Combine Signatures Within an Action

info

Only available on the test network

Overview

When you sign a message with using Lit's PKPs, signature shares are typically combined client-side to form the complete signature. The signAndCombineEcdsa function allows you to combine these shares within a Lit Action, meaning they will remain within the confines of each Lit node's Trusted Execution Environment (TEE) and not exposed to the end client.

When you call signAndCombineEcdsa, the signature shares are collected from each Lit node before they are combined on a single node.

Signing a message

const code = `(async () => {
  // sign "hello world" and allow all the nodes to combine the signature and return it to the action.
  const utf8Encode = new TextEncoder();
  const toSign = utf8Encode.encode('Hello World');

  // Will use the authentication provided to the `executeJs` call from the sdk on the client.
  const signature = await Lit.Actions.signAndCombineEcdsa({
    toSign,
    publicKey,
    sigName,
  });
  
  // Set the response from the action as the signature share which will not need combination on the client
  Lit.Actions.setResponse({ response: JSON.stringify(signature) });
})()`;

const client = new LitNodeClient({
    litNetwork: 'cayenne',
});
await client.connect();
const res = await client.executeJs({
    code,
    sessionSigs: {} // your session
    jsParams: {
      publicKey: "<your pkp public key>",
      sigName: 'fooSig',
    }
});

console.log("response from singing in a transaction: ", res);

Signing a Transaction

With the built in EthersJS we are able to take advantage of the serializeTxnForSigning implementations and serialize a transaction, which is then signed, combined and then sent back to the client.

const code = `(async () => {
  const sigName = "sig1";
  // example transaction
  let txn = {
      to: "0x70997970C51812dc3A010C7d01b50e0d17dc79C8",
      value: 1000000000000000,
      gasPrice: 20000000000,
      nonce: 0,
  };

  // using ether's serializeTransaction
  // https://docs.ethers.org/v5/api/utils/transactions/#transactions--functions
  const serializedTx = ethers.utils.serializeTransaction(txn);
  let hash = utils.keccak256(ethers.utils.toUtf8Bytes(serializedTx));
  // encode the message into an uint8array for signing
  const toSign = await new TextEncoder().encode(hash);
  const signature = await Lit.Actions.signAndCombineEcdsa({
      toSign,
      publicKey,
      sigName,
  });

  // here we're setting the response to the signature output, but there's no need to do this
  // if your use case requires the signature to not be seen by the client
  Lit.Actions.setResponse({
    response: signature
  });
})();
`;

const client = new LitNodeClient({
    litNetwork: 'cayenne',
});

await client.connect();
const res = await client.executeJs({
    code,
    sessionSigs: {} // your session
    jsParams: {
      publicKey: "<your pkp public key>",
    }
});
console.log("result from singing in a lit action", res);

Using Signatures from Within A Lit Action

For an example of how you may use the signature from signAndCombineEcdsa from within the Lit Action see here.