Kez/kez-chat/document.md

# KEZ Chat & File Share — Design Document

**Status:** Pre-implementation planning. No code yet.
**Last updated:** 2026-05-24
**Goal:** A Keybase-class chat + file sharing experience built on the KEZ
identity stack, with NATS for messaging and Iroh for file transfer.

---

## 1. What this is

A real-time chat + file-sharing application with verified identities.

- Users get human-friendly handles like `@tudisco@kez.lat`.
- The handle is bound to a KEZ ed25519 primary key; the same key
  authenticates to the chat infrastructure.
- Conversations are end-to-end encrypted; the broker is dumb.
- Files are visible in the sender's "shared files" list but only
  downloaded when a recipient actually wants them. No background sync.
- Identity is portable: the underlying key + sigchain survives the home
  server going dark. Handles can be migrated to other servers later.

This is the Keybase model rebuilt on a decentralized substrate:
- **Identity layer** → KEZ (instead of Keybase's central account system)
- **Chat layer** → NATS with client-side E2E (instead of Keybase Chat)
- **File layer** → Iroh peer-to-peer with content addressing (instead of KBFS)

---

## 2. Three-layer architecture

```
┌─────────────────────────────────────────────────────────────┐
│                kez-chat application                         │
│           (chat UI, file browser, profile views)            │
└────┬──────────────┬─────────────────────┬───────────────────┘
     │              │                     │
     ▼              ▼                     ▼
┌─────────┐   ┌──────────┐         ┌────────────────┐
│   KEZ   │   │   NATS   │         │     Iroh       │
│         │   │          │         │                │
│ ↓ who   │   │ ↓ chat   │         │ ↓ file blobs   │
│ ↓ what  │   │ ↓ tickets│         │ ↓ on-demand    │
│   they  │   │ ↓ presence         │ ↓ NAT travers. │
│   own   │   │ ↓ small  │         │ ↓ E2E in QUIC  │
│ ↓ where │   │   stuff  │         │                │
│   they  │   │          │         │                │
│   listen│   │ dumb     │         │                │
│         │   │ broker;  │         │                │
│         │   │ clients  │         │                │
│         │   │ do E2E   │         │                │
└─────────┘   └──────────┘         └────────────────┘
     │              ▲                       ▲
     └─────────── sigchain ──────────────────┘
       (handle → KEZ primary → endpoints
        and links to other identities)
```

Each layer does one thing well. Each is replaceable without touching the
others. The KEZ sigchain is the bridge that ties them together — it tells
a verifier "this user's broker is X, their Iroh nodes are Y₁ and Y₂."

---

## 3. Identity & username model

### 3.1 Handles

Handles look like email and Mastodon addresses:

```
@tudisco@kez.lat
@chris@kez.lat
@alice@chris.com         ← custom domain, opted out of default (future)
```

`kez.lat` is the placeholder default home server domain. We'll lock in
the real production domain before launch.

For v0, **the handle namespace is global** — registration is on the one
default home server. Federation (multiple servers with their own
namespaces) is deliberately not in v0, but the design must not preclude
it. See §3.5.

In the UI, since there's only one home server in v0, handles are
displayed bare (`@tudisco`). The `@kez.lat` suffix is implied and stored
internally.

### 3.2 Key generation tied to username

When a user creates an account:

1. App generates a **fresh ed25519 keypair** locally.
   - This is the user's KEZ primary key.
   - It's also their NATS nkey for the chat broker (same key, same algorithm).
   - It's also their Iroh node identity (same primitive again).
2. App **registers `@username`** on the home server's handle registry.
   - Sends a signed registration request proving control of the private key.
   - Registry rejects squatting (first-come-first-served).
3. App **initializes a sigchain** for the new primary.
   - First event: `add_endpoint` advertising the NATS broker the app will use.
   - Second event: `add_endpoint` advertising the Iroh NodeId of the local device.
4. App **uploads the sigchain** to the deployed `kez-sig-server`.

After this flow the user has a fully working KEZ identity:
- `@tudisco@kez.lat` resolves via the handle registry to their primary key.
- That key's sigchain (on `kez-sig-server`) advertises their NATS broker and Iroh nodes.
- Other users can verify them and reach them.

### 3.3 Why ed25519 only for this app

Both KEZ primary types work in general, but the chat app **requires** ed25519:

- **NATS nkeys are ed25519.** Direct alignment: the user's KEZ primary key
  is their NATS credential. No second auth scheme.
- **Iroh node IDs are ed25519.** Same primitive, native fit.
- **One key type to manage.** Users with a pre-existing nostr key can
  still attach it to their KEZ sigchain as a verifiable claim (so they're
  cross-referenced on nostr too), but the primary that runs the app is
  ed25519. The nostr key never participates in chat or file transport.

### 3.4 Account recovery: paper backup (Keybase-style)

The user's ed25519 private key is the only thing that can prove their
identity. Lose it, lose the account.

Recovery model for v0:

- On account creation, the app converts the 32-byte ed25519 seed to a
  **mnemonic phrase** (BIP-39 style, 24 words). Standard, well-tested
  word lists, deterministic encoding.
- App **forces the user to write it down** before continuing — shows
  the words, asks for confirmation, then asks them to retype a few
  random words back to prove they recorded it.
- App stores the seed locally in OS-protected storage (Keychain,
  Credential Manager, libsecret). Mnemonic is shown only at creation
  and on-demand from settings.
- **Lost device flow:** user installs the app on a new device, types
  their mnemonic, app regenerates the same ed25519 keypair, then pulls
  the sigchain from `kez-sig-server` to restore their identity state.
- The handle is still theirs because the registry knows the primary key.

No server-side recovery. No email reset. No customer support. Same model
Bitcoin wallets and Keybase used — user holds the seed phrase, user is
responsible for it.

### 3.5 Federation-ready design (not in v0)

For v0 we have **one** home server (`kez.lat`). All handles live there.
To make sure we don't paint ourselves into a corner:

1. **Internal representation of a handle is always the qualified form**
   (`tudisco@kez.lat`), never just `tudisco`. The UI strips the suffix
   for display; storage always keeps the full form.
2. **Handle resolution is HTTP-based**, not hard-coded. The chat app
   looks up `chris@kez.lat` by hitting `https://kez.lat/v1/u/chris`.
   When federation lands, looking up `chris@example.com` hits
   `https://example.com/v1/u/chris` instead.
3. **WebFinger endpoint included from v0** — so cross-server discovery
   already works via standard tooling, even if our app only uses our
   own server for now.
4. **Sigchain endpoint URLs are fully qualified.** A user's sigchain
   lives at `https://sig.kez.lat/v1/sigchains/ed25519/<hex>` — when
   another server's user wants to verify ours, the URL is right there.

The v0 chat app might hard-code "lookups go to `kez.lat`" for now;
flipping that to "lookups go to whatever's after the `@`" is a config
change later, not a redesign.

---

## 4. The home server (`kez-chat-server`)

A single Rust binary, deployed as one container alongside other
microservices (NATS broker, sigchain server).

### 4.1 What it does (and what it doesn't)

| Responsibility | This server? |
|---|---|
| **Handle registry** | ✅ Yes |
| **NATS auth callout** | ✅ Yes |
| **WebFinger endpoint** | ✅ Yes |
| **HTTP API for clients** | ✅ Yes |
| **Sigchain storage** | ❌ No — defer to `kez-sig-server` (separate container) |
| **NATS broker** | ❌ No — separate `nats-server` (Go) container |
| **Iroh pinning** | ❌ No for v0 — files transfer P2P when both peers are online. Pinning is a future tier. |
| **Channel verification (gist/dns/etc.)** | ❌ No — clients do it locally via `kez-channels`. KEZ system is only used for identity, not as part of chat. |

The chat server is deliberately small. Microservices: each service does
one thing, deployed independently. Operator runs three containers
(chat-server + nats-server + sig-server). When pinning lands later, that
becomes a fourth optional container.

### 4.2 Process / deployment model

NATS is **not part of our deployment.** The operator runs NATS however
they want (Synadia Cloud, their own cluster, a friend's broker, a single
local container) and gives the chat-server a URL. Same idea as a
database: we connect to one; we don't ship one.

```
                External infrastructure
                (operator's responsibility)
                ┌──────────────────────┐
                │     NATS broker      │
                │   + JetStream        │
                │   somewhere          │
                └─────────▲─────▲──────┘
                          │     │
       chat-server ──────┘     │ ◄────── client app
       (auth callout)           │         (publish/subscribe)
                                │
┌─────────────── our deployment ─────────────────┐
│                                                │
│  ┌─────────────────┐   ┌────────────────┐      │
│  │ kez-chat-server │   │ kez-sig-server │      │
│  │   (Rust)        │   │   (Rust)       │      │
│  │                 │   │                │      │
│  │ ↓ handles       │   │ ↓ sigchain     │      │
│  │ ↓ nats auth     │   │   storage      │      │
│  │ ↓ HTTP API      │   │                │      │
│  └─────────────────┘   └────────────────┘      │
│         ▲                      ▲               │
└─────────┼──────────────────────┼───────────────┘
          │                      │
   ┌──────┴──────────────────────┴────────────────────────┐
   │  Chat app (per user, runs on phone/desktop)          │
   │                                                      │
   │  • talks to the operator's NATS broker (NATS proto)  │
   │  • talks to kez-chat-server over HTTPS               │
   │  • talks to kez-sig-server over HTTPS                │
   │  • runs local iroh::Node for file send/receive       │
   └──────────────────────────────────────────────────────┘
```

The chat-server orchestrates auth against whatever NATS broker is
configured, but doesn't run, host, supervise, or ship NATS in any form.

### 4.3 docker-compose sketch (our two services only)

```yaml
# deploy/docker-compose.yml — what we ship
services:
  chat-server:
    build: .                   # kez-chat-server Rust binary
    environment:
      NATS_URL: ${NATS_URL}    # operator points us at their NATS broker
      SIG_SERVER_URL: http://sig-server:7878
      DB_PATH: /data/handles.db
      AUTH_CALLOUT_NKEY_PATH: /etc/kez/auth-callout.nkey
    volumes:
      - chat-data:/data
      - ./auth-callout.nkey:/etc/kez/auth-callout.nkey:ro
    depends_on: [sig-server]
    ports:
      - "8080:8080"            # HTTP API for clients

  sig-server:
    image: kez-sig-server:latest   # the existing rust-sig-server
    environment:
      KEZ_DB: /data/sigchains.db
    volumes:
      - sig-data:/data
    ports:
      - "7878:7878"

volumes:
  chat-data:
  sig-data:
```

**NATS is not in this file.** The operator brings their own — running
on a different host, in a different compose project, on Synadia Cloud,
or wherever. They give us `NATS_URL` and a place to put our auth
callout endpoint URL in their `nats.conf`.

What the operator needs to add on the NATS side (in **their** config):

```conf
# nats.conf — added to whatever NATS deployment the operator runs
authorization {
  auth_callout {
    issuer: "<our auth-callout signing nkey public part>"
    auth_users: ["AUTHUSER"]      # placeholder identity NATS uses
    account: "DEFAULT"
  }
}
```

The chat-server signs auth-callout responses with a long-lived nkey
that NATS trusts. When a client connects to NATS with their KEZ
ed25519 key, NATS forwards the auth request to our chat-server,
which checks the handle registry and signs a yes/no response.

We provide a reference `nats.conf` snippet in the docs. The operator
patches it into their own NATS deployment.

For local development, see Appendix A.

### 4.4 Endpoints

```
GET   /v1/healthz
GET   /v1/u/:handle                      handle → { primary, sigchain_url, endpoints }
POST  /v1/register                       claim a handle (signed body)
GET   /.well-known/webfinger?resource=acct:tudisco@kez.lat

# NATS auth callout (called BY nats-server, not by users)
POST  /internal/nats/auth                verify nkey signature, return permissions
```

Sigchain endpoints are **not** on this server — clients talk directly to
`kez-sig-server` for those.

---

## 5. End-to-end flows

### 5.1 Account creation — `@tudisco@kez.lat`

```
1. User opens chat app, clicks "Create account"
2. App: generates ed25519 keypair locally
3. App: converts seed to 24-word mnemonic, makes user write it down,
   verifies recall before continuing
4. App: user picks handle "tudisco"
5. App → chat-server:
     POST /v1/register
     { "handle": "tudisco",
       "primary": "ed25519:<pubkey-hex>",
       "registration_sig": "<sig over canonical message>" }
6. Server: validates signature, checks handle is free, stores in registry
7. Server: 201 Created
8. App: initializes sigchain locally, signs:
     - add_endpoint { protocol: "nats", url: "...", inbox: "kez.inbox.<pk>" }
     - add_endpoint { protocol: "iroh", node_id: "<local iroh id>" }
9. App → sig-server: POST /v1/sigchains/ed25519/<pk>/events  (one per event)
10. App: connects to nats-server with nkey auth (signed challenge,
    nats-server invokes chat-server's auth callout, gets back yes/no
    + allowed subjects)
11. App: subscribes to JetStream durable consumer on its inbox subject
12. Done — @tudisco@kez.lat is live and reachable
```

### 5.2 Adding a contact

```
1. Tudisco types "@chris" in app
2. App → chat-server: GET /v1/u/chris
   Returns: { primary: "ed25519:abc...", sigchain_url: "https://sig.kez.lat/..." }
3. App → sig-server (URL from above): fetch sigchain
4. App walks events to extract:
     - NATS broker URL + inbox subject (from add_endpoint nats)
     - Iroh node IDs (from add_endpoint iroh)
     - Other identity claims (github:chris, dns:chris.com, etc. — for display)
5. App caches LOCALLY: { "@chris@kez.lat" => ed25519:abc..., endpoints: {...} }
   (TOFU — trust on first use)
```

### 5.3 Sending a chat message

```
1. Tudisco types "hello" to Chris
2. App looks up Chris's primary key + NATS endpoint from local cache
3. App derives a per-message symmetric key:
     X25519(tudisco_priv, chris_pub) → HKDF → 32-byte ChaCha20-Poly1305 key
4. App encrypts "hello" with that key (+ random nonce)
5. App signs ciphertext with tudisco's KEZ primary
6. App publishes to subject `kez.inbox.<chris-pubkey-hex>` on the NATS
   broker, JetStream-published so the broker stores it durably
7. Chris's app (subscribed via durable consumer) receives the message
   whenever next online — broker buffers it if offline
8. Chris's app verifies signature against tudisco's key, decrypts,
   shows "tudisco: hello"
```

The broker sees:
- An nkey-authenticated client publishing encrypted bytes to a subject
- It does NOT see: who's reading the subject, message contents, sender
  identity (sender identity is in the signed payload, not the NATS frame)

### 5.4 Sharing a file (v0: both peers online)

```
1. Tudisco drags `report.pdf` into the chat with Chris
2. App imports the blob into local Iroh node → BLAKE3 hash + ticket
3. App optionally adds an entry to tudisco's "shared files" manifest
   (visible if Chris later browses tudisco's profile)
4. App generates a per-file symmetric content key
5. App encrypts the blob in place (or stores both plaintext + encrypted —
   detail for later) with the content key
6. App wraps the content key for chris's KEZ key (X25519 → HKDF)
7. App sends a NATS message to chris's inbox:
     { type: "file_share",
       filename: "report.pdf",
       size: 1234567,
       iroh_ticket: "blobac://...",
       wrapped_content_key: "..." }
   (same encryption as chat messages, so chris can read this)
8. Chris's app sees the notification: "tudisco shared report.pdf (1.2 MB)"
   File NOT downloaded yet.
9. Chris clicks Download.
10. Chris's app opens an Iroh connection to tudisco's NodeId (from
    tudisco's sigchain), pulls the blob via the ticket, decrypts with
    the unwrapped content key, verifies BLAKE3 hash. File appears.
```

**v0 limitation:** If tudisco is offline at step 10, chris waits.
Iroh will retry; download starts when tudisco's node comes back.
Pinning (the server holding a copy) is **not** in v0 — we accept this
limitation in exchange for zero server-side storage cost and the
simplest possible architecture.

### 5.5 Browsing someone's files (Keybase-style)

```
1. Chris opens tudisco's profile
2. App resolves @tudisco → primary → sigchain
3. Sigchain has a `set_shared_files` op pointing at a manifest blob hash
4. App fetches the manifest blob via Iroh (small, fast)
5. App decrypts entries wrapped for chris's key, ignores ones it can't
   decrypt (those are wrapped for other people)
6. App renders the visible entries: name, size, share date,
   thumbnail (if present)
7. Chris clicks an entry → flow continues like §5.4 step 9
```

Manifest is small (KB-scale); blobs are MB-to-GB. Browsing is cheap;
fetching is per-file deliberate. **Recipient never auto-syncs.**

---

## 6. Project & folder layout

### 6.1 Where this project lives

```
/Kez
├── rust-lib/                ← (proposed refactor) shared Rust libraries
│   ├── Cargo.toml           workspace
│   └── crates/
│       ├── kez-core/        moved from rust/crates/
│       └── kez-channels/    moved from rust/crates/
│
├── rust/                    ← Rust CLI (kez binary)
│   └── crates/
│       └── kez-cli/         depends on ../../rust-lib/crates/...
│
├── rust-sig-server/         ← existing sigchain storage (reused as-is)
│
├── kez-chat/                ← THIS PROJECT
│   ├── document.md          (this file)
│   ├── Cargo.toml
│   ├── src/
│   │   ├── main.rs          binary entry
│   │   ├── handles.rs       handle registry (sqlite-backed)
│   │   ├── nats_auth.rs     NATS auth callout endpoint
│   │   ├── webfinger.rs     WebFinger discovery endpoint
│   │   └── api.rs           axum routes + state
│   ├── deploy/
│   │   ├── docker-compose.yml   chat-server + nats + sig-server
│   │   ├── nats.conf            with auth_callout config
│   │   └── systemd/             alternative deployment
│   └── tests/
│       └── http.rs              integration tests
│
├── nodejs/                  ← (unchanged)
└── crosstest.sh             ← (path updates if rust-lib moves)
```

### 6.2 The `rust-lib/` proposal — share code, no duplication

Right now, `kez-core` and `kez-channels` live inside `rust/crates/`. The
sig-server and the new chat-server both want to use them. Today's
downstream path-dep is:

```toml
kez-core = { path = "../rust/crates/kez-core" }
```

…which works but reaches into another project's crate tree.

**Recommendation:** move the pure libraries out into a top-level
`rust-lib/` workspace. The CLI stays in `rust/`. Downstream servers
depend on `../rust-lib/crates/kez-core`. Clean structure, no
duplication, no confusion about which folder owns what.

Refactor steps:

- `mv rust/crates/kez-core rust-lib/crates/`
- `mv rust/crates/kez-channels rust-lib/crates/`
- Create `rust-lib/Cargo.toml` (workspace).
- Update `rust/Cargo.toml` to have just kez-cli.
- Update path deps in: `rust/crates/kez-cli/Cargo.toml`,
  `rust-sig-server/Cargo.toml`.
- Update `crosstest.sh` if any paths are hardcoded.

**Suggested order:** do the refactor *before* starting kez-chat so we
import cleanly from the start.

### 6.3 Dependencies (planned)

| Crate | Why |
|---|---|
| `kez-core` (path) | Identity types, ed25519, signing |
| `kez-channels` (path) | Verify users' linked accounts when displayed |
| `axum` 0.8 | HTTP API |
| `tokio` | Async runtime |
| `rusqlite` (bundled) | Handle registry |
| `async-nats` | NATS client (admin work + the auth callout glue) |
| `serde` / `serde_json` | Standard |
| `thiserror` / `anyhow` | Standard |
| `tracing` / `tracing-subscriber` | Logging |
| `tower-http` | CORS, request tracing |
| `clap` | CLI args |

**Not** depended on by the chat-server:
- `iroh` — server doesn't run an Iroh node in v0 (no pinning)
- nats-server (Go) — separate container, not a Rust dep

### 6.4 NATS broker — external dependency

NATS is **not part of our project**. It's external infrastructure the
operator provides, the same way they'd provide a database or an SMTP
relay. We ship:

- An `async-nats` client used by the chat-server (admin/utility work)
- An auth-callout HTTP endpoint that NATS calls during client connection
- A documented `nats.conf` snippet operators add to their NATS deployment
- A reference local-dev setup (Appendix A) for running NATS yourself
  while developing

What we require from the operator's NATS:

| Requirement | Why |
|---|---|
| **NATS 2.10+** (for auth_callout) | We rely on auth callout to bridge KEZ identity into NATS |
| **JetStream enabled** | For offline message buffering (durable consumers) |
| **TCP reachable** from chat-server and clients | Standard |
| **TLS** (in production) | Standard |
| **auth_callout configured** to hit our endpoint | Required for client auth |

That's it. Operator can run a single Docker container, a clustered
production deployment, or a managed service — we don't care, as long
as `NATS_URL` and the callout config are correct.

Why fully external rather than alongside us:

- NATS is a serious piece of infrastructure with its own scaling and
  operational concerns. Bundling it implies we're responsible for it.
  We're not.
- Operators with existing NATS deployments can reuse them. No "now run
  our copy of NATS too."
- Different teams might run different NATS topologies (single instance,
  cluster, mesh, leaf nodes). None of that is our problem.
- Swapping NATS implementations or moving to a managed provider is a
  config change, not a code change.

### 6.5 Iroh — client-side only

Clients run a local Iroh node for sending and receiving files. The
**chat-server does NOT run an Iroh node** in v0.

Implication: when @tudisco shares a file with @chris, the bytes go
directly from tudisco's device to chris's device via Iroh. If tudisco
is offline, chris waits. There's no fallback to a server-stored copy.

This is the simplest possible operational model. Pinning (server-side
fallback storage) is a future addition (§8).

---

## 7. MVP scope

### Server (`kez-chat-server`)

- [ ] HTTP API scaffold (axum + tokio)
- [ ] Handle registry (POST /register, GET /u/:handle)
- [ ] Registration signature validation (uses kez-core)
- [ ] WebFinger endpoint
- [ ] NATS auth callout (POST /internal/nats/auth)
- [ ] Healthz / metrics
- [ ] Integration tests against real nats-server + sig-server in a
  test docker-compose

### Deployment

- [ ] docker-compose.yml (chat + nats + sig-server)
- [ ] nats.conf with auth_callout configured
- [ ] systemd alternative deployment recipe
- [ ] README with TLS / reverse proxy guidance

### Client (`kez-chat-cli` — separate project later)

Out of scope for the server work, but the **server isn't usable without**
at least a CLI client that does:
- [ ] Account creation (key gen + mnemonic backup + handle registration)
- [ ] Contact lookup + verification
- [ ] Send / receive 1:1 chat messages (E2E via NATS)
- [ ] Send / receive files (E2E via Iroh)
- [ ] Browse @user shared-files manifest

UI app comes after CLI proves the flow works.

---

## 8. Out of scope (v0)

- **Iroh pinning** (sender must be online for receiver to fetch)
- **Group chat** (only 1:1 for v0)
- **Forward secrecy / ratcheting** (Double Ratchet, MLS) — chat is
  encrypted but each message uses the same X25519-derived key per pair
- **Voice / video calls**
- **Multi-device key sync** — one device per user in v0
- **Account recovery beyond mnemonic** — paper backup is the only recovery
- **Federation across home servers** — one server (kez.lat) in v0;
  design preserves the option
- **Channel-based identity verification** — the CLI already does
  `kez verify id ...`; not duplicated in the chat-server. Users add
  KEZ channel proofs (gist, dns, etc.) via the existing CLI separately.
- **Avatars / display names** — defer the design. For v0 the UI shows
  the handle and that's enough.

---

## 9. The one remaining open question

**Manifest format** for "@chris's shared files":

| Option | How | Tradeoff |
|---|---|---|
| **A. Signed JSON blob, hash in sigchain** | Manifest is a JSON blob stored on Iroh. A new sigchain op `set_shared_files` commits the latest manifest hash. Recipients walk the sigchain → find the pointer → fetch the manifest blob from Iroh. | Simpler. No Iroh Docs dep. Sigchain anchors the version (signed). Update = new sigchain event. |
| **B. Iroh Doc** | Manifest is a mutable CRDT document. Recipients subscribe; updates sync in near-real-time. | Fancier UX (live updates). Requires Iroh Docs subsystem (heavier dep, less stable). |

**Recommended default: A.** Simpler, fewer moving parts, reuses
primitives we already have. We can upgrade to B later if real users
need real-time profile feed updates.

Settle yes/no on this and the design is locked.

---

## Decisions locked from earlier discussion

| Question | Decision |
|---|---|
| Bundle sigchain in chat-server? | **No.** Use existing `kez-sig-server`. Microservices. |
| Bundle NATS into Rust server? | **No.** NATS is external infrastructure the operator provides. We don't ship, embed, or supervise it. We connect to whatever broker `NATS_URL` points at. |
| KEZ + nostr coexistence for chat? | **No nostr in chat.** KEZ is identity-only; nostr only as a verifiable claim in someone's sigchain, not as transport. |
| Handle scope: federation or global? | **Global for v0**, federation-ready design (see §3.5). |
| Recovery if key lost? | **Paper backup (24-word mnemonic), Keybase-style.** No server-side recovery. |
| Iroh pinning in v0? | **No.** Sender must be online for receiver to fetch. Pinning is a future tier. |

---

## 10. Risks & honest concerns

1. **NATS auth callout integration depth.** Documented but fiddly.
   nkey signature verification is straightforward; the per-user subject
   permission glue needs care. Test cases for "user can publish to
   their own inbox only" / "user can subscribe to their own inbox
   only" matter.

2. **Iroh is pre-1.0.** Pin a version. Migration is a chore but only
   touches client code, not identity. Identity stays stable (KEZ).

3. **Single-device assumption.** Real users have phones AND laptops.
   v0 assumes one device per primary. Designing multi-device is a
   real follow-up.

4. **No offline file delivery.** A natural user complaint will be
   "Chris sent me a file but he's offline now." We've made the trade
   knowingly; document the limitation clearly in-app ("File will
   download when @chris is back online").

5. **Handle squatting.** First-come-first-served. Mitigations:
   - Rate-limit registration by IP
   - Reserve some handles (`@admin`, common project names)
   - Accept that some squatting will happen; document the policy

6. **NAT traversal.** Iroh handles it with relays. Test on hostile
   networks (corporate firewalls, mobile carriers with CGNAT) before
   claiming "just works."

7. **Operational cost.** Three containers (chat + nats + sig-server)
   + bandwidth + a domain. Cheap at small scale, scales with users.
   Need a "running kez.lat for 1k users — what does it cost?" answer
   before community adoption.

---

## 11. The plan, sequenced

When we start building:

1. **Refactor: move `kez-core` + `kez-channels` to `rust-lib/`.**
   Small but unblocks clean imports from kez-chat.

2. **Scaffold `kez-chat-server`** (axum + tokio + sqlite + tracing).
   Handle registry + WebFinger first — these unblock client-side
   account creation.

3. **NATS auth callout.** Bring up a NATS broker for development (see
   Appendix A), configure its auth_callout to hit our chat-server's
   `/internal/nats/auth`. End-to-end: a client can register a handle
   and then connect to NATS authenticated by its KEZ key.

4. **Minimal `kez-chat-cli` client** (separate project) that does:
   - `kez-chat register tudisco`
   - `kez-chat add @chris`
   - `kez-chat send @chris "hello"`
   - `kez-chat listen`
   No UI. Enough to prove the chat flow works end-to-end against
   the server.

5. **Iroh integration in the client** (not the server).
   - Client runs a local Iroh node
   - `kez-chat share @chris ./file.pdf`
   - `kez-chat fetch <ticket>`

6. **Shared-files manifest.** New `set_shared_files` sigchain op.
   `kez-chat browse @tudisco` lists his shared files.

7. **Deployment recipe.** docker-compose, systemd, deployment doc.

8. **Then** start the GUI app. Could be Tauri (Rust + web frontend),
   Iced (pure Rust UI), or something else.

---

## 12. One-paragraph summary

`kez-chat` is a Keybase-class chat and file-sharing app built on the
KEZ identity stack. Users get `@username@kez.lat` handles backed by an
ed25519 primary key. The same key authenticates to a NATS broker
(chat, presence, file tickets — broker is dumb, clients do E2E with
ChaCha20-Poly1305 over X25519-derived keys) and identifies an Iroh
node (P2P bulk transfer, content-addressed blobs, on-demand fetch).
**Our project ships two services**: a thin Rust `kez-chat-server`
that handles the handle registry + NATS auth callout + HTTP API, and
the existing `kez-sig-server` that stores sigchains. **NATS is
external infrastructure the operator provides** — we never ship,
embed, or supervise it. The chat-server does not run an Iroh node
and does not pin files in v0; file transfer is pure P2P between
online peers. Account recovery is via a 24-word paper-backup
mnemonic. Federation across home servers is deferred but the design
keeps it as a flip-the-switch future change.

---

## Appendix A: running a NATS broker locally for development

NATS is not part of our project, but you need one running to test the
chat-server end-to-end. Easiest path during development:

```sh
docker run -d --name kez-dev-nats \
  -p 4222:4222 -p 8222:8222 \
  -v "$PWD/dev-nats.conf:/etc/nats/nats.conf:ro" \
  nats:latest -c /etc/nats/nats.conf --jetstream
```

Where `dev-nats.conf` enables the auth callout pointing at your
locally-running chat-server (e.g. `http://host.docker.internal:8080/internal/nats/auth`).

A full reference `dev-nats.conf` will live at `deploy/dev-nats.conf`
when we start building. This appendix exists so developers have a
one-liner to spin up NATS for testing; **it is not the production
deployment recipe** (operators run their own NATS however they want).

For production: see the NATS docs (https://docs.nats.io). Our project
has no opinion beyond "must be 2.10+ with JetStream + auth_callout
configured to hit our endpoint."