the hybrid — the default for OpenSSL 3.5+ — completes the handshake. Note that X25519MLKEM768 names a key-share encoding, not a shared-secret rule: Zig's std MlKem768X25519 is X-Wing, which encodes it identically and produces a different secret.

RFC 8446 defines Transport Layer Security Version 1.3, delivering major security and performance improvements over previous TLS versions.

Key Improvements in TLS 1.3

1. 1-RTT Handshake: Halves handshake latency compared to TLS 1.2 by guessing the key exchange algorithm in the initial ClientHello.
2. Removed Deprecated Primitives: Obsoletes static RSA key exchange, Diffie-Hellman static groups, SHA-1, MD5, and CBC ciphers in favor of Ephemeral Diffie-Hellman (ECDHE) with AEAD ciphers.
3. Encrypted Handshake: Certificate messages and extensions are encrypted immediately following the ServerHello.
4. Resumption (0-RTT): Pre-shared key resumption allows early data transmission on the very first round trip.

Handshake Diagram

Client                                               Server
ClientHello (KeyShare, SNI, ALPN) -------->
                                              ServerHello (KeyShare)
                                              {EncryptedExtensions}
                                              {Certificate}
                                              {CertificateVerify}
                                  <--------   {Finished}
{Finished}                        -------->
[Application Data]                <------->   [Application Data]

Key Updates

Both native roles implement post-handshake KeyUpdate (RFC 8446 Section 4.6.3): receive keys rotate on arrival, requested updates are answered automatically, and each sending direction enforces the RFC 9846 limit of 2^48 - 1 updates. The server exposes Connection.sendKeyUpdate() to initiate an update; clients handle updates while reading.

Related

• Protocol: TLS 1.2
• Guide: TLS
• Security: TLS