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Version: v2.x.x

Signed Chain Data

Cross chain communication and authentication without provisioning access beforehand‚Äč

If you'd like to request that the Lit Network sign the result of a smart contract function call, you can do that using the getSignedChainDataToken function of the LitNodeClient.

This will perform a smart contract function RPC call, sign the response, and then return a JWT. You may send this JWT to a server, or, send it into a smart contract to enable a cross-chain communication use-case. Solidity code to verify the signature is not yet available.

To call a function, you need the smart contract ABI. For this example, we will use the Chainlink price oracle smart contract.

const aggregatorV3InterfaceABI = [
{
inputs: [],
name: "decimals",
outputs: [{ internalType: "uint8", name: "", type: "uint8" }],
stateMutability: "view",
type: "function",
},
{
inputs: [],
name: "description",
outputs: [{ internalType: "string", name: "", type: "string" }],
stateMutability: "view",
type: "function",
},
{
inputs: [
{ internalType: "uint80", name: "_roundId", type: "uint80" },
],
name: "getRoundData",
outputs: [
{ internalType: "uint80", name: "roundId", type: "uint80" },
{ internalType: "int256", name: "answer", type: "int256" },
{ internalType: "uint256", name: "startedAt", type: "uint256" },
{ internalType: "uint256", name: "updatedAt", type: "uint256" },
{
internalType: "uint80",
name: "answeredInRound",
type: "uint80",
},
],
stateMutability: "view",
type: "function",
},
{
inputs: [],
name: "latestRoundData",
outputs: [
{ internalType: "uint80", name: "roundId", type: "uint80" },
{ internalType: "int256", name: "answer", type: "int256" },
{ internalType: "uint256", name: "startedAt", type: "uint256" },
{ internalType: "uint256", name: "updatedAt", type: "uint256" },
{
internalType: "uint80",
name: "answeredInRound",
type: "uint80",
},
],
stateMutability: "view",
type: "function",
},
{
inputs: [],
name: "version",
outputs: [{ internalType: "uint256", name: "", type: "uint256" }],
stateMutability: "view",
type: "function",
},
];

Next, you'll need the smart contract address

const addr = "0x5f4eC3Df9cbd43714FE2740f5E3616155c5b8419";

Encode the call data using the encodeCallData function

const callData = LitJsSdk.encodeCallData({
abi: aggregatorV3InterfaceABI,
functionName: "latestRoundData",
functionParams: [],
});

Request the signed chain data token from the Lit Network. Note that this requires a connected LitNodeClient.

const jwt = await litNodeClient.getSignedChainDataToken({
callRequests,
chain: 'ethereum',
});

You can then extract the various parts of the JWT, and verify the signature, using the verifyJwt function.

const { verified, header, payload, signature } = LitJsSdk.verifyJwt({
jwt,
});

The responses to the function calls live in the payload.callResponses array. You can decode them using the decodeCallResult function.

const decoded = LitJsSdk.decodeCallResult({
abi: aggregatorV3InterfaceABI,
functionName: "latestRoundData",
data: payload.callResponses[0],
});

At this point, the function call response is in the decoded variable. For this smart contract call, the current price of ETH in USD is in decoded.answer as a BigNumber.

You can now verify the signature and be certain that the smart contract function call returned that result at the time it was called (if the signature is genuine).