# dimpl

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.

## Goals
- **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.

### Non‑goals
- **DTLS 1.0**
- **Async** (the crate is Sans‑IO and event‑loop agnostic)
- **no_std** (at least not without allocation)
- **RSA**
- **DHE**

### Regarding DTLS 1.3 and the future of this crate

dimpl was built as a support package for [str0m](https://github.com/algesten/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.

## Cryptography surface
- **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.

### 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

## 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(
        cfg,
        cert.certificate,
        cert.private_key,
    );
    dtls.set_active(true); // client role
    dtls
}

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

#### MSRV
Rust 1.81.0

#### Status
- 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.

[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

[`Dtls::handle_packet`]: https://docs.rs/dimpl/0.1.0/dimpl/struct.Dtls.html#method.handle_packet
[`Dtls::poll_output`]: https://docs.rs/dimpl/0.1.0/dimpl/struct.Dtls.html#method.poll_output
[`Dtls::handle_timeout`]: https://docs.rs/dimpl/0.1.0/dimpl/struct.Dtls.html#method.handle_timeout
[`Output`]: https://docs.rs/dimpl/0.1.0/dimpl/enum.Output.html
[`Output::PeerCert`]: https://docs.rs/dimpl/0.1.0/dimpl/enum.Output.html#variant.PeerCert


License: MIT OR Apache-2.0
