# 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
Three constructors control which DTLS version is used:
- [`Dtls::new_12`][new_12] — explicit DTLS 1.2
- [`Dtls::new_13`][new_13] — explicit DTLS 1.3
- [`Dtls::new_auto`][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`
- **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).
- Not supported: PSK cipher suites.
### Certificate model
During the handshake the engine emits
[`Output::PeerCert`][peer_cert] 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`][handle_packet] — feed an entire
received UDP datagram.
- [`Dtls::poll_output`][poll_output] — drain pending output:
DTLS records, timers, events.
- [`Dtls::handle_timeout`][handle_timeout] — trigger
retransmissions/time‑based progress.
The output is an [`Output`][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
## Example (Sans‑IO loop)
```rust
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_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);
```
#### 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).
[new_12]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.new_12
[new_13]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.new_13
[new_auto]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.new_auto
[peer_cert]: https://docs.rs/dimpl/latest/dimpl/enum.Output.html#variant.PeerCert
[handle_packet]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.handle_packet
[poll_output]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.poll_output
[handle_timeout]: https://docs.rs/dimpl/latest/dimpl/struct.Dtls.html#method.handle_timeout
[output]: https://docs.rs/dimpl/latest/dimpl/enum.Output.html
[RFC 5764]: https://www.rfc-editor.org/rfc/rfc5764
[RFC 7714]: https://www.rfc-editor.org/rfc/rfc7714
[RFC 7627]: https://www.rfc-editor.org/rfc/rfc7627
License: MIT OR Apache-2.0