dimpl — DTLS 1.2 implementation (Sans‑IO, Sync)

dimpl is a focused DTLS 1.2 implementation aimed at WebRTC. It is a Sans‑IO state machine you embed into your own UDP/RTC event loop: you feed incoming datagrams, poll for outgoing records or timers, and wire up certificate verification and SRTP key export yourself.

• DTLS 1.2: Implements the DTLS 1.2 handshake and record layer used by WebRTC.
• Safety: forbid(unsafe_code) throughout the crate.
• Minimal Rust‑only deps: Uses small, well‑maintained Rust crypto crates.
• Low overhead: Tight control over allocations and buffers; Sans‑IO integration.

• DTLS 1.0
• Async (the crate is Sans‑IO and event‑loop agnostic)
• no_std (at least not without allocation)
• RSA
• DHE

dimpl was built as a support package for str0m, with WebRTC as its primary use case, which currently uses DTLS 1.2. The author is not a cryptography expert; however, our understanding is that DTLS 1.2 is acceptable provided we narrow the protocol's scope—for example, by supporting only specific cipher suites and hash algorithms and by requiring the Extended Master Secret extension.

If you are interested in extending this crate to support DTLS 1.3 and/or additional cipher suites or hash algorithms, we welcome collaboration, but we are not planning to lead such initiatives.

• Cipher suites (TLS 1.2 over DTLS)
  • ECDHE_ECDSA_AES256_GCM_SHA384
  • ECDHE_ECDSA_AES128_GCM_SHA256
• AEAD: AES‑GCM 128/256 only (no CBC/EtM modes).
• Key exchange: ECDHE (P‑256/P‑384)
• Signatures: ECDSA P‑256/SHA‑256, ECDSA P‑384/SHA‑384
• DTLS‑SRTP: Exports keying material for SRTP_AEAD_AES_256_GCM, SRTP_AEAD_AES_128_GCM, and SRTP_AES128_CM_SHA1_80 (RFC 5764, RFC 7714).
• Extended Master Secret (RFC 7627) is negotiated and enforced.
• Not supported: PSK cipher suites.

During the handshake the engine emits Output::PeerCert with the peer's leaf certificate (DER). The crate uses that certificate to verify DTLS handshake messages, but it does not perform any PKI validation. Your application is responsible for validating the peer certificate according to your policy (fingerprint, chain building, name/EKU checks, pinning, etc.).

Drive the engine with three calls:

• Dtls::handle_packet — feed an entire received UDP datagram.
• Dtls::poll_output — drain pending output: DTLS records, timers, events.
• Dtls::handle_timeout — trigger retransmissions/time‑based progress.

The output is an Output enum with borrowed references into your provided buffer:

• Packet(&[u8]): send on your UDP socket
• Timeout(Instant): schedule a timer and call handle_timeout at/after it
• Connected: handshake complete
• PeerCert(&[u8]): peer leaf certificate (DER) — validate in your app
• KeyingMaterial(KeyingMaterial, SrtpProfile): DTLS‑SRTP export
• ApplicationData(&[u8]): plaintext received from peer

use std::sync::Arc;
use std::time::Instant;

use dimpl::{certificate, Config, Dtls, Output};

// Stub I/O to keep the example focused on the state machine
enum Event { Udp(Vec<u8>), Timer(Instant) }
fn wait_next_event(_next_wake: Option<Instant>) -> Event { Event::Udp(Vec::new()) }
fn send_udp(_bytes: &[u8]) {}

fn example_event_loop(mut dtls: Dtls) -> Result<(), dimpl::Error> {
    let mut next_wake: Option<Instant> = None;
    loop {
        // Drain engine output until we have to wait for I/O or a timer
        let mut out_buf = vec![0u8; 2048];
        loop {
            match dtls.poll_output(&mut out_buf) {
                Output::Packet(p) => send_udp(p),
                Output::Timeout(t) => { next_wake = Some(t); break; }
                Output::Connected => {
                    // DTLS established — application may start sending
                }
                Output::PeerCert(_der) => {
                    // Inspect peer leaf certificate if desired
                }
                Output::KeyingMaterial(_km, _profile) => {
                    // Provide to SRTP stack
                }
                Output::ApplicationData(_data) => {
                    // Deliver plaintext to application
                }
            }
        }

        // Block waiting for either UDP input or the scheduled timeout
        match wait_next_event(next_wake) {
            Event::Udp(pkt) => dtls.handle_packet(&pkt)?,
            Event::Timer(now) => dtls.handle_timeout(now)?,
        }
    }
}

fn mk_dtls_client() -> Dtls {
    let cert = certificate::generate_self_signed_certificate().unwrap();
    let cfg = Arc::new(Config::default());
    let mut dtls = Dtls::new(cfg, cert);
    dtls.set_active(true); // client role
    dtls
}

// Putting it together
let dtls = mk_dtls_client();
let _ = example_event_loop(dtls);

Rust 1.81.0

• Session resumption is not implemented (WebRTC does a full handshake on ICE restart).
• Renegotiation is not implemented (WebRTC does full restart).
• Only DTLS 1.2 is accepted/advertised.

License: MIT OR Apache-2.0