# [zat](https://zat.dev)

AT Protocol building blocks for zig.

<details>
<summary><strong>this readme is an ATProto record</strong></summary>

> [view in zat.dev's repository](https://at-me.zzstoatzz.io/view?handle=zat.dev)

zat publishes these docs as [`site.standard.document`](https://standard.site) records, signed by its DID.

</details>

## install

requires zig 0.16+.

```bash
zig fetch --save https://tangled.org/zat.dev/zat/archive/main
```

then in `build.zig`:

```zig
const zat = b.dependency("zat", .{}).module("zat");
exe.root_module.addImport("zat", zat);
```

## what's here

<details>
<summary><strong>string primitives</strong> - parsing and validation for atproto identifiers</summary>

- **Tid** - timestamp identifiers (base32-sortable)
- **Did** - decentralized identifiers
- **Handle** - domain-based handles
- **Nsid** - namespaced identifiers (lexicon types)
- **Rkey** - record keys
- **AtUri** - `at://` URIs

```zig
const zat = @import("zat");

if (zat.AtUri.parse(uri_string)) |uri| {
    const authority = uri.authority();
    const collection = uri.collection();
    const rkey = uri.rkey();
}
```

</details>

<details>
<summary><strong>identity resolution</strong> - resolve handles and DIDs to documents</summary>

```zig
// handle → DID
var handle_resolver = zat.HandleResolver.init(io, allocator);
defer handle_resolver.deinit();
const did = try handle_resolver.resolve(zat.Handle.parse("bsky.app").?);
defer allocator.free(did);

// DID → document
var did_resolver = zat.DidResolver.init(io, allocator);
defer did_resolver.deinit();
var doc = try did_resolver.resolve(zat.Did.parse("did:plc:z72i7hdynmk6r22z27h6tvur").?);
defer doc.deinit();

const pds = doc.pdsEndpoint();       // "https://..."
const key = doc.signingKey();         // verification method
```

supports did:plc (via plc.directory) and did:web. handle resolution via HTTP well-known and DNS TXT.

</details>

<details>
<summary><strong>CBOR codec</strong> - DAG-CBOR encoding and decoding</summary>

```zig
// decode
const decoded = try zat.cbor.decode(allocator, bytes);
defer decoded.deinit();

// navigate values
const text = decoded.value.getStr("text");
const cid = decoded.value.getCid("data");

// encode (deterministic key ordering)
const encoded = try zat.cbor.encodeAlloc(allocator, value);
defer allocator.free(encoded);
```

full DAG-CBOR support: maps, arrays, byte strings, text strings, integers, floats, booleans, null, CID tags (tag 42). deterministic encoding with sorted keys for signature verification.

</details>

<details>
<summary><strong>CAR codec</strong> - Content Addressable aRchive parsing with CID verification</summary>

```zig
// parse with SHA-256 CID verification (default)
const parsed = try zat.car.read(allocator, car_bytes);
defer parsed.deinit();

const root_cid = parsed.roots[0];
for (parsed.blocks.items) |block| {
    // block.cid_raw, block.data
}

// skip verification for trusted local data
const fast = try zat.car.readWithOptions(allocator, car_bytes, .{
    .verify_block_hashes = false,
});
```

enforces size limits (configurable `max_size`, `max_blocks`) matching indigo's production defaults.

</details>

<details>
<summary><strong>MST</strong> - Merkle Search Tree</summary>

```zig
var tree = zat.mst.Mst.init(allocator);
defer tree.deinit();

try tree.put(allocator, "app.bsky.feed.post/abc123", value_cid);
const found = tree.get("app.bsky.feed.post/abc123");
try tree.delete(allocator, "app.bsky.feed.post/abc123");

// compute root CID (serialize → hash → CID)
const root = try tree.rootCid(allocator);
```

the core data structure of an atproto repo. key layer derived from leading zero bits of SHA-256(key), nodes serialized with prefix compression.

</details>

<details>
<summary><strong>crypto</strong> - signing, verification, key encoding</summary>

```zig
// JWT verification
var token = try zat.Jwt.parse(allocator, token_string);
defer token.deinit();
try token.verify(public_key_multibase);

// ECDSA signature verification (P-256 and secp256k1)
try zat.jwt.verifySecp256k1(hash, signature, public_key);
try zat.jwt.verifyP256(hash, signature, public_key);

// multibase/multicodec key parsing
const key_bytes = try zat.multibase.decode(allocator, "zQ3sh...");
defer allocator.free(key_bytes);
const parsed = try zat.multicodec.parsePublicKey(key_bytes);
// parsed.key_type: .secp256k1 or .p256
// parsed.raw: 33-byte compressed public key
```

ES256 (P-256) and ES256K (secp256k1) with low-S normalization. RFC 6979 deterministic signing. `did:key` construction and multibase encoding.

</details>

<details>
<summary><strong>OAuth client helpers</strong> - ATProto OAuth ceremony without app storage policy</summary>

```zig
var transport = zat.HttpTransport.init(io, allocator);
defer transport.deinit();

const authserver = try zat.oauth.discoverAuthorizationServer(allocator, &transport, pds_url);
defer allocator.free(authserver);

var metadata = try zat.oauth.fetchAuthorizationServerMetadata(allocator, &transport, authserver);
defer metadata.deinit(allocator);

var secrets = try zat.oauth.prepareAuthRequestSecrets(allocator, io);
defer secrets.deinit(allocator);

var par = try zat.oauth.sendParRequest(allocator, io, &transport, .{
    .par_url = metadata.pushed_authorization_request_endpoint,
    .authserver_issuer = metadata.issuer,
    .client_id = client_id,
    .redirect_uri = redirect_uri,
    .scope = "atproto repo:example.app.record",
    .state = secrets.state,
    .pkce_challenge = secrets.pkce_challenge,
    .login_hint = handle,
    .client_keypair = &client_keypair,
    .dpop_keypair = &secrets.dpop_keypair,
});
defer par.deinit(allocator);
```

also includes client metadata JSON generation, authorization URL formatting, code/refresh token exchange, DPoP nonce retry, and DPoP-authenticated resource requests. cookies, sessions, redirects, and persistence stay with your application or web framework.

</details>

<details>
<summary><strong>commit verification</strong> - check a repo CAR against the account's signing key</summary>

```zig
// given CAR bytes and the account's public key (see identity resolution)
const result = try zat.verifyCommitCar(allocator, car_bytes, public_key, .{
    .expected_did = did,
    .require_complete_repo = true, // reject CARs with missing MST/record blocks
});
// result.commit_did, result.commit_rev, result.commit_cid, result.record_count
```

atproto repos are self-authenticating, so anything that accepts one from the network verifies it before trusting it: checks the commit signature, confirms the MST root matches the commit's `data` CID, and validates every block hash. `verifyCommitDiff` does the same for the partial CARs on firehose `#commit` events. this is how [zds](https://tangled.org/zat.dev/zds) guards `com.atproto.repo.importRepo` — an uploaded CAR is verified against the account's DID document before the PDS accepts it.

`signCommit` is the produce side: it builds and signs the canonical commit block, and round-trips through `verifyCommitCar` in the test suite.

</details>

<details>
<summary><strong>firehose client</strong> - raw CBOR event stream from relay</summary>

```zig
var client = zat.FirehoseClient.init(io, allocator, .{});
defer client.deinit();

const Handler = struct {
    pub fn onEvent(_: *@This(), event: zat.FirehoseClient.Event) void {
        switch (event.header.type) {
            .commit => {
                // event.body.blocks, event.body.ops, ...
            },
            else => {},
        }
    }
};
var handler: Handler = .{};
try client.subscribe(&handler);
```

connects to `com.atproto.sync.subscribeRepos` via WebSocket. decodes binary CBOR frames into typed events. round-robin host rotation with backoff.

</details>

<details>
<summary><strong>jetstream client</strong> - typed JSON event stream</summary>

```zig
var client = zat.JetstreamClient.init(io, allocator, .{
    .wanted_collections = &.{"app.bsky.feed.post"},
});
defer client.deinit();

const Handler = struct {
    pub fn onEvent(_: *@This(), event: zat.JetstreamClient.Event) void {
        if (event.commit) |commit| {
            const record = commit.record;
            // process...
            _ = record;
        }
    }
};
var handler: Handler = .{};
try client.subscribe(&handler);
```

connects to jetstream (bluesky's JSON event stream). typed events, automatic reconnection with cursor tracking, round-robin across community relays.

</details>

<details>
<summary><strong>xrpc client</strong> - call AT Protocol endpoints</summary>

```zig
var client = zat.XrpcClient.init(io, allocator, "https://bsky.social");
defer client.deinit();

const nsid = zat.Nsid.parse("app.bsky.actor.getProfile").?;
var response = try client.query(nsid, params);
defer response.deinit();

if (response.ok()) {
    var json = try response.json();
    defer json.deinit();
    // use json.value
}
```

</details>

<details>
<summary><strong>json helpers</strong> - navigate nested json without verbose if-chains</summary>

```zig
// runtime paths for one-offs:
const uri = zat.json.getString(value, "embed.external.uri");
const count = zat.json.getInt(value, "meta.count");

// comptime extraction for complex structures:
const FeedPost = struct {
    uri: []const u8,
    cid: []const u8,
    record: struct {
        text: []const u8 = "",
    },
};
const post = try zat.json.extractAt(FeedPost, allocator, value, .{"post"});
```

</details>

## used by

<details>
<summary><strong>downstream projects</strong> — what's building on zat</summary>

### firehose consumers (Jetstream)

| project | what it does |
|---|---|
| [labelz](https://tangled.org/zzstoatzz.io/labelz) | AT Protocol labeler — keyword-matched, secp256k1-signed, served over XRPC. exists to pressure-test zat as an SDK ([live](https://labelz.fly.dev/health)) |
| [coral](https://tangled.org/zzstoatzz.io/coral) | named-entity recognition over the firehose ([live](https://coral.waow.tech)) |
| [pollz](https://tangled.org/zzstoatzz.io/pollz) | polls on AT Protocol ([live](https://pollz.waow.tech)) |
| [typeahead](https://tangled.org/zzstoatzz.io/typeahead) | community actor search ([live](https://typeahead.waow.tech)) |
| [find-bufo](https://tangled.org/zzstoatzz.io/find-bufo) | bsky bot that quote-posts matching bufo images for opt-in followers |

### CBOR / CAR / MST users

| project | what it does |
|---|---|
| [zlay](https://tangled.org/zzstoatzz.io/zlay) | AT Protocol relay — direct PDS crawl, signature validation, inline collection index ([live](https://zlay.waow.tech/_health)) |
| [atproto-bench](https://tangled.org/zzstoatzz.io/atproto-bench) | cross-SDK relay benchmarks (zig vs go vs rust vs python) |
| [ken](https://tangled.org/zzstoatzz.io/ken) | semantic search over your atproto repo — vector index of records, search by meaning ([live](https://ken.waow.tech)) |

### identity + XRPC users

| project | what it does |
|---|---|
| [music-atmosphere-feed](https://tangled.org/zzstoatzz.io/music-atmosphere-feed) | bsky feed generator for music links — JWT auth for personalized feed |
| [leaflet-search](https://tangled.org/zzstoatzz.io/leaflet-search) | search across leaflet, pckt, offprint, and other standard.site publishers |

</details>

## benchmarks

zat is benchmarked against the Go (indigo, atmos), Rust (rsky, shrike, jacquard), Python, and Elixir ecosystems in [atproto-bench](https://tangled.org/zzstoatzz.io/atproto-bench):

- **firehose decode** (CID-verified): 226k frames/sec (zat) vs 62k (atmos) vs 53k (rsky) vs 20k (indigo)
- **MST insert + root**: 5.7M records/sec, producing the same root bytes as atmos and shrike
- **signature verify**: 21.6k verifies/sec — ahead of Go (12k–19k), behind Rust (~27k); ECDSA dominates every SDK's pipeline
- **end-to-end repo verification** (handle → signature-checked repo): ~300ms compute (zat) vs ~410ms (indigo) vs ~422ms (rsky crates)

## specs

validation follows [atproto.com/specs](https://atproto.com/specs/atp). passes the [atproto interop test suite](https://github.com/bluesky-social/atproto-interop-tests) (syntax, crypto, MST vectors).

## versioning

pre-1.0 semver:
- `0.x.0` - new features (backwards compatible)
- `0.x.y` - bug fixes

breaking changes bump the minor version and are documented in commit messages.

## license

[MIT](https://tangled.org/zat.dev/zat/blob/main/LICENSE)

---

[devlog](devlog/) · [changelog](CHANGELOG.md)