alkssh/README.md

# alkssh

SSH adapter for the alk family: a thin wrapper around
[russh](https://docs.rs/russh) terminating the vanilla-ssh wire and
bridging to/from alkcall channels, the SSH counterpart of the
alkhttp adapter (`family` ADR-003). Vanilla clients terminate vanilla
ssh; family protocol halves (alktty, alktunnels, alkgit) receive their
own chunk planes; the door itself speaks no family protocol and owns
no deployment policy.

The crate is a **door, not a backend**: no binary, no front doors; a
downstream app (alkforge) embeds it, enables the features it wants,
and brings identity/ACL configuration and the family fabric handles.
It is two-directional: a server mount serves vanilla ssh into the
family, and a dial points family ops at a vanilla `sshd`.

## The fail-closed feature graph

`default = []` by design: **a default build has no capabilities and
can serve nothing but clean declines.** Every channel-type mapping is
a cargo feature; each feature is flat and independent (no feature
implies another):

| Feature | Maps | Family protocol |
|---|---|---|
| `session` | pty / shell / exec (pipe) rows → chunk-framed session | alktty |
| `git` | the exec-only git rows (`git-{upload,receive}-pack`) | alkgit |
| `tunnel` | `direct-tcpip` / `direct-streamlocal`, the `-R` legs | alktunnels |
| `sftp` | reserved decline placeholder (OQ-13), always declines until the alkfs mapping lands | n/a |

Non-mapped surface (x11, `auth-agent`, the `env` request, git *subsystem*
requests, `sftp` until alkfs) declines fail-closed in every build:
never a half-capability, never a silent fallback, and never advertised;
an unfeatured channel type is declined, not advertised. A request
whose config fields are unset declines too (an unset seam or shell
argv configures nothing; the feature alone is not a capability).

## Usage

The assembly injects the seams: an alkcall
`IdentityProvider` (verified fingerprint → identity), key sources, and
the per-feature family handles. The examples below are the crate's
compile-checked doctests (see the crate docs in `src/lib.rs`).

A server mount (per accepted TCP connection, run the door):

```rust,no_run
use std::sync::Arc;

use async_trait::async_trait;

struct Identity;

impl alkcall::core::auth::IdentityProvider for Identity {
    fn resolve_from_fingerprint(&self, _: &str) -> Option<alkcall::core::auth::Identity> {
        None // the assembly's registry decides
    }

    fn resolve_from_token(&self, _: &alkcall::core::auth::AuthToken) -> Option<alkcall::core::auth::Identity> {
        None
    }
}

struct HostKeys;

#[async_trait]
impl alkssh::HostKeySource for HostKeys {
    async fn load(&self) -> Result<Vec<russh::keys::PrivateKey>, alkssh::KeySourceError> {
        Err(alkssh::KeySourceError::Absent) // sketch; vault-backed in prod
    }
}

let serve = alkssh::ServeConfig::new(Arc::new(Identity), Arc::new(HostKeys));
let adapter = alkssh::SshAdapter::new(serve).await?;

// per accepted TCP connection from the assembly's listener:
// adapter.handle(stream).await?;
```

A dial, family ops pointed upstream at a vanilla server:

```rust,no_run
use std::sync::Arc;

use async_trait::async_trait;

struct DialKey;

#[async_trait]
impl alkssh::KeySource for DialKey {
    async fn load(&self) -> Result<russh::keys::PrivateKey, alkssh::KeySourceError> {
        Err(alkssh::KeySourceError::Absent) // sketch; vault-backed in prod
    }
}

let conn = alkssh::dial(alkssh::DialConfig::new(
    "hub.example:22",
    "runner",
    Arc::new(DialKey),
    Arc::new(alkssh::RejectAll),
))
.await?;

// op rows (under their features): conn.exec, conn.tunnel_direct,
// conn.git_service, conn.reverse_listen; see the crate docs.
conn.close().await; // clean disconnect; drop is never the clean path
```

Minimal builds compile and doctest with no features enabled:

```console
$ cargo build
$ cargo test
```

Every capability row additionally rides its feature build (the
matrix: no features / `git` / `session` / `tunnel` / `sftp` /
`--all-features`).

## Key custody and security posture

- **Keys are sources, never static material** (alknet-main's `main`
  key model, ADR-005): the dial/host key seams are
  decrypt-at-use handles over an injected
  [`alkvault`](https://docs.rs/alkvault) service; keys are held
  encrypted at rest, decrypted at use, zeroized through the seam.
  The vault is a secret store on this path, not a key generator.
- **No env vars, no config-file key material, no secrets on the
  wire.** The standing family posture (family ADR-002 decision 5).
- **Publickey-only authentication**, exactly one attempt; no
  password, keyboard-interactive, cert, or fallback methods exist on
  the surface.
- **Host-key trust is assembly policy**: the crate ships a
  `HostKeyPolicy` seam with a fail-closed default (`RejectAll`; a
  dial with it connects to nothing) and **no accept-all built-in**;
  any permissive policy is the assembly's to write.
- **Injected identity**: every wire-exposed op is checked against
  alkcall's `AccessControl::check(peer_identity)`; internal ops are
  never wire-callable; there is no unauthenticated capability.
- **Bounded resources everywhere** (`Limits`: wall clock always
  set, channel caps, listener caps, window/packet budgets, auth
  attempt limit): the crate never constructs an unbounded session.
  Known, recorded limits ride the published docs (the crate-root
  rustdoc §Known limits).
- **Every disabled feature declines**: the decline matrix is contract
  territory tested in every build.

## Documentation

- Crate docs: `cargo doc --open` (the crate-root docs carry the
  feature graph and the known-limits records).
- `docs/architecture/`: the authoritative contract: the spec
  (`overview.md`, `mapping.md`, `client.md`, `auth.md`), ADRs
  001–006, and the open-questions tracker.

## License

MIT OR Apache-2.0; see `LICENSE-MIT` / `LICENSE-APACHE`.