alknet/docs/architecture/decisions/004-auth-as-shared-core.md

ADR-004: Auth as Shared Core (IdentityProvider)

Status

Accepted

Context

The previous architecture had authentication spread across multiple layers: CredentialProvider with four phases (A–D), AuthProtocol as an irpc service, server_auth and client_auth as separate modules, and IdentityProvider as a trait in alknet-core. Different interface types presented credentials differently — SSH used key fingerprints, HTTP used Bearer tokens, DNS used query labels — but the resolution was ad-hoc and tied to the three-layer model.

The ALPN dispatch model simplifies this: every handler receives the same AuthContext, but the credential extraction (how a handler learns who the peer is) differs per ALPN. The resolution (turning a credential into an Identity) should be shared across all handlers.

Decision

Authentication and identity resolution live in alknet-core as shared infrastructure. Each handler presents credentials differently, but all resolve through the same IdentityProvider:

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

Credential presentation per handler:

Handler	Credential presentation	Resolves via
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()
WebTransportAdapter	AuthToken in CONNECT headers	resolve_from_token()
GitAdapter	Signed push certificate	resolve_from_fingerprint()

Auth resolution is hybrid — the endpoint resolves what it can, and handlers resolve what they must:

1. Endpoint-level resolution (before handle() is called): If the TLS handshake provides a client certificate, the endpoint resolves the fingerprint to an Identity and passes it in AuthContext. This is the case for SSH (where the key exchange happens at the protocol level, but the TLS layer may also provide information).

2. Handler-level resolution (inside handle()): For protocols that carry credentials in application frames (AuthToken in the first call frame, Bearer header in HTTP), the handler extracts the credential from the stream and calls IdentityProvider to resolve it. The handler then enriches or replaces the partial AuthContext with the fully resolved Identity.

The AuthContext passed to handle() may be partial — containing only transport-level information if no TLS client certificate was provided. Handlers must not assume AuthContext contains a fully resolved Identity. Each handler knows its own credential extraction protocol and is responsible for completing authentication.

The CredentialProvider concept from the previous architecture is simplified: there is no phase progression (A–D). The IdentityProvider has two resolution paths — fingerprint and token — and a ConfigIdentityProvider implementation that draws from static and dynamic config.

alknet-secret remains independent. It does not depend on alknet-core or IdentityProvider. The secret service provides derived keys on request; identity resolution is a separate concern.

Consequences

Positive:

• Unified identity model — every handler resolves identities the same way through IdentityProvider
• Handlers own their credential extraction — SSH reads key fingerprints, call reads AuthTokens, HTTP reads Bearer headers
• Endpoint provides what it can for free (TLS-level auth), handlers complete what they need
• Adding a new credential type is adding a method to IdentityProvider, not a new phase
• alknet-secret stays standalone — no coupling between key derivation and identity resolution
• AuthContext is a value type — easy to construct in tests, can be partial for handler-level testing

Negative:

• IdentityProvider is in alknet-core — any change to it recompiles all handlers (mitigated: the trait should be stable; implementation changes don't force recompiles)
• Two resolution paths (fingerprint, token) may not cover all future auth schemes (mitigated: the trait can be extended, or a handler can do custom resolution after the initial AuthContext)
• Handlers must handle partial AuthContext — the endpoint may not have resolved an Identity, so handlers must be prepared to do credential extraction themselves
• WebTransport and browser-based auth needs careful design — AuthToken in CONNECT headers requires the token to be available before the stream is established

References

• Pivot proposal: docs/research/pivot/alpn-service-architecture.md
• ADR-002: ProtocolHandler trait
• ADR-003: Crate decomposition
• ADR-005: irpc as call protocol foundation
• The previous architecture had equivalent decisions in ADR-023 (unified auth) and ADR-029 (identity as core type), which are archived in the reference implementation at /workspace/@alkdev/alknet-main/.