Lucia

Tutorial: Username and password auth in SvelteKit

Before starting, make sure you've set up your database and middleware as described in the Getting started page.

An example project based on this tutorial is also available. You can clone the example locally or open it in StackBlitz.

npx degit https://github.com/lucia-auth/examples/tree/main/sveltekit/username-and-password <directory_name>

Update database

Add a username and password_hash column to your user table.

column type attributes
username string unique
password_hash string

Create a DatabaseUserAttributes interface in the module declaration and add your database columns. By default, Lucia will not expose any database columns to the User type. To add a username field to it, use the getUserAttributes() option.

// src/lib/server/auth.ts
import { Lucia } from "lucia";
import { dev } from "$app/environment";

export const lucia = new Lucia(adapter, {
	sessionCookie: {
		attributes: {
			secure: !dev
		}
	},
	getUserAttributes: (attributes) => {
		return {
			// attributes has the type of DatabaseUserAttributes
			username: attributes.username
		};
	}
});

declare module "lucia" {
	interface Register {
		Lucia: typeof lucia;
		DatabaseUserAttributes: DatabaseUserAttributes;
	}
}

interface DatabaseUserAttributes {
	username: string;
}

Sign up user

Create routes/signup/+page.svelte and set up a basic form.

<!-- routes/signup/+page.svelte -->
<script lang="ts">
	import { enhance } from "$app/forms";
</script>

<h1>Sign up</h1>
<form method="post" use:enhance>
	<label for="username">Username</label>
	<input name="username" id="username" /><br />
	<label for="password">Password</label>
	<input type="password" name="password" id="password" /><br />
    <button>Continue</button>
</form>

Create a form action in routes/signup/+page.server.ts. First, do a very basic input validation. Hash the password, generate a new user ID, and create a new user. If successful, create a new session with Lucia.createSession() and set a new session cookie.

// routes/signup/+page.server.ts
import { lucia } from "$lib/server/auth";
import { fail, redirect } from "@sveltejs/kit";
import { generateIdFromEntropySize } from "lucia";
import { hash } from "@node-rs/argon2";

import type { Actions } from "./$types";

export const actions: Actions = {
	default: async (event) => {
		const formData = await event.request.formData();
		const username = formData.get("username");
		const password = formData.get("password");
		// username must be between 4 ~ 31 characters, and only consists of lowercase letters, 0-9, -, and _
		// keep in mind some database (e.g. mysql) are case insensitive
		if (
			typeof username !== "string" ||
			username.length < 3 ||
			username.length > 31 ||
			!/^[a-z0-9_-]+$/.test(username)
		) {
			return fail(400, {
				message: "Invalid username"
			});
		}
		if (typeof password !== "string" || password.length < 6 || password.length > 255) {
			return fail(400, {
				message: "Invalid password"
			});
		}

		const userId = generateIdFromEntropySize(10); // 16 characters long
		const passwordHash = await hash(password, {
			// recommended minimum parameters
			memoryCost: 19456,
			timeCost: 2,
			outputLen: 32,
			parallelism: 1
		});

		// TODO: check if username is already used
		await db.table("user").insert({
			id: userId,
			username: username,
			password_hash: passwordHash
		});

		const session = await lucia.createSession(userId, {});
		const sessionCookie = lucia.createSessionCookie(session.id);
		event.cookies.set(sessionCookie.name, sessionCookie.value, {
			path: ".",
			...sessionCookie.attributes
		});

		redirect(302, "/");
	}
};

Argon2id should be your first choice for hashing passwords, followed by Scrypt and Bcrypt. Hashing is by definition computationally expensive so you should use the most performant option for your runtime.

  • For Node.js we recommend using @node-rs/argon2.
  • For Bun, we recommend using Bun.password.
  • Use Deno-specific packages for Deno.
  • For other runtimes (e.g. Cloudflare Workers), your choice is very limited. @noble/hashes provides pure-js implementations of various hashing algorithms, but because it's written in JS, you may hit into CPU limitations of your service. If possible, avoid these runtimes when you need to hash passwords.

Make sure to check the recommended minimum parameters for your hashing algorithm.

Sign in user

Create routes/login/+page.svelte and set up a basic form.

<!-- routes/login/+page.svelte -->
<script lang="ts">
	import { enhance } from "$app/forms";
</script>

<h1>Sign in</h1>
<form method="post" use:enhance>
	<label for="username">Username</label>
	<input name="username" id="username" /><br />
	<label for="password">Password</label>
	<input type="password" name="password" id="password" /><br />
	<button>Continue</button>
</form>

Create a form action in routes/login/+page.server.ts. First, do a very basic input validation. Get the user with the username and verify the password. If successful, create a new session with Lucia.createSession() and set a new session cookie.

// routes/login/+page.server.ts
import { lucia } from "$lib/server/auth";
import { fail, redirect } from "@sveltejs/kit";
import { verify } from "@node-rs/argon2";

import type { Actions } from "./$types";

export const actions: Actions = {
	default: async (event) => {
		const formData = await event.request.formData();
		const username = formData.get("username");
		const password = formData.get("password");

		if (
			typeof username !== "string" ||
			username.length < 3 ||
			username.length > 31 ||
			!/^[a-z0-9_-]+$/.test(username)
		) {
			return fail(400, {
				message: "Invalid username"
			});
		}
		if (typeof password !== "string" || password.length < 6 || password.length > 255) {
			return fail(400, {
				message: "Invalid password"
			});
		}

		const existingUser = await db
			.table("username")
			.where("username", "=", username.toLowerCase())
			.get();
		if (!existingUser) {
			// NOTE:
			// Returning immediately allows malicious actors to figure out valid usernames from response times,
			// allowing them to only focus on guessing passwords in brute-force attacks.
			// As a preventive measure, you may want to hash passwords even for invalid usernames.
			// However, valid usernames can be already be revealed with the signup page among other methods.
			// It will also be much more resource intensive.
			// Since protecting against this is non-trivial,
			// it is crucial your implementation is protected against brute-force attacks with login throttling etc.
			// If usernames are public, you may outright tell the user that the username is invalid.
			return fail(400, {
				message: "Incorrect username or password"
			});
		}

		const validPassword = await verify(existingUser.password_hash, password, {
			memoryCost: 19456,
			timeCost: 2,
			outputLen: 32,
			parallelism: 1
		});
		if (!validPassword) {
			return fail(400, {
				message: "Incorrect username or password"
			});
		}

		const session = await lucia.createSession(existingUser.id, {});
		const sessionCookie = lucia.createSessionCookie(session.id);
		event.cookies.set(sessionCookie.name, sessionCookie.value, {
			path: ".",
			...sessionCookie.attributes
		});

		redirect(302, "/");
	}
};

Validate requests

You can validate requests by checking locals.user. The field user.username is available since we defined the getUserAttributes() option. You can protect pages, such as /, by redirecting unauthenticated users to the login page.

// +page.server.ts
import type { PageServerLoad, Actions } from "./$types";

export const load: PageServerLoad = async (event) => {
	if (!event.locals.user) redirect(302, "/login");
	return {
		username: event.locals.user.username
	};
};

Sign out user

Sign out users by invalidating their session with Lucia.invalidateSession(). Make sure to remove their session cookie by setting a blank session cookie created with Lucia.createBlankSessionCookie().

// routes/+page.server.ts
import { lucia } from "$lib/server/auth";
import { fail, redirect } from "@sveltejs/kit";

import type { Actions, PageServerLoad } from "./$types";

export const load: PageServerLoad = async ({ locals }) => {
	// ...
};

export const actions: Actions = {
	default: async (event) => {
		if (!event.locals.session) {
			return fail(401);
		}
		await lucia.invalidateSession(event.locals.session.id);
		const sessionCookie = lucia.createBlankSessionCookie();
		event.cookies.set(sessionCookie.name, sessionCookie.value, {
			path: ".",
			...sessionCookie.attributes
		});
		redirect(302, "/login");
	}
};

<!-- routes/+page.svelte -->
<script lang="ts">
	import { enhance } from "$app/forms";
</script>

<form method="post" use:enhance>
    <button>Sign out</button>
</form>