better-auth-siws — Sign-In With Solana (SIWS) for Better Auth

Simple SIWS plugin for Better Auth. It provides two endpoints — start and verify — and a helper to build the canonical SIWS message.

• Server plugin: siwsPlugin
• Client plugin: siwsClientPlugin
• Message helper: buildSiwsMessage

Installation

pnpm add better-auth-siws better-auth bs58

You will also need a wallet on the client that can signMessage (e.g. via @solana/wallet-adapter). For tests or Node-only examples, you can use @solana/web3.js or tweetnacl.

---

Server setup (Better Auth)

import { betterAuth } from "better-auth";
import { siwsPlugin } from "better-auth-siws";

export const auth = betterAuth({
  baseURL: "https://app.example.com/api/auth",
  security: {
    trustedOrigins: ["https://app.example.com"],
  },
  plugins: [
    siwsPlugin({
      domain: "app.example.com",          // required; must match your app domain (no protocol)
      statement: "Sign in with Solana to MyApp.", // optional
      nonceTtlSeconds: 300,                // optional; default 300 seconds
    }),
  ],
});

The plugin registers two endpoints under your baseURL:

• POST /siws/start → returns { nonce, domain, uri }
• POST /siws/verify → verifies signature, creates a Better Auth session, and sets cookies

When used through Better Auth, these are exposed as auth.api.start and auth.api.verify.

---

Client setup (Better Auth client)

import { createAuthClient } from "better-auth/client";
import { siwsClientPlugin } from "better-auth-siws/client";

export const clientAuth = createAuthClient({
  baseURL: "https://app.example.com/api/auth",
  plugins: [siwsClientPlugin()],
});

This exposes clientAuth.api.start and clientAuth.api.verify in the browser.

---

End-to-end flow (browser)

Below is a minimal browser flow using a wallet that supports signMessage.

import bs58 from "bs58";
import { buildSiwsMessage } from "better-auth-siws";
import { clientAuth } from "./clientAuth"; // your file that creates the client

async function signInWithSolana(wallet: { publicKey: { toBase58: () => string }, signMessage: (data: Uint8Array) => Promise<Uint8Array> }) {
  // 1) Ask server for a nonce
  const address = wallet.publicKey.toBase58();
  const { nonce, domain, uri } = await clientAuth.api.start({
    body: { address },
  });

  // 2) Build the canonical SIWS message
  const message = buildSiwsMessage({
    domain,
    address,
    uri,
    nonce,
    issuedAt: new Date().toISOString(),
    // optionally add: statement, expirationTime, resources
  });

  // 3) Have the wallet sign the message
  const signatureBytes = await wallet.signMessage(new TextEncoder().encode(message));
  const signature = bs58.encode(signatureBytes);

  // 4) Verify with the server → sets Better Auth session cookies on success
  const result = await clientAuth.api.verify({
    body: { address, message, signature },
  });

  // `result` includes the user and session; cookies are set by the server response
  return result;
}

Cookies are set by the server via Better Auth, so subsequent requests are authenticated automatically.

---

Node-only example (using @solana/web3.js)

import { betterAuth } from "better-auth";
import { buildSiwsMessage, siwsPlugin } from "better-auth-siws";
import { Keypair } from "@solana/web3.js";
import bs58 from "bs58";
import nacl from "tweetnacl";

const auth = betterAuth({
  baseURL: "http://localhost:3000/api/auth",
  security: { trustedOrigins: ["http://localhost:3000"] },
  plugins: [siwsPlugin({ domain: "app.example.com" })],
});

async function nodeFlow() {
  const kp = Keypair.generate();
  const address = kp.publicKey.toBase58();

  const { nonce, domain, uri } = await auth.api.start({
    body: { address },
    headers: { origin: "http://localhost:3000" },
  });

  const message = buildSiwsMessage({
    domain,
    address,
    uri,
    nonce,
    issuedAt: new Date().toISOString(),
  });

  const signature = bs58.encode(
    nacl.sign.detached(new TextEncoder().encode(message), kp.secretKey)
  );

  const result = await auth.api.verify({
    body: { address, message, signature },
    headers: { origin: "http://localhost:3000" },
  });

  console.log(result);
}

---

API Reference

siwsPlugin(options)

interface SiwsOptions {
  domain: string;            // e.g., "app.example.com" (no protocol)
  statement?: string;
  nonceTtlSeconds?: number;  // default 300
}

• start (POST /siws/start): accepts { address }, returns { nonce, domain, uri } and stores a one-time nonce.
• verify (POST /siws/verify): accepts { address, message, signature }, verifies signature, binds domain, creates a session, and sets cookies.

siwsClientPlugin()

Registers the same endpoints on the client instance so you can call clientAuth.api.start/verify from the browser.

buildSiwsMessage(input)

const message = buildSiwsMessage({
  domain: string,
  address: string,      // base58
  uri: string,          // server baseURL
  statement?: string,
  nonce: string,
  issuedAt: string,     // ISO timestamp
  expirationTime?: string,
  resources?: string[],
});

Returns a canonical SIWS message string that the wallet must sign.

---

cURL

# Start
curl -X POST \
  -H "Content-Type: application/json" \
  -d '{"address":"<BASE58_PUBLIC_KEY>"}' \
  https://app.example.com/api/auth/siws/start

# Verify (after you build the message and produce base58 signature)
curl -X POST \
  -H "Content-Type: application/json" \
  -d '{
    "address":"<BASE58_PUBLIC_KEY>",
    "message":"<RAW_MESSAGE_STRING>",
    "signature":"<BASE58_SIGNATURE>"
  }' \
  https://app.example.com/api/auth/siws/verify -i

---

Notes & Troubleshooting

• Domain binding: The message must start with your configured domain. If you rename domains or use multiple environments, ensure options.domain matches where your client runs.
• Nonce TTL: Defaults to 300s. verify deletes the nonce after one use.
• Cookies: On success, the server sets Better Auth session cookies. Ensure the client is on the same site or that your fetch layer forwards and accepts cookies as needed.
• Invalid signature (401): Usually the signature was produced by a different key than the address (or message mutated). Rebuild message and re-sign.
• Nonce invalid/expired (400): Call start again and sign a fresh message.

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License

MIT © João Veiga