alkssh/tasks/implementation/w1-host-key-policy.md

---
id: w1-host-key-policy
name: Wave 1 — the HostKeyPolicy seam (OQ-05, policy-blind)
status: completed
depends_on: [w1-manifest]
scope: narrow
risk: low
impact: project
level: implementation
---

## Description

Land the client-side host-key policy seam (`auth.md` §Host-key policy,
OQ-05 resolution) — the trust-decision contract the dial surface calls:

- **The `HostKeyPolicy` trait** — the crate surfaces the presented host
  key (host, port, fingerprint) to an assembly-provided policy that
  decides accept/reject. The two OQ-05 cases are the *implementor's*:
  case-1 pins (known beforehand — hard-decline on mismatch) and case-2
  policies (unknown beforehand — prompt-and-record, learn-and-record,
  provider-metadata match, accept-and-continue for ephemeral fleets).
  The crate is **policy-blind**: it never decides trust, never
  hard-codes a trust model.
- **The crate default is fail-closed**: reject unknown (russh's own
  reject-all default, kept as the built-in default policy). **No
  accept-all built-in crate option** — a permissive case-2 policy is
  the assembly's to write.
- The trait receives the fingerprint string (the wave-1 fingerprint
  computation is the caller's input path, wave-4 dial); the exact
  decision type (bool vs a richer verdict carrying a reason) is the
  task's to pin against the russh `check_server_key` hook shape at
  this pin — flagged for the wave-1 gate.
- The russh client-handler consumption (`check_server_key` wiring) is
  wave-4 dial-surface material — this task lands the seam and its
  default only.

## Acceptance Criteria

- [x] `HostKeyPolicy` trait in place; the presented-key context
      (host, port, fingerprint) is the decision input
- [x] Built-in default policy rejects unknown keys (fail-closed);
      no accept-all option exists in the crate
- [x] Doc comments carry the two-case model and the policy-blind rule
- [x] Unit tests: default policy rejects; a custom policy's verdict
      is honored
- [x] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
      `cargo fmt --check` pass

## References

- docs/architecture/auth.md §Host-key policy (OQ-05 resolution)
- docs/architecture/client.md §The dial surface, §What the crate does
  not include (trust decisions)
- docs/architecture/decisions/005-at-rest-key-custody.md (decision 6 —
  the client host-*pin* is public config, never vault material)
- docs/research/auth-bridge.md §Host keys and `check_server_key`

## Notes

> Landed `src/host_key.rs` (`HostKeyPolicy`, `PresentedHostKey`,
> `RejectAll`) + lib.rs re-exports. Design decisions pinned against
> the russh 0.65 hook shape (`Handler::check_server_key ->
> Result<bool, Self::Error>`, async via `impl Future`, default
> `Ok(false)` — verified in the russh 0.65.0 source):
>
> - **The verdict type is `bool`** (the flagged pin): the hook's bool
>   is the trust decision and its `Err` channel is the handler's
>   wire/IO-failure territory — a trust decline has no typed reason
>   channel on this surface. A decline is a disposition, not an
>   error (`src/error.rs` contract); decline reasons are the
>   policy's own record (case-2 policies log their decisions), not a
>   crate surface. Wave-4 wiring maps `false` → russh's hard,
>   clean `Error::UnknownKey` decline (mismatch = hard decline,
>   per auth.md).
> - **The trait is async and dyn-compatible**: `#[async_trait]`,
>   `decide(&self, &PresentedHostKey) -> bool`, `Send + Sync +
>   'static` — the family injected-handle pattern (`Arc<dyn
>   HostKeyPolicy>`, the alkcall `IdentityProvider`/`OperationEnv`
>   precedent), which w1-config's dial-config handle and
>   w1-test-doubles' inspectable double both require. State-holding
>   (learn-and-record) policies record by interior mutability
>   behind the shared `&self` handle.
> - **New dep `async-trait = "0.1"`** (MIT OR Apache-2.0; the only
>   way to get a dyn-compatible async trait at this toolchain —
>   native AFIT is not object-safe, and russh 0.65 removed its own
>   `async-trait` feature, per ADR-002's context). Precedented and
>   deduped: alkcall already carries `async-trait 0.1`, so the tree
>   adds no new transitive package. **Flagged for the wave-1 gate**
>   (manifest addition outside w1-manifest, same class as the
>   verdict-type pin).
> - **`RejectAll` is the built-in default** (russh's reject-all
>   default, kept); doc comments carry the two-case model, the
>   policy-blind rule, the no-accept-all rule, the fail-closed
>   disposition (an undecided policy declines), and the host-pin-is-
>   public-config note (ADR-005 decision 6).
> - The `PresentedHostKey.fingerprint` doc pins exact string
>   equality against assembly pins; the string is the single
>   fingerprint path's format, computed at wave-4 wiring.

## Summary

> Seam landed: `src/host_key.rs` with the `HostKeyPolicy` trait
> (`async fn decide(&self, &PresentedHostKey) -> bool`,
> dyn-compatible via `#[async_trait]`), the `PresentedHostKey`
> (host/port/fingerprint) decision input, and the fail-closed
> `RejectAll` built-in default; no accept-all option exists in the
> crate. Verdict type pinned to russh 0.65's `check_server_key`
> shape (bool = the trust decision; decline reasons live with the
> policy, not the crate). `async-trait = "0.1"` added for the
> dyn-compatible async handle (alkcall-precedented, tree-deduped).
> Tests: default rejects; custom verdict honored; both honored
> through an `Arc<dyn Handle>`; the whole presented context reaches
> the policy. Verified: `cargo test` (14), `cargo test
> --all-features`, `cargo clippy --all-targets -- -D warnings`,
> `cargo fmt --check` all pass. `taskgraph validate` clean.