alknet/docs/architecture/crates/core/auth.md

status, last_updated

status	last_updated
draft	2026-06-21

Authentication

AuthContext, Identity, IdentityProvider, AuthToken, and the resolution flow.

See ADR-004 and ADR-011 for rationale.

AuthContext

Created by the endpoint for each incoming connection. Passed to ProtocolHandler::handle() as an immutable reference.

#[derive(Clone)]
pub struct AuthContext {
    /// The peer's authenticated identity, if resolved by the endpoint.
    /// None means the endpoint has no identity information for this connection.
    pub identity: Option<Identity>,

    /// The negotiated ALPN for this connection. Always present.
    pub alpn: Vec<u8>,

    /// The peer's remote address, if available. Informational (NAT/proxy).
    pub remote_addr: Option<SocketAddr>,

    /// SHA-256 fingerprint of the TLS client certificate, if presented.
    /// Set by the endpoint during TLS handshake. Handlers may use this for
    /// fingerprint-based auth even when IdentityProvider returns None.
    pub tls_client_fingerprint: Option<String>,
}

Construction by the endpoint

The endpoint constructs AuthContext from the QUIC connection:

1. alpn: From connection.alpn() — always present after TLS handshake.
2. remote_addr: From connection.remote_addr() — may be None for iroh connections.
3. tls_client_fingerprint: Extracted from the TLS session's client certificate, if one was presented.
4. identity: If a TLS client fingerprint is available, the endpoint calls IdentityProvider::resolve_from_fingerprint(). If it resolves, identity = Some(resolved). If not, identity = None.

Handler-level resolution

Handlers that require authentication extract protocol-specific credentials and call IdentityProvider inside handle(). When identity is resolved, the handler stores it on the Connection for observability:

// Example: CallAdapter extracting an AuthToken from the first frame
async fn handle(&self, connection: Connection, auth: &AuthContext) -> Result<(), HandlerError> {
    let identity = match &auth.identity {
        Some(id) => id.clone(),  // Endpoint already resolved identity
        None => {
            let stream = connection.accept_bi().await?;
            let token = extract_auth_token(stream).await?;
            self.identity_provider
                .resolve_from_token(&token)
                .ok_or(HandlerError::AuthRequired)?
        }
    };
    connection.set_identity(identity);  // Store for observability (OQ-11)
    // ... proceed with authenticated identity
}

Handlers that don't require authentication (e.g., DNS resolver, health check) can ignore auth.identity entirely and don't call set_identity.

Two Identity Scopes

There are two distinct identity scopes that must not be conflated:

Scope	Where it's set	Where it's stored	What it represents	Used for
Connection-level	Handler in handle()	Connection (via set_identity)	Who opened this QUIC connection	Observability, logging, audit
Per-request	CallAdapter per call.requested	OperationContext.identity	Who is making this specific call	ACL (ADR-015)

The connection-level identity is stable — set once when the handler resolves it. The per-request identity is dynamic — resolved per call.requested, potentially different across requests on the same connection (if different auth tokens are used). The per-request identity takes precedence for ACL on OperationContext; the connection-level identity is for observability only, not for ACL.

Connection exposes set_identity via interior mutability — the handler sets it once when resolved, the endpoint and observability layers read it. The identity is write-once-read-many.

AuthContext is Clone and immutable

• derive(Clone) allows handlers to clone AuthContext for per-stream or per-channel contexts.
• handle() receives &AuthContext — immutable. Handlers that resolve identity create local variables, they don't mutate the shared context. This prevents cross-contamination between streams on the same connection.

Identity

The authenticated peer identity. Carries authorization information.

#[derive(Debug, Clone, PartialEq)]
pub struct Identity {
    /// Unique identifier string. Fingerprint, key prefix, or principal name.
    pub id: String,

    /// Authorization scopes. e.g., ["relay:connect", "secrets:derive"]
    pub scopes: Vec<String>,

    /// Named resource lists. e.g., {"service": ["gitea", "registry"]}
    pub resources: HashMap<String, Vec<String>>,
}

This is the same structure as the reference implementation (alknet-main/crates/alknet-core/src/auth/identity.rs), minus the russh dependency. The id field is ALPN-agnostic:

• SSH key auth: "SHA256:abc123..." (key fingerprint)
• API key auth: "alk_test" (key prefix)
• Certificate auth: "username" (principal name)

AuthToken

Opaque authentication token carried in protocol frames.

#[derive(Debug, Clone)]
pub struct AuthToken {
    pub raw: Vec<u8>,
}

Unchanged from the reference implementation. The handler that extracted it knows its encoding (UTF-8 string, binary token, etc.).

IdentityProvider

Trait for resolving credentials to identities. Implemented by ConfigIdentityProvider.

pub trait IdentityProvider: Send + Sync + 'static {
    fn resolve_from_fingerprint(&self, fingerprint: &str) -> Option<Identity>;
    fn resolve_from_token(&self, token: &AuthToken) -> Option<Identity>;
}

• resolve_from_fingerprint(): Used by the endpoint (TLS client cert) and by SSH (key fingerprint).
• resolve_from_token(): Used by call protocol (AuthToken in first frame) and HTTP (Bearer header).

Both methods return Option<Identity> — None means the credential is not recognized.

ConfigIdentityProvider

The default implementation. Resolves identities from DynamicConfig:

pub struct ConfigIdentityProvider {
    dynamic: Arc<ArcSwap<DynamicConfig>>,
}

The "Config" prefix indicates that identities are resolved from configuration (as opposed to a database or external service). This reads from ArcSwap<DynamicConfig>, which is hot-reloadable — not from StaticConfig. An alternative name would be DynamicConfigIdentityProvider to make this clearer, but ConfigIdentityProvider is consistent with the reference implementation and the naming is unlikely to cause confusion in practice.

How it resolves:

• Fingerprint: Look up in DynamicConfig::auth::authorized_keys_fingerprints. If found, return Identity { id: fingerprint, scopes: ["relay:connect"], resources: {} }.
• Token: Parse as UTF-8. If it starts with alk_, look up in DynamicConfig::auth::api_keys by prefix match + SHA-256 hash. If found and not expired, return Identity { id: prefix, scopes: entry.scopes, resources: entry.resources }.

Changes to DynamicConfig via ConfigReloadHandle are reflected immediately — ConfigIdentityProvider reads from ArcSwap on every call.

Resolution Flow

Endpoint-level (before handle())

QUIC connection arrives
  → TLS handshake (ALPN negotiation)
  → Extract TLS client certificate fingerprint (if presented)
  → If fingerprint present: IdentityProvider::resolve_from_fingerprint()
    → Some(identity): auth.identity = Some(identity)
    → None: auth.identity = None
  → Construct AuthContext { identity, alpn, remote_addr, tls_client_fingerprint }
  → Look up handler by alpn
  → tokio::spawn(handler.handle(connection, &auth))

Handler-level (inside handle())

Handler receives &AuthContext
  → If auth.identity is Some: use it (endpoint already resolved)
  → If auth.identity is None and handler requires auth:
    → Extract protocol-specific credential (AuthToken, SSH key, etc.)
    → Call IdentityProvider::resolve_from_token() or resolve_from_fingerprint()
    → If resolved: use the Identity
    → If not resolved: return HandlerError::AuthRequired
  → If handler doesn't require auth: proceed without identity

IdentityProvider Injection

Handlers need access to IdentityProvider to resolve credentials inside handle(). Since ProtocolHandler::handle() doesn't receive an IdentityProvider parameter, each handler must obtain it through constructor injection:

// Example: SshAdapter holds an Arc<dyn IdentityProvider>
pub struct SshAdapter {
    identity_provider: Arc<dyn IdentityProvider>,
    // ... other handler-specific state
}

#[async_trait]
impl ProtocolHandler for SshAdapter {
    fn alpn(&self) -> &'static [u8] { b"alknet/ssh" }

    async fn handle(&self, connection: Connection, auth: &AuthContext) -> Result<(), HandlerError> {
        let identity = match &auth.identity {
            Some(id) => id.clone(),
            None => {
                // Extract SSH key fingerprint, resolve via identity_provider
                let fingerprint = extract_ssh_fingerprint(&connection).await?;
                self.identity_provider
                    .resolve_from_fingerprint(&fingerprint)
                    .ok_or(HandlerError::AuthRequired)?
            }
        };
        // ...
    }
}

The CLI binary constructs each handler with Arc::clone(&identity_provider) and passes it when building the HandlerRegistry. This is the assembly pattern: the CLI (the only crate that depends on all handlers) wires dependencies together.

The endpoint's AlknetEndpoint also holds Arc<dyn IdentityProvider> for endpoint-level auth resolution (TLS client certificate fingerprints), but handlers don't receive it from the endpoint — they receive it at construction time from the CLI.

Handler	Credential source	Resolution method
SshAdapter	SSH public key handshake	resolve_from_fingerprint()
CallAdapter	AuthToken in first frame	resolve_from_token()
HttpAdapter	Authorization: Bearer header	resolve_from_token()
DnsAdapter	AuthToken in query labels	resolve_from_token()
GitAdapter	Signed push certificate	resolve_from_fingerprint()
SftpAdapter	SSH key (shares with SshAdapter)	resolve_from_fingerprint()

Key Differences from Reference Implementation

Aspect	Reference	New Model
Auth resolution	Inside SSH handler, before handle()	Hybrid: endpoint resolves TLS-level, handler resolves protocol-level
AuthContext type	None (just Arc<ArcSwap<DynamicConfig>> + IdentityProvider)	Explicit struct with optional fields
Identity.id	Always a fingerprint or API key prefix	Same, but ALPN-agnostic documentation
ConfigIdentityProvider	Depends on russh for PublicKey types	No russh dependency; fingerprints stored as strings
Credential phases	A–D phases in CredentialProvider	Two paths: fingerprint and token. No phases.

Design Decisions

Decision	ADR	Summary
Hybrid auth model	ADR-004	Endpoint resolves TLS-level, handler resolves protocol-level
AuthContext with optional Identity	ADR-011	Explicit None, not "partially authenticated"
AuthContext is immutable in handle()	ADR-011	Handlers create local variables for resolved identity
Two resolution paths	ADR-004	Fingerprint and token, not phased auth
Handler stores resolved identity on Connection	OQ-11 (resolved)	connection.set_identity() — write-once-read-many for observability

Open Questions

None. All auth-related open questions are resolved.