wg-app-link/README.md

# wg-app-link

The private link between a phone and a machine you run, extracted from the
two projects that had each written it.

`dev-updater` serves locally-built APKs to a phone. `ai-app` runs model
sessions for one. They have nothing in common above the waterline — and
underneath they are the same program: a server bound to a WireGuard
interface so it is not on the LAN, presenting a certificate from a CA the
app pins, answering only requests carrying a bearer token that was enrolled
by scanning a QR code off the terminal, and keeping its state in
owner-only files outside the repo.

That link was written twice. This is it written once.

> **Status: proposal with a working core.** Nothing has been removed from
> either project yet. The Rust modules here are real, tested and
> lint-clean; the rest of this file is the case for what should follow and
> what should not.

## The evidence

Measured, not estimated — `difflib` over the two working trees on
2026-08-28:

| file | dev-updater | ai-app | identical |
|---|---|---|---|
| `EnrollmentScanActivity.kt` | 32 lines | 32 lines | **98%** |
| `ServerConfig.kt` | 139 | 122 | **89%** |
| `auth.rs` | 250 | 230 | **83%** |
| `PinnedCert.kt` | 60 | 59 | **75%** |
| `certs.rs` | 218 | 199 | **70%** |
| RON `format` module | — | — | **byte-identical** |

The percentages understate it, because the differences are almost entirely
a product name and a type parameter:

- **`EnrollmentScanActivity.kt`** differs by its `package` line. That is all.
- **`PinnedCert.kt`** differs by `package` and one string, `"dev-updater-dev-ca"` against `"ai-app-dev-ca"`.
- **`certs.rs`** differs by the organisation name in the certificate, and by ai-app having factored the file-mode handling into a `private` module that dev-updater still has inline in two places.
- **`auth.rs`** differs by the state type the middleware is generic over (`AppState` against `SessionManager`), and by dev-updater keeping `print_enrollment` in `auth.rs` where ai-app keeps the same function in `main.rs`.
- **`wg_address` and `local_addresses`** are the same logic in both, in files that are otherwise 8% alike.

ai-app's own `Cargo.toml` already says the quiet part: RON is used there
because it is *"the same choice, and the same house rules, as the sibling
dev-updater project's config."*

## Why this is worth doing, in one observation

**The two copies have each drifted into holding an improvement the other
lacks.** Not hypothetically — as of the day this was written:

| improvement | in dev-updater | in ai-app |
|---|---|---|
| `private::` module owning every file mode | no, inline twice | yes |
| `SharedPreferences.edit { }` instead of the deprecated builder | no | yes |
| `existingTokenKey()` — reads the key without creating one, so a stored blob with no key means "not enrolled" rather than leaving a stray key behind | yes | no |
| `hasLocalNetworkPermission` — tells a denied permission apart from an unreachable server, which are identical at the socket | yes | no |
| Catppuccin theme with colour-by-consequence buttons | yes | no, ad-hoc `Color(0xFF…)` per screen |

Every one of those is a fix somebody made once, in one repo, that the other
will either never get or get by being written a third time. That is the
cost this repo removes, and it is already being paid.

## It found a bug in all three before shipping a line

`private` was lifted from ai-app because that copy was the better one. Two
sessions then read it as a module rather than as scattered helpers, and
found that `OpenOptions::mode` applies **only when a file is created** — so
opening one that already exists keeps whatever mode it had.

Both projects rewrite existing files holding private material. `certs.rs`
reissues the TLS leaf on every start, so a `leaf-key.pem` that had ever
existed world-readable would have stayed that way for the rest of its life,
with every start looking like it was setting the mode. The config's temp
file is the other: normally fresh, but one left by a crashed save is
reopened with its old mode and then renamed over the file holding the
enrolled token hashes.

It was latent in both — the modes on both machines were checked, and were
correct — but it was present three times: dev-updater, ai-app, and this
crate's first commit.

The reason nobody caught it is the argument for this repo, and it is not
about duplication. The correct reasoning was already written down, in a
comment, three functions above the code that needed it: `create_dir` sets
the mode a second time and explains exactly why. It stayed invisible for as
long as nobody had cause to read those functions as a set. Two readers
looking at one module found it in under an hour.

Fixed here (`67ac924`), in ai-app (`d0b6b66`), and in dev-updater
(`bdaade4`), which also adopted the `private` module wholesale — the first
time the sharing paid rather than merely being argued for.

## What is here

`server/` — the `wg-app-link` crate. Six modules, each extracted only after
diffing the two copies and finding nothing but a name between them.

- **`netif`** — `wg_address()`, which fails closed when the tunnel is down, and `local_addresses()` for the certificate's SANs. The product name is a parameter so the failure reads as advice rather than as a library complaining. Both split the *lookup* from the *decision*, so the failure path can be tested on a machine that has a tunnel — every machine this runs on does.
- **`enroll`** — token generation, hex-SHA-256 storage, constant-time comparison, the `<scheme>://enroll?…` URI, and the terminal QR. The URI scheme is the parameter, because it is what routes a scan back to the right app.
- **`certs`** — the CA generated once and never replaced, the leaf reissued every start. `product` names the organisation and common name and is the whole of what is per-project. Shared because being written twice is worst here: a trust anchor built two ways can be built differently two ways, and the difference surfaces as an opaque handshake failure on a phone.
- **`format`** — the two RON house rules. This was byte-identical in both projects, which made it the clearest thing in the evidence table and the easiest deletion.
- **`private`** — owner-only files and directories, taken from ai-app's version, with an `append_file` alongside `create_file` because a transcript must never be truncated by being opened.
- **`xdg`** — where each project's state lives: `config_home(product)` and `data_home(product)`, resolving the XDG variable, ignoring it unless absolute, and falling back under `$HOME`. Shared because the *reason* is shared and is not obvious from the code — the repo is a mount that resolves at different absolute paths on each side, so config inside it records paths that work on only one, and a CA private key inside it would let the untrusted side mint a leaf the pinned app trusts.

`app/` — the Android library, `com.example.wgapplink`. The same test applied
to Kotlin: the four files appeared in both apps and differed only in
comments and a product name.

- **`PinnedTls`** — trusts one CA and not the system store, so a genuine certificate for another host is refused as firmly as a self-signed one. The PEM is constructed with, not read, because each app generates its own at build time from the machine doing the build.
- **`ServerStore`** — the enrollment: preferences, the `<scheme>://enroll?…` URI, and the token sealed under an Android Keystore key. Two parameters, both per-app and both load-bearing — see the warning below.
- **`EnrollmentScanActivity`** — zxing's capture activity with the 10% framing inset and the laser decorations removed, so framing is never the user's problem.
- **`localNetworkAllowed`** — whether `ACCESS_LOCAL_NETWORK` was granted. Android 17 made it mandatory and a denial is invisible at the socket, so without asking, a blocked app and an unreachable server produce the same timeout.

**Adopting the app half takes two lines and one catalog entry.** In
`settings.gradle.kts`:

```kotlin
include(":link")
project(":link").projectDir = file("../wg-app-link/app")
```

and in the version catalog, because a subproject resolves plugin versions
from the build including it rather than from its own:

```toml
androidLibrary = { id = "com.android.library", version.ref = "agp" }
```

It also needs `androidx-core-ktx` and `zxing-embedded` in that catalog, and
`alias(libs.plugins.androidLibrary) apply false` in the root build file.
Consumed as a subproject rather than a published artifact so the two stay
locked to whatever commit the submodule points at — the same arrangement
the Rust half uses with a path dependency.

> **The Keystore alias is persisted. Carry the existing value over exactly.**
> `ServerStore(scheme, keyAlias)` takes both because both are per-app, but
> they fail differently. A wrong scheme means a scanned QR is ignored, which
> is visible immediately. A wrong `keyAlias` means the app can no longer
> unseal the token it already stored, so an enrolled phone silently reads as
> not enrolled with nothing on screen to say why. ai-app's is
> `aiapp-token-key`; dev-updater's is `dev-updater-token-key`.

**Not moved: the `PinnedCaCertificate.kt` generator.** It is ~40 lines of
Gradle in each app's build file, and all three things it varies — the
environment variable, the certificate path, and the package the constant is
emitted into — are per-app, so sharing it means parameterising the whole of
it and introducing a composite build that neither project has today. The
one bug it has had is fixed identically in both copies: a PEM constant must
start at the opening quotes, or `CertificateFactory` loses its `-----BEGIN`
preamble sniff, tries DER, and fails at runtime with an ASN.1 decode error
nowhere near the generator. Worth revisiting if it ever needs a second fix.

## What should follow, and what should not

**Should follow, in this order.** Each is already near-identical:

1. ~~The Kotlin `EnrollmentScanActivity`, `PinnedCert`, and the enrollment/Keystore half of `ServerConfig`.~~ Done — see `app/` above. It paid as predicted: ai-app's `ServerConfig` had gained `localNetworkAllowed` and the KTX `edit` block while dev-updater's had not, so the two had drifted in both directions exactly as the evidence suggested.
2. Atomic owner-only config save. Both do temp-file-then-rename with the mode set before the rename; only the schema differs.

**Should not.** Naming these is the point of the exercise:

- **The auth middleware itself.** It is generic over each project's state type. Share the primitives (`enroll`), let each keep the six lines that wire them to its own state — the alternative is a trait that exists only to let one function be shared.
- **The HTTP clients.** dev-updater's `DownloadServer.kt` and ai-app's `Api.kt` are 14% alike and 135 against 494 lines. They have diverged because they are genuinely different programs. A shared *pinned transport* underneath them may be worth it later; a shared API is not.
- **Config schemas.** Shared house rules, separate contents.
- **Anything above the link.** Projects, builds, sessions, providers. If a third project would not want it, it does not belong here.

## Two decisions worth stating, rather than inheriting

**`WG_INTERFACE` is a constant, not a parameter.** For these two projects a
second answer would mean two ideas of what "the tunnel" is, and there is
one tunnel. That reasoning is about *these* projects rather than about the
world, so it is the first thing a third user should expect to change.

**`local_addresses` puts `10.0.2.2` in every certificate.** It is the alias
an Android emulator reaches its host by, and not an address the machine
owns — so this is a SAN for something that is not this host, present so a
debug build can reach a server running beside it. Inherited from ai-app
and correct there; written down here because promoting it makes it every
future project's default rather than one project's choice.

## A review heuristic this repo keeps proving

**When you find a rule stated somewhere, grep for its siblings.** Three
separate bugs today were the correct rule already written down, applied to
one member of a set and not the others:

- `create_dir` set its mode a second time and explained why; `create_file`
  and `append_file`, three functions below, did not.
- ai-app explained one ssh failure and not its sibling.
- dev-updater's acceptance gate compared components by *declaration*, and
  `BuildState::matches` — which meant the same thing — compared them by
  full equality, so writing a measured field replaced a running build's
  state.

None was found by a test. All three were found by a second reader looking
at code as a set, which is the thing this crate is actually buying.

## The name

`wg-app-link`: the link between an app and the machine it talks to, over
WireGuard.

Two earlier names were worse. `wg-server-app` names the two projects this
came out of rather than what it is, which stops being useful the moment a
third one uses it. `wg-link` was worse still, and not merely vague —
`wg` and `ip link` are both real network tools, so it reads as something
that manages a WireGuard interface. `app` is what rules that reading out.

The `wg-` prefix does slightly oversell WireGuard's share of the code:
only `netif` is about the tunnel, while `enroll`, `private` and the
certificate work are not. It earns its place anyway, because binding the
tunnel is the architectural premise the rest follows from — it is why
there is no LAN exposure to defend, and why a plain-HTTP bootstrap port is
safe to offer at all.

## Testing

`./run-tests.sh`. The crate is warning-clean under `cargo clippy
--all-targets` and formatted with plain `cargo fmt` at its defaults, per
the house rules.