Ryan Hooper

— On the schedule —

DTLS 1.3 in OpenSSL

DTLS 1.3 brings significant protocol improvements over its predecessor: a variable-length unified header reduces record overhead from 13 bytes to 3-5 bytes by encoding the epoch in 2 bits and omitting redundant version and content-type fields; sequence numbers are now encrypted using AES-ECB or ChaCha20-derived masks to prevent traffic analysis; and an explicit ACK mechanism replaces implicit acknowledgment for reliable delivery of critical handshake messages like client certificates, KeyUpdate, and NewSessionTicket. This presentation details the OpenSSL implementation of these features and introduces the DTLS SSL Listener—a new server-side solution that provides automatic connection demultiplexing by source address, built-in cookie validation via HelloVerifyRequest (DTLS 1.2) and HelloRetryRequest (DTLS 1.3) to prevent amplification attacks, and a unified interface matching the QUIC listener API. We conclude with practical code examples showing how to establish DTLS 1.3 connections using traditional APIs and how to build multi-client servers using the listener interface with configurable timeouts and both blocking and non-blocking accept modes.