docs(adr-027): TLS identity redesign — ACME + RawKey decoupling

ADR-027 resolves the architectural gap surfaced when ACME integration
became a concrete target:

1. TlsIdentity::Acme variant — static config data (domains, cache_dir,
   directory, contact) with async AcmeState constructed at endpoint
   setup via two-phase TlsSetup (not stuffed into the Clone-able enum).

2. TlsIdentity::RawKey decoupled from the iroh feature — uses
   Ed25519SecretKey (alknet-core-owned wrapper over ed25519_dalek)
   instead of iroh::SecretKey. Raw-key TLS identity (RFC 7250, the
   default for most alknet nodes) now works in quinn-only builds.
   iroh transport converts via SecretKey::from_bytes.

3. ACME feature-gated behind new acme feature (rustls-acme optional
   dep). Non-ACME builds don't compile it.

4. dispatch_quinn guard for acme-tls/1 challenge connections — TLS-ALPN-01
   is handled at the rustls cert resolver layer during the handshake;
   the guard closes challenge connections gracefully instead of logging
   a misleading "no handler" warning.

Research confirmed QUIC (quinn) handles ACME challenges differently than
TCP (reverse-proxy): quinn gives no ClientHello peek hook, but the
challenge is fully answered at the cert resolution step before the
connection surfaces to the application. No handler registration needed.

Spec updates: config.md, endpoint.md, open-questions.md (OQ-12),
overview.md + README.md (ADR index), ADR-010 (cross-ref).

Tasks: core/rawkey-decouple-from-iroh (gen 1, no deps),
core/acme-integration (gen 2, depends on rawkey). Graph: 36 tasks.
This commit is contained in:
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---
id: core/rawkey-decouple-from-iroh
name: Decouple TlsIdentity::RawKey from the iroh feature (ADR-027)
status: pending
depends_on: []
scope: narrow
risk: medium
impact: component
level: implementation
---
## Description
`TlsIdentity::RawKey(iroh::SecretKey)` is gated `#[cfg(feature = "iroh")]`
and the `RawKeyCertResolver` / `Ed25519SigningKey` rustls impls are gated
`#[cfg(all(feature = "quinn", feature = "iroh"))]`. This means quinn-only
builds (the default feature set) cannot use RFC 7250 raw-key identity —
the mode described as "default for most alknet nodes" (OQ-12, ADR-027).
The coupling is artificial: `iroh::SecretKey` is a thin newtype over
`ed25519_dalek::SigningKey`. The alknet code uses only `.public().as_bytes()`,
`.sign(msg)`, and `.clone()`. This task replaces `iroh::SecretKey` with an
alknet-core-owned `Ed25519SecretKey` wrapper, un-gates the raw-key TLS
path from the `iroh` feature, and updates the iroh transport to convert.
See ADR-027 for the full design rationale.
### Implementation steps
1. **Add `ed25519-dalek` as a direct dependency** of alknet-core in
`Cargo.toml`. It's already in the lockfile (transitive via iroh).
Version: `2.2` (match what's in `Cargo.lock`).
2. **Introduce `Ed25519SecretKey`** in `config.rs` (or a new
`tls.rs` module if config.rs is getting large):
```rust
#[derive(Clone)]
pub struct Ed25519SecretKey(ed25519_dalek::SigningKey);
impl Ed25519SecretKey {
pub fn generate() -> Self { ... }
pub fn from_bytes(bytes: &[u8; 32]) -> Self { ... }
pub fn as_bytes(&self) -> &[u8; 32] { ... }
pub fn public(&self) -> ed25519_dalek::VerifyingKey { ... }
}
```
Add `ZeroizeOnDrop` (the key is secret material). Add a redacting
`Debug` impl (like `Secret<T>` in types.rs). Do NOT derive `Debug` —
the raw key bytes must not be printed.
3. **Change `TlsIdentity::RawKey`** from `RawKey(iroh::SecretKey)` to
`RawKey(Ed25519SecretKey)`. Remove the `#[cfg(feature = "iroh")]` gate
— `RawKey` is available in all builds.
4. **Rewire `Ed25519SigningKey`** in `endpoint.rs`:
- Change the inner field from `iroh::SecretKey` to `Ed25519SecretKey`
(or `ed25519_dalek::SigningKey`).
- `spki_public_key()`: use `self.key.public().as_bytes()` (same logic,
different key type — `ed25519_dalek::VerifyingKey` has `as_bytes()`).
- `sign()`: use `self.key.sign(message)` → ed25519-dalek's
`SigningKey::sign` returns `Signature` which has `to_bytes()`.
- Change the cfg gate from `#[cfg(all(feature = "quinn", feature =
"iroh"))]` to `#[cfg(feature = "quinn")]` on `RawKeyCertResolver`,
`Ed25519SigningKey`, and all related impls.
5. **Update `build_iroh_endpoint`**: when `TlsIdentity::RawKey(key)` is
present, convert to `iroh::SecretKey::from_bytes(key.as_bytes())`
before passing to `iroh::Endpoint::builder().secret_key(...)`. This
conversion is `#[cfg(feature = "iroh")]` only.
6. **Update `build_rustls_server_config`**: the `RawKey` arm changes
from `#[cfg(feature = "iroh")]` to always-available (within the
`#[cfg(feature = "quinn")]` function). The `RawKeyCertResolver::new`
takes `&Ed25519SecretKey` instead of `&iroh::SecretKey`.
7. **Update all tests** that construct `TlsIdentity::RawKey`:
- `endpoint.rs` tests: `iroh::SecretKey::generate(&mut csprng)` →
`Ed25519SecretKey::generate()`.
- Any test in `config.rs` that constructs `RawKey`.
### What NOT to change
- `TlsIdentity::X509` and `SelfSigned` — untouched by this task.
- The `endpoint-request-client-cert` task (server config client auth) —
independent, can proceed in parallel or before/after this task.
- ACME — separate follow-up task (`core/acme-integration`).
## Acceptance Criteria
- [ ] `ed25519-dalek` is a direct dependency of alknet-core
- [ ] `Ed25519SecretKey` type exists with `generate`, `from_bytes`, `as_bytes`, `public`; redacting `Debug`; `ZeroizeOnDrop`
- [ ] `TlsIdentity::RawKey` uses `Ed25519SecretKey`, not `iroh::SecretKey`
- [ ] `TlsIdentity::RawKey` is not gated behind `#[cfg(feature = "iroh")]`
- [ ] `RawKeyCertResolver` and `Ed25519SigningKey` are gated `#[cfg(feature = "quinn")]` only (not `all(feature = "quinn", feature = "iroh")`)
- [ ] `build_iroh_endpoint` converts `Ed25519SecretKey` → `iroh::SecretKey::from_bytes`
- [ ] `cargo build -p alknet-core --features quinn` (no iroh) succeeds with `TlsIdentity::RawKey` usable
- [ ] `cargo build -p alknet-core --all-features` succeeds
- [ ] `cargo test -p alknet-core --all-features` succeeds
- [ ] `cargo test -p alknet-core --features quinn` succeeds (quinn-only, no iroh)
- [ ] `cargo clippy -p alknet-core --all-features --all-targets` clean
- [ ] `cargo clippy -p alknet-core --features quinn --all-targets` clean
## References
- ADR-027 — full design rationale
- crates/alknet-core/src/config.rs:33-41 — `TlsIdentity` enum
- crates/alknet-core/src/endpoint.rs:593-689 — `RawKeyCertResolver`, `Ed25519SigningKey`
- crates/alknet-core/src/endpoint.rs:511-538 — `build_iroh_endpoint` (conversion site)
- crates/alknet-core/src/endpoint.rs:484-495 — `build_rustls_server_config` RawKey arm
- /workspace/iroh/iroh-base/src/key.rs:261 — `iroh::SecretKey(SigningKey)` newtype
## Notes
> This is the foundation task for ADR-027. The ACME task
> (`core/acme-integration`) depends on this one because both modify
> `TlsIdentity` and `build_rustls_server_config`. Doing decoupling first
> means the ACME task builds on the cleaned-up enum without iroh coupling.