docs: document UV update wiring and add license, msrv, cable rows (#294)

Documents the PIN and user-verification update channel that every consumer must wire to complete a ceremony, which had no documentation before. Adds a license section, an MSRV note, a crate-level overview for the docs landing page, and the missing cloud-assisted hybrid transport row, and removes a stale setup step. Documentation only.

Author: AlfioEmanueleFresta

README.md

@@ -51,22 +51,25 @@ Validating the relying party ID against the calling origin requires the [Public
 | ---------------------------- | --------------------- | --------------------- |
 | **USB (HID)**                | 🟢 Supported (hidapi) | 🟢 Supported (hidapi) |
 | **Bluetooth Low Energy**     | 🟢 Supported (bluez)  | 🟢 Supported (bluez)  |
-| **NFC**                      | 🟢 Supported (pcsc or libnfc) | 🟢 Supported (pcsc or libnfc) |
+| **NFC** [^nfc-optin]         | 🟢 Supported (pcsc or libnfc) | 🟢 Supported (pcsc or libnfc) |
 | **TPM 2.0 (Platform)**       | 🟠 Planned ([#4][#4]) | 🟠 Planned ([#4][#4]) |
+| **Hybrid (caBLE v2, cloud-assisted)** | N/A          | 🟢 Supported          |
 | **CTAP 2.3 hybrid (QR-initiated, BLE only)**    | N/A                   | 🟢 Supported          |
 
+USB, BLE, and the two hybrid transports build with the default features. NFC is
+opt-in: the crate ships with `default = []` for the NFC stack, so enable the
+`nfc-backend-pcsc` feature (pure userspace, recommended) or `nfc-backend-libnfc`
+(requires the `libnfc` system library) to compile it in.
+
+[^nfc-optin]: Off by default. Enable `nfc-backend-pcsc` and/or `nfc-backend-libnfc`.
+
 ## Example programs
 
 Examples live in [`libwebauthn/examples/`](libwebauthn/examples) and are grouped by purpose:
 `ceremony/` for register and authenticate flows, `features/` for per-feature demos
 (extensions, preflight, PRF, device selection), and `management/` for CTAP2 admin
 operations. All examples share helpers from `examples/common/`.
 
-```
-$ cd libwebauthn
-$ git submodule update --init
-```
-
 The basic ceremony examples (register + authenticate) cover all transports. The
 WebAuthn examples consume and emit JSON per the [WebAuthn IDL][webauthn].
 
@@ -103,6 +106,17 @@ $ cargo run --example cred_management_hid
 $ cargo run --example persistent_cred_management_hid
 ```
 
+## Minimum Supported Rust Version (MSRV)
+
+libwebauthn uses Rust edition 2021 and tracks recent stable Rust. CI builds and
+tests on the current stable toolchain. There is no separate older MSRV floor: if
+a recent stable `rustc` builds the crate, it is supported.
+
+## License
+
+Licensed under the GNU Lesser General Public License v2.1 or later
+(`LGPL-2.1-or-later`). See [COPYING](COPYING) for the full text.
+
 ## Contributing
 
 We welcome contributions!

libwebauthn/src/lib.rs

@@ -36,8 +36,9 @@
 //!     SystemPublicSuffixList,
 //! };
 //! use libwebauthn::transport::hid::list_devices;
-//! use libwebauthn::transport::{ChannelSettings, Device};
+//! use libwebauthn::transport::{Channel, ChannelSettings, Device};
 //! use libwebauthn::webauthn::WebAuthn;
+//! use libwebauthn::UvUpdate;
 //!
 //! # async fn run() -> Result<(), Box<dyn std::error::Error>> {
 //! // 1. Enumerate authenticators on your transport of choice (HID shown here).
@@ -47,7 +48,22 @@
 //!     // 2. Open a channel to the device.
 //!     let mut channel = device.channel(ChannelSettings::default()).await?;
 //!
-//!     // 3. Build a request from its WebAuthn IDL JSON.
+//!     // 3. Drive user-verification updates on a separate task: each update
+//!     //    carries the means to answer it. See `examples/ceremony/`.
+//!     let mut updates = channel.get_ux_update_receiver();
+//!     tokio::spawn(async move {
+//!         while let Ok(update) = updates.recv().await {
+//!             match update {
+//!                 UvUpdate::PresenceRequired => println!("Touch your authenticator"),
+//!                 UvUpdate::PinRequired(request) => {
+//!                     let _ = request.send_pin("the user's PIN");
+//!                 }
+//!                 _ => {}
+//!             }
+//!         }
+//!     });
+//!
+//!     // 4. Build a request from its WebAuthn IDL JSON.
 //!     let origin: RequestOrigin = "https://example.org".try_into().expect("invalid origin");
 //!     let psl = SystemPublicSuffixList::auto().expect("public suffix list unavailable");
 //!     let settings = RequestSettings {
@@ -60,7 +76,7 @@
 //!     let request =
 //!         MakeCredentialRequest::prepare(&origin, request_json, &settings).await?;
 //!
-//!     // 4. Run the ceremony on the channel.
+//!     // 5. Run the ceremony on the channel.
 //!     let _response = channel.webauthn_make_credential(&request).await?;
 //! }
 //! # Ok(())

libwebauthn/src/transport/channel.rs

@@ -44,12 +44,15 @@ pub trait Channel: Send + Sync + Display + Ctap2AuthTokenStore {
     /// UX updates for this channel, must include UV updates.
     type UxUpdate: Send + Sync + Debug + From<UvUpdate>;
 
+    /// Broadcast sender fanning UX updates out to subscribed receivers.
     fn get_ux_update_sender(&self) -> &broadcast::Sender<Self::UxUpdate>;
 
+    /// Subscribe to this channel's UX updates; drive the receiver on a separate task so the ceremony can make progress.
     fn get_ux_update_receiver(&self) -> broadcast::Receiver<Self::UxUpdate> {
         self.get_ux_update_sender().subscribe()
     }
 
+    /// Broadcast a UX update to all current receivers.
     #[instrument(skip(self))]
     async fn send_ux_update(&mut self, state: Self::UxUpdate) {
         trace!("Sending UX update");