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@atcute/oauth-browser-client

minimal OAuth browser client implementation for AT Protocol.

  • only the bare minimum: enough code to get authentication reasonably working, with only one happy path is supported (only ES256 keys for DPoP. PKCE and DPoP-bound PAR is required.)
  • does not use IndexedDB: makes the library work under Safari's lockdown mode, and has less maintenance headache overall, but it also means this is "less secure" (it won't be able to use non-exportable keys as recommended by DPoP specification.)
  • not well-tested: it has been used in personal projects and by friends for quite some time, but hasn't seen any use outside of that. using the reference implementation is recommended if you are unsure about the implications presented here.

usage

setup

initialize the client by importing and calling configureOAuth with the client ID and redirect URL. this call should be placed before any other calls you make with this library.

import { configureOAuth } from '@atcute/oauth-browser-client';

configureOAuth({
	metadata: {
		client_id: 'https://example.com/oauth/client-metadata.json',
		redirect_uri: 'https://example.com/oauth/callback',
	},
});

starting an authorization flow

Caution

the built-in handle resolution makes use of Bluesky-hosted services to return the intended DID, and this can mean sharing the user's IP address and the handle identifiers to Bluesky.

while Bluesky has a declared privacy policy, both developers and users need to be informed and aware of the privacy implications of this arrangement. read this guide on how you can implement your own resolution code.

if your application involves asking for the user's handle or DID, you can use resolveFromIdentity which resolves the user's identity to get its PDS, and the metadata of its authorization server.

import { resolveFromIdentity } from '@atcute/oauth-browser-client';

const { identity, metadata } = await resolveFromIdentity('mary.my.id');

alternatively, if it involves asking for the user's PDS, then you can use resolveFromService which just grabs the authorization server metadata.

import { resolveFromService } from '@atcute/oauth-browser-client';

const { metadata } = await resolveFromService('bsky.social');

we can then proceed with authorization by calling createAuthorizationUrl with the resolved metadata (and identity, if using resolveFromIdentity) along with the scope of the authorization, which should either match the one in your client metadata, or a reduced set of it.

import { createAuthorizationUrl } from '@atcute/oauth-browser-client';

// passing `identity` is optional,
// it allows for the login form to be autofilled with the user's handle or DID
const authUrl = await createAuthorizationUrl({
	metadata: metadata,
	identity: identity,
	scope: 'atproto transition:generic transition:chat.bsky',
});

// recommended to wait for the browser to persist local storage before proceeding
await sleep(200);

// redirect the user to sign in and authorize the app
window.location.assign(authUrl);

// if this is on an async function, ideally the function should never ever resolve.
// the only way it should resolve at this point is if the user aborted the authorization
// by returning back to this page (thanks to back-forward page caching)
await new Promise((_resolve, reject) => {
	const listener = () => {
		reject(new Error(`user aborted the login request`));
	};

	window.addEventListener('pageshow', listener, { once: true });
});

finalizing authorization

once the user has been redirected to your redirect URL, we can call finalizeAuthorization with the parameters that have been provided.

import { XRPC } from '@atcute/client';
import { OAuthUserAgent, finalizeAuthorization } from '@atcute/oauth-browser-client';

// `createAuthorizationUrl` asks for the server to redirect here with the
// parameters assigned in the hash, not the search string.
const params = new URLSearchParams(location.hash.slice(1));

// this is optional, but after retrieving the parameters, we should ideally
// scrub it from history to prevent this authorization state to be replayed,
// just for good measure.
history.replaceState(null, '', location.pathname + location.search);

// you'd be given a session object that you can then pass to OAuthUserAgent!
const session = await finalizeAuthorization(params);

// now you can start making requests!
const agent = new OAuthUserAgent(session);

// pass it onto the XRPC so you can make RPC calls with the PDS.
{
	const rpc = new XRPC({ handler: agent });

	const { data } = await rpc.get('com.atproto.identity.resolveHandle', {
		params: {
			handle: 'mary.my.id',
		},
	});
}

// or, use it directly!
{
	const response = await agent.handle('/xrpc/com.atproto.identity.resolveHandle?handle=mary.my.id');
}

the session object returned by finalizeAuthorization should not be stored anywhere else, as it is already persisted in the internal database. you are expected to keep track of who's signed in and who was last signed in for your own UI, as the sessions stored by the database is not guaranteed to be permanent (mostly if they don't come with a refresh token.)

resuming existing sessions

you can resume existing sessions by calling getSession with the DID identifier you intend to resume.

import { XRPC } from '@atcute/client';
import { OAuthUserAgent, getSession } from '@atcute/oauth-browser-client';

const session = await getSession('did:plc:ia76kvnndjutgedggx2ibrem', { allowStale: true });

const agent = new OAuthUserAgent(session);
const rpc = new XRPC({ handler: agent });

removing sessions

you can manually remove sessions via deleteStoredSession, but ideally, you should revoke the token first before doing so.

import { OAuthUserAgent, deleteStoredSession, getSession } from '@atcute/oauth-browser-client';

const did = 'did:plc:ia76kvnndjutgedggx2ibrem';

try {
	const session = await getSession(did, { allowStale: true });
	const agent = new OAuthUserAgent(session);

	await agent.signOut();
} catch (err) {
	// `signOut` also deletes the session, we only serve as fallback if it fails.
	deleteStoredSession(did);
}

additional guide

configuring your Vite project

you might want to configure the server options in your Vite config so you'll never end up visiting your app in localhost, which is specifically forbidden by AT Protocol's OAuth, let's change it so it'll always use 127.0.0.1:

/// vite.config.ts
import { defineConfig } from 'vite';

const SERVER_HOST = '127.0.0.1';
const SERVER_PORT = 12520;

export default defineConfig({
	server: {
		host: SERVER_HOST,
		port: SERVER_PORT,
	},
});

additionally, to make it easier to develop locally and deploy to production, you should consider adding a plugin that'll inject the necessary values for you through environment variables:

/// vite.config.ts
import metadata from './public/oauth/client-metadata.json' with { type: 'json' };

export default defineConfig({
	// ...

	plugins: [
		// injects OAuth-related environment variables
		{
			config(_conf, { command }) {
				if (command === 'build') {
					process.env.VITE_OAUTH_CLIENT_ID = metadata.client_id;
					process.env.VITE_OAUTH_REDIRECT_URI = metadata.redirect_uris[0];
				} else {
					const redirectUri = (() => {
						const url = new URL(metadata.redirect_uris[0]);
						return `http://${SERVER_HOST}:${SERVER_PORT}${url.pathname}`;
					})();

					const clientId =
						`http://localhost` +
						`?redirect_uri=${encodeURIComponent(redirectUri)}` +
						`&scope=${encodeURIComponent(metadata.scope)}`;

					process.env.VITE_DEV_SERVER_PORT = '' + SERVER_PORT;
					process.env.VITE_OAUTH_CLIENT_ID = clientId;
					process.env.VITE_OAUTH_REDIRECT_URI = redirectUri;
				}

				process.env.VITE_CLIENT_URI = metadata.client_uri;
				process.env.VITE_OAUTH_SCOPE = metadata.scope;
			},
		},
	],
});

we'll augment the type declarations to get type-checking on it:

/// src/vite-env.d.ts

interface ImportMetaEnv {
	readonly VITE_DEV_SERVER_PORT?: string;
	readonly VITE_CLIENT_URI: string;
	readonly VITE_OAUTH_CLIENT_ID: string;
	readonly VITE_OAUTH_REDIRECT_URI: string;
	readonly VITE_OAUTH_SCOPE: string;
}

interface ImportMeta {
	readonly env: ImportMetaEnv;
}

et voilà! you can now use this to configure the client.

configureOAuth({
	metadata: {
		client_id: import.meta.env.VITE_OAUTH_CLIENT_ID,
		redirect_uri: import.meta.env.VITE_OAUTH_REDIRECT_URI,
	},
});

// ... later during sign-in process
const authUrl = await createAuthorizationUrl({
	metadata: metadata,
	identity: identity,
	scope: import.meta.env.VITE_OAUTH_SCOPE,
});

adjust the code here as necessary, the plugin adds more environment variables than what is actually needed, you can remove them if you don't think you'd need it.

doing handle resolution

there are two ways that a handle can be verified:

  1. HTTP verification: there is a file at /.well-known/atproto-did containing your account's DID
  2. DNS verification: there is an _atproto TXT record containing your account's DID

you'd want to resolve both of these. if both methods return a response but does not match each other then it should ideally be thrown.

verify that the DID matches the intended format

const isDid = (did: string): did is At.DID => {
	return /^did:([a-z]+):([a-zA-Z0-9._:%-]*[a-zA-Z0-9._-])$/.test(did);
};

pass this resolved DID to resolveFromIdentity, and carry on as per usual.

HTTP handle resolution

this is very straightforward, make a request to https://<handle>/.well-known/atproto-did without following redirects. check if the response status is 200 and trim off any excess whitespaces.

some web servers might not set a permissible CORS header to access this resource, in which case there is nothing that can be done, unless you'd want to proxy the requests.

const resolveHandleViaHttp = async (handle: string): Promise<At.DID> => {
	const url = new URL('/.well-known/atproto-did', `https://${handle}`);

	const response = await fetch(url, { redirect: 'error' });
	if (!response.ok) {
		throw new ResolverError(`domain is unreachable`);
	}

	const text = await response.text();

	const did = text.split('\n')[0]!.trim();
	if (isDid(did)) {
		return did;
	}

	throw new ResolverError(`failed to resolve ${handle}`);
};

DNS handle resolution

as websites can't do DNS resolution on their own, we'd have to rely on DNS-over-HTTPS (DoH) services. it should be noted that this can have privacy implications of its own, please read through the privacy policy of whichever DoH service you end up using and make the user aware of it as well.

for this example, we'll be using Cloudflare's DoH resolver for Firefox (privacy policy) as it has support for application/dns-json format which allows us to query and see the responses in JSON.

const SUBDOMAIN = '_atproto';
const PREFIX = 'did=';

const resolveHandleViaDoH = async (handle: string): Promise<At.DID> => {
	const url = new URL('https://mozilla.cloudflare-dns.com/dns-query');
	url.searchParams.set('type', 'TXT');
	url.searchParams.set('name', `${SUBDOMAIN}.${handle}`);

	const response = await fetch(url, {
		method: 'GET',
		headers: { accept: 'application/dns-json' },
		redirect: 'follow',
	});

	const type = response.headers.get('content-type')?.trim();
	if (!response.ok) {
		const message = type?.startsWith('text/plain')
			? await response.text()
			: `failed to resolve ${handle}`;

		throw new ResolverError(message);
	}

	if (type !== 'application/dns-json') {
		throw new ResolverError(`unexpected response from DoH server`);
	}

	const result = asResult(await response.json());
	const answers = result.Answer?.filter(isAnswerTxt).map(extractTxtData) ?? [];

	for (let i = 0; i < answers.length; i++) {
		// skip if the line does not start with "did="
		if (!answers[i].startsWith(PREFIX)) {
			continue;
		}

		// ensure there is no other entry starting with "did="
		for (let j = i + 1; j < answers.length; j++) {
			if (answers[j].startsWith(PREFIX)) {
				break;
			}
		}

		const did = answers[i].slice(PREFIX.length);
		if (isDid(did)) {
			return did;
		}

		break;
	}

	throw new ResolverError(`failed to resolve ${handle}`);
};

type Result = { Status: number; Answer?: Answer[] };
const isResult = (result: unknown): result is Result => {
	if (result === null || typeof result !== 'object') {
		return false;
	}

	return (
		'Status' in result &&
		typeof result.Status === 'number' &&
		(!('Answer' in result) || (Array.isArray(result.Answer) && result.Answer.every(isAnswer)))
	);
};
const asResult = (result: unknown): Result => {
	if (!isResult(result)) {
		throw new TypeError(`unexpected DoH response`);
	}

	return result;
};

type Answer = { name: string; type: number; data: string; TTL: number };
const isAnswer = (answer: unknown): answer is Answer => {
	if (answer === null || typeof answer !== 'object') {
		return false;
	}

	return (
		'name' in answer &&
		typeof answer.name === 'string' &&
		'type' in answer &&
		typeof answer.type === 'number' &&
		'data' in answer &&
		typeof answer.data === 'string' &&
		'TTL' in answer &&
		typeof answer.TTL === 'number'
	);
};

type AnswerTxt = Answer & { type: 16 };
const isAnswerTxt = (answer: Answer): answer is AnswerTxt => {
	return answer.type === 16;
};

const extractTxtData = (answer: AnswerTxt): string => {
	return answer.data.replace(/^"|"$/g, '').replace(/\\"/g, '"');
};

using your PDS for handle resolution

alternatively, if you operate your own PDS, you can make use of it as a handle resolver.

const resolveHandleViaPds = async (handle: string): Promise<At.DID> => {
	const rpc = new XRPC({ handler: simpleFetchHandler({ service: `https://my-pds.example.com` }) });

	const { data } = await rpc.get('com.atproto.identity.resolveHandle', {
		params: {
			handle: handle,
		},
	});

	return data.did;
};