dimpl

dimpl — DTLS 1.2 and 1.3 implementation (Sans‑IO, Sync)

dimpl is a DTLS 1.2 and 1.3 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.

Goals

• DTLS 1.2 and 1.3: Implements the DTLS 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.

Non‑goals

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

Version selection

Four constructors control which DTLS version is used:

• Dtls::new_12 — explicit DTLS 1.2 (certificate‑based)
• Dtls::new_12_psk — explicit DTLS 1.2 (PSK, no certificates)
• Dtls::new_13 — explicit DTLS 1.3
• Dtls::new_auto — auto‑sense: the first incoming ClientHello determines the version (based on the supported_versions extension)

Cryptography surface

• Cipher suites (TLS 1.2 over DTLS)
  • ECDHE_ECDSA_AES256_GCM_SHA384
  • ECDHE_ECDSA_AES128_GCM_SHA256
  • ECDHE_ECDSA_CHACHA20_POLY1305_SHA256
• PSK cipher suites (TLS 1.2 over DTLS)
  • PSK_AES128_CCM_8
• Cipher suites (TLS 1.3 over DTLS)
  • TLS_AES_128_GCM_SHA256
  • TLS_AES_256_GCM_SHA384
  • TLS_CHACHA20_POLY1305_SHA256
• AEAD: AES‑GCM 128/256, ChaCha20‑Poly1305 (no CBC/EtM modes).
• Key exchange: ECDHE (P‑256/P‑384), X25519
• 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 (DTLS 1.2).

Certificate model

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.).

Sans‑IO integration model

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
• CloseNotify: peer sent a close_notify alert (graceful shutdown)

Error handling

Every Error returned by the public API (handle_packet, handle_timeout, send_application_data, and close) is fatal: the connection is no longer usable and must be thrown away. The engine has no recoverable error states, so the correct response is always to drop the Dtls instance — and start a fresh handshake if you still need a connection.

Transient, non‑fatal conditions inherent to running over an unreliable transport — malformed datagrams, replayed or out‑of‑window records, and other parser noise — are handled internally and never surface as an Error. Such packets are discarded (logged at debug!) while the connection keeps running. You therefore never need to distinguish "retry" from "give up": a returned Error always means give up on this connection.

Example (Sans‑IO loop)

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
                }
                Output::CloseNotify => {
                    // Peer initiated graceful shutdown — leave the event loop
                    return Ok(());
                }
                _ => {}
            }
        }

        // 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_12(cfg, cert, Instant::now());
    dtls.set_active(true); // client role
    dtls
}

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

Example (PSK client)

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

use dimpl::{Config, Dtls, PskResolver};

struct MyPsk;

impl PskResolver for MyPsk {
    fn resolve(&self, identity: &[u8]) -> Option<Vec<u8>> {
        if identity == b"device-01" {
            Some(b"shared-secret-key".to_vec())
        } else {
            None
        }
    }
}

let config = Arc::new(
    Config::builder()
        .with_psk_client(b"device-01".to_vec(), Arc::new(MyPsk))
        .build()
        .unwrap(),
);

let mut dtls = Dtls::new_12_psk(config, Instant::now());
dtls.set_active(true); // client role

MSRV

Rust 1.85.0

Status

• Session resumption is not implemented (WebRTC does a full handshake on ICE restart).
• Renegotiation is not implemented (WebRTC does full restart).

License: MIT OR Apache-2.0