Wallet Signatures

An AuthSig is a wallet signature obtained from a user. Wallet signatures are required to communicate with the Lit Nodes and authorize requests.

Format of an AuthSig

You can use any signature compliant with EIP-4361, also known as Sign in with Ethereum (SIWE), for the AuthSig. However, the signature must be presented in an AuthSig object formatted like so:

{
  "sig": "0x18720b54cf0d29d618a90793d5e76f4838f04b559b02f1f01568d8e81c26ae9536e11bb90ad311b79a5bc56149b14103038e5e03fee83931a146d93d150eb0f61c",
  "derivedVia": "web3.eth.personal.sign",
  "signedMessage": "localhost wants you to sign in with your Ethereum account:\n0x1cD4147AF045AdCADe6eAC4883b9310FD286d95a\n\nThis is a test statement.  You can put anything you want here.\n\nURI: https://localhost/login\nVersion: 1\nChain ID: 1\nNonce: gzdlw7mR57zMcGFzz\nIssued At: 2022-04-15T22:58:44.754Z",
  "address": "0x1cD4147AF045AdCADe6eAC4883b9310FD286d95a"
}

In the AuthSig data structure:

• sig is the signature produced by signing the signedMessage
• derivedVia is the method used to derive the signature (e.g., "web3.eth.personal.sign")
• signedMessage is the original message that was signed
• address is the public key address that was used to create the signature

You can refer to the AuthSig type definition in the Lit JS SDK V2.

Obtaining an AuthSig in the browser

Using checkAndSignAuthMessage

The Lit SDK checkAndSignAuthMessage() function provides a convenient way to obtain an AuthSig from an externally-owned account in a browser environment.

import { checkAndSignAuthMessage } from "@lit-protocol/lit-node-client@serrano";

const authSig = await checkAndSignAuthMessage({
  chain: "ethereum",
});

When called, checkAndSignAuthMessage triggers a wallet selection popup in the user's browser. The user is then asked to sign a message, confirming ownership of their crypto address. The signature of the signed message is returned as the authSig variable.

The function also stores the AuthSig in local storage, removing the need for the user to sign the message again. However, if the signature expires or becomes too old, the user may be prompted to sign the message again.

checkAndSignAuthMessage checks the currently selected chain in the user's wallet. If user's wallet supports it, the function sends a request to the user's wallet to change to the chain specified in the checkAndSignAuthMessage() function call. This ensures that the user is interacting with the correct blockchain network.

Using signAndSaveAuthMessage

If you prefer to implement your own wallet selection interface, you can call the signAndSaveAuthMessage() function, which offers more customization. To use this function, pass in an instance of an ethers.js Web3Provider object, the wallet address, the chain ID, and the signature expiration time.

import { ethConnect } from "@lit-protocol/auth-browser";

const authSig = await ethConnect.signAndSaveAuthMessage({
  web3: web3Provider,
  account: walletAddress,
  chainId: 1,
  expiration: new Date(Date.now() + 1000 * 60 * 60 * 24).toISOString(),
});

note

Be sure to import cosmosConnect and solConnect for Cosmos and Solana respectively.

Using EIP-1271 for Account Abstraction

In general, smart contracts can't produce an AuthSig since they don't possess a private key. However, you can generate an AuthSig for smart contracts using EIP-1271, a standard for verifying signatures when the account is a smart contract.

Following the same data structure as above, you can format your smart contract AuthSig like so:

• sig is the actual hex-encoded signature
• derivedVia must be "EIP1271" to inform the nodes that this AuthSig is for smart contracts
• signedMessage is any string that you want to pass to the isValidSignature(bytes32 _hash, bytes memory _signature) as the _hash argument
• address is the address of the smart contract

note

The smart contract must implement the isValidSignature(bytes32 _hash, bytes memory _signature) function since the Lit Nodes will call this function to validate the AuthSig. Refer to the EIP-1271 docs to understand the isValidSignature function.

You can present the smart contract AuthSig object to the Lit Nodes just like any other AuthSig.

Check out this React project for an example of how to generate and use a smart contract AuthSig.

Clearing Local Storage

If you want to clear the AuthSig stored in local storage, you can call the disconnectWeb3 method.

Obtaining an AuthSig on the server-side

If you want to obtain an AuthSig on the server-side, you can instantiate an ethers.Signer to sign a SIWE message, which will produce a signature that can be used in an AuthSig object.

const LitJsSdk = require("@lit-protocol/lit-node-client-nodejs");
const { ethers } = require("ethers");
const siwe = require("siwe");

async function main() {
  // Initialize LitNodeClient
  const litNodeClient = new LitJsSdk.LitNodeClientNodeJs();
  await litNodeClient.connect();

  // Initialize the signer
  const wallet = new ethers.Wallet("<Your private key>");
  const address = ethers.utils.getAddress(await wallet.getAddress());

  // Craft the SIWE message
  const domain = "localhost";
  const origin = "https://localhost/login";
  const statement =
    "This is a test statement.  You can put anything you want here.";
  const siweMessage = new siwe.SiweMessage({
    domain,
    address: address,
    statement,
    uri: origin,
    version: "1",
    chainId: "1",
  });
  const messageToSign = siweMessage.prepareMessage();

  // Sign the message and format the authSig
  const signature = await wallet.signMessage(messageToSign);

  const authSig = {
    sig: signature,
    derivedVia: "web3.eth.personal.sign",
    signedMessage: messageToSign,
    address: address,
  };

  console.log(authSig);
}

main();