# EVMTransaction > Retrieve the transaction data from Ethereum, Flare, or Songbird. > For the complete documentation index, see [llms.txt](/llms.txt). Markdown versions of documentation pages are available by appending `.md` to the page URL. Source: https://dev.flare.network/fdc/guides/hardhat/evm-transaction The [`EVMTransaction`](/fdc/attestation-types/evm-transaction) attestation type enables data collection about a transaction on an EVM chain. The currently supported chain are: `ETH`, `FLR`, and `SGB`. You can learn more about it in the official [specification repo](/fdc/attestation-types/evm-transaction). We will now demonstrate how the FDC protocol can be used to collect the data of a given Ethereum transaction. The transaction we will be observing has the hash `0x4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c`; this is an arbitrary transaction that we acquired from the Sepolia Ethereum testnet [explorer](https://sepolia.etherscan.io/). The same procedure works for all supported sources, `ETH`, `FLR`, and `SGB`. The source then requires only a slight modification; we will remind you of that when it comes up in the guide. In this guide, we will be following the steps outlined in the [FDC Overview](/fdc/overview). We will define a `scripts/fdcExample/EVMTransaction.ts` file that will encapsulate the whole process. The shared helpers used below live in the starter under `scripts/utils/fdc.ts` and `scripts/utils/getters.ts`. scripts/fdcExample/EVMTransaction.ts ``` import { run, web3 } from "hardhat";import { TransferEventListenerInstance } from "../../typechain-types";import { prepareAttestationRequestBase, submitAttestationRequest, retrieveDataAndProofBase,} from "../utils/fdc";const EVMTransaction = artifacts.require("TransferEventListener");const { VERIFIER_URL_TESTNET, VERIFIER_API_KEY_TESTNET, COSTON2_DA_LAYER_URL } = process.env;...async function main() { const data = await prepareAttestationRequest(transactionHash); console.log("Data:", data, "\n"); const abiEncodedRequest = data.abiEncodedRequest; const roundId = await submitAttestationRequest(abiEncodedRequest); const proof = await retrieveDataAndProof(abiEncodedRequest, roundId); const eventListener: TransferEventListenerInstance = await deployAndVerifyContract(); await interactWithContract(eventListener, proof);}main().then((data) => { process.exit(0);}); ``` The function names mostly mirror the steps described in the [FDC guide](/fdc/overview). ## Prepare request[​](#prepare-request "Direct link to Prepare request") In this guide we will demonstrate how to prepare an attestation request through a verifier server. At the end of the section we will provide a breakdown of the abi encoded request; thus we will demonstrate how it can be constructed manually. To use the verifier server, we send a request to its `prepareRequest` endpoint. A JSON request to the verifier follows the same structure for all attestation types, with field values varying between types. ### Required Fields[​](#required-fields "Direct link to Required Fields") - `attestationType` is the UTF8 hex string encoding of the attestation type name, zero-padded to 32 bytes. - `sourceId` is the UTF8 hex string encoding of the data source identifier name, zero-padded to 32 bytes. - `requestBody` is different for each attestation type. In the case of `EVMTransaction`, `requestBody` is a JSON containing the fields: - `transactionHash`: hash (address) of the observed transaction - `requiredConfirmations`: the depth of the block containing the transaction at which it is considered confirmed, i.e. when the transaction itself is considered confirmed; as `uint16` - `provideInput`: a `bool` determining whether the `input` field is included in the response - `listEvents`: a `bool` determining whether the `events` field is included in the response - `logIndices`: an `uint32` array of indices of the events to be included in the response; if `listEvents` is set to false `false` and this field is not `[]`, the attestation request will fail ### Reference Documentation[​](#reference-documentation "Direct link to Reference Documentation") - [EVMTransaction Specification](/fdc/attestation-types/evm-transaction) - [Verifier Interactive Docs](https://fdc-verifiers-testnet.flare.network/verifier/api-doc#/) ### Example Values[​](#example-values "Direct link to Example Values") - `transactionHash`: `0x4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c` - `requiredConfirmations`: `1` - `provideInput`: `true` - `listEvents`: `true` - `logIndices`: `[]` - `urlTypeBase`: string `eth` - Replace `eth` with `sgb` or `flr` for other chains. scripts/fdcExample/EVMTransaction.ts ``` // Request dataconst transactionHash = "0x4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c";// Configuration constantsconst attestationTypeBase = "EVMTransaction";const sourceIdBase = "testETH";const verifierUrlBase = VERIFIER_URL_TESTNET;const urlTypeBase = "eth"; ``` ### Encoding Functions[​](#encoding-functions "Direct link to Encoding Functions") To encode values into UTF8 hex: - `toUtf8HexString`: Converts a string to UTF8 hex. - `toHex`: Zero-right-pads the string to 32 bytes. These functions are included in the [Base library](https://github.com/flare-foundation/flare-hardhat-starter/blob/master/scripts/fdcExample/Base.ts) within the [example repository](https://github.com/flare-foundation/flare-hardhat-starter/), but they can also be defined locally in your contract or script. scripts/utils/fdc.ts ``` function toHex(data: string) { var result = ""; for (var i = 0; i < data.length; i++) { result += data.charCodeAt(i).toString(16); } return result.padEnd(64, "0");} ``` scripts/utils/fdc.ts ``` function toUtf8HexString(data: string) { return "0x" + toHex(data);} ``` Because of the `console.log` commands it will produce JSON strings that represent valid requests; we can then pass this to the [interactive verifier](https://fdc-verifiers-testnet.flare.network/verifier/api-doc/) to check what the response will be. The process of posting a request to a verifier server is identical for all attestation types. It differs only in values used. For that reason we define a base function that the `prepareAttestationRequest` function will call. The `prepareAttestationRequestBase` function formulates a request for the verifier server, and posts it to the given URL. scripts/utils/fdc.ts ``` async function prepareAttestationRequestBase( url: string, apiKey: string, attestationTypeBase: string, sourceIdBase: string, requestBody: any,) { console.log("Url:", url, "\n"); const attestationType = toUtf8HexString(attestationTypeBase); const sourceId = toUtf8HexString(sourceIdBase); const request = { attestationType: attestationType, sourceId: sourceId, requestBody: requestBody, }; console.log("Prepared request:\n", request, "\n"); const response = await fetch(url, { method: "POST", headers: { "X-API-KEY": apiKey, "Content-Type": "application/json", }, body: JSON.stringify(request), }); if (response.status != 200) { throw new Error( `Response status is not OK, status ${response.status} ${response.statusText}\n`, ); } console.log("Response status is OK\n"); return await response.json();} ``` In the example repository, it is once again included within the [Base](https://github.com/flare-foundation/flare-foundry-starter/blob/master/scripts/fdcExample/Base.s.sol) library file. We construct the URL by appending to the verifier address `https://fdc-verifiers-testnet.flare.network/` the path `/verifier/eth/EVMTransaction/prepareRequest`. If we were using another source, we would replace the string `eth` with `sgb` or `flr` accordingly (we would also have to replace `testETH` with `testSGB` or `testFLR`). Thus, the function that prepares the verifier request looks like: scripts/fdcExample/EVMTransaction.ts ``` async function prepareAttestationRequest(transactionHash: string) { const requiredConfirmations = "1"; const provideInput = true; const listEvents = true; const logIndices: string[] = []; const requestBody = { transactionHash: transactionHash, requiredConfirmations: requiredConfirmations, provideInput: provideInput, listEvents: listEvents, logIndices: logIndices, }; const url = `${verifierUrlBase}/verifier/${urlTypeBase}/EVMTransaction/prepareRequest`; const apiKey = VERIFIER_API_KEY_TESTNET!; return await prepareAttestationRequestBase( url, apiKey, attestationTypeBase, sourceIdBase, requestBody, );} ```
Understanding the abiEncodedRequest. Understanding the `abiEncodedRequest`. If everything went right, the `abiEncodedRequest` should look something like this (minus the line breaks - we split it after the `0x` symbol and then after every 64 characters (32 bytes), for the sake of clarity). ``` 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 ``` Let's break it down line by line: - **First line:** `toUtf8HexString("EVMTransaction")` - **Second line:** `toUtf8HexString("testETH")` - **Third line:** message integrity code (MIC), a hash of the whole response salted with a string `"Flare"`, ensures the integrity of the attestation - **Remaining lines:** ABI encoded `EVMTransaction.RequestBody` Solidity struct What this demonstrates is that, with some effort, the `abiEncodedRequest` can be constructed manually.
## Submit request to FDC[​](#submit-request-to-fdc "Direct link to Submit request to FDC") This step transitions from offchain request preparation to onchain interaction with the FDC protocol. We will submit the validated request to the blockchain using deployed official Flare smart contracts. To streamline the process of accessing these, the [Flare smart contracts periphery package](https://www.npmjs.com/package/@flarenetwork/flare-periphery-contracts) is shipped with the `ContractRegistry` library. The `IFlareContractRegistry` deployed at `0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019` on every Flare-family network resolves names to live contract addresses. We use it to discover: - `FdcHub`: for posting the request to - `FdcRequestFeeConfigurations`: calculates the fee of the request - `FlareSystemsManager`: for calculating the round ID - `Relay`: confirms the round has finalized - `FdcVerification`: exposes the FDC protocol id and verification entry points The starter resolves these from TypeScript via small wrappers in `scripts/utils/getters.ts` that call `IFlareContractRegistry.getContractAddressByName(...)` and return a typechain instance. To submit the attestation request, we first access the deployed `FdcHub` contract. We determine the fee for our attestation type, and then request the attestation of the FDC, paying the required fee. Lastly, we calculate the voting round Id from the transaction that carried the attestation request; we will need it to query the data and proof. scripts/utils/fdc.ts ``` async function submitAttestationRequest(abiEncodedRequest: string) { const fdcHub = await getFdcHub(); const requestFee = await getFdcRequestFee(abiEncodedRequest); const transaction = await fdcHub.requestAttestation(abiEncodedRequest, { value: requestFee, }); console.log("Submitted request:", transaction.tx, "\n"); const roundId = await calculateRoundId(transaction); console.log( `Check round progress at: https://${hre.network.name}-systems-explorer.flare.rocks/voting-round/${roundId}?tab=fdc\n`, ); return roundId;} ``` scripts/utils/getters.ts ``` async function getFdcHub() { const FlareContractRegistry = artifacts.require("IFlareContractRegistry"); const registry: IFlareContractRegistryInstance = await FlareContractRegistry.at( "0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019", ); const fdcHubAddress: string = await registry.getContractAddressByName("FdcHub"); return await FdcHub.at(fdcHubAddress);} ``` The request fee is obtained from the `fdcRequestFeeConfigurations` contract. We once again resolve its address through the `IFlareContractRegistry`. scripts/utils/fdc.ts ``` async function getFdcRequestFee(abiEncodedRequest: string) { const FlareContractRegistry = artifacts.require("IFlareContractRegistry"); const registry: IFlareContractRegistryInstance = await FlareContractRegistry.at( "0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019", ); const fdcRequestFeeConfigurationsAddress: string = await registry.getContractAddressByName("FdcRequestFeeConfigurations"); const fdcRequestFeeConfigurations: IFdcRequestFeeConfigurationsInstance = await FdcRequestFeeConfigurations.at(fdcRequestFeeConfigurationsAddress); return await fdcRequestFeeConfigurations.getRequestFee(abiEncodedRequest);} ``` The round ID is calculate from the timestamp of the block, containing the transaction requesting attestation. We first subtract from the block timestamp the timestamp of the first voting epoch. Then, we divide the number by the duration of the voting epoch (90 seconds). Instead of hard-coding them, we retrieve these values from another official Flare contract, the `flareSystemsManager`. scripts/utils/fdc.ts ``` async function calculateRoundId(transaction: any) { const blockNumber = transaction.receipt.blockNumber; const block = await ethers.provider.getBlock(blockNumber); const blockTimestamp = BigInt(block!.timestamp); const flareSystemsManager: IFlareSystemsManagerInstance = await getFlareSystemsManager(); const firsVotingRoundStartTs = BigInt( await flareSystemsManager.firstVotingRoundStartTs(), ); const votingEpochDurationSeconds = BigInt( await flareSystemsManager.votingEpochDurationSeconds(), ); console.log("Block timestamp:", blockTimestamp, "\n"); console.log("First voting round start ts:", firsVotingRoundStartTs, "\n"); console.log( "Voting epoch duration seconds:", votingEpochDurationSeconds, "\n", ); const roundId = Number( (blockTimestamp - firsVotingRoundStartTs) / votingEpochDurationSeconds, ); console.log("Calculated round id:", roundId, "\n"); console.log( "Received round id:", Number(await flareSystemsManager.getCurrentVotingEpochId()), "\n", ); return roundId;} ``` We obtain the `flareSystemsManager` contract through the `IFlareContractRegistry`. scripts/utils/getters.ts ``` async function getFlareSystemsManager() { const FlareContractRegistry = artifacts.require("IFlareContractRegistry"); const registry: IFlareContractRegistryInstance = await FlareContractRegistry.at( "0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019", ); const flareSystemsManagerAddress: string = await registry.getContractAddressByName("FlareSystemsManager"); return await FlareSystemsManager.at(flareSystemsManagerAddress);} ``` ## Retrieve data and proof[​](#retrieve-data-and-proof "Direct link to Retrieve data and proof") To retrieve the data and proof, we must first wait for the voting round in which the attestation request was submitted to finalize; this takes no more than 180 seconds, but is on average much less. After the round has been finalized, we post a request to a DA Layer provider. We can check if the request was submitted successfully on the [AttestationRequests](https://coston2-systems-explorer.flare.rocks/attestation-request) page on the Flare Systems Explorer website. To check if the round has been finalized, go to [Finalizations](https://coston2-systems-explorer.flare.rocks/finalizations) page. If you want to learn more about how the FDC protocol works, check [here](/fdc/overview). Because the process includes waiting for the voting round to finalize, we prepare a `sleep` function. The function pauses the execution of the script for a given number of milliseconds. scripts/utils/fdc.ts ``` function sleep(ms: number) { return new Promise((resolve) => setTimeout(resolve, ms));} ``` The only difference between the `retrieveDataAndProof` functions of all six attestation types is the URL of the DA Layer server. For that reason, we will define as separate `retrieveDataAndProofBase` function that will handle most of the logic. The function waits for the round to finalize - rechecking every 10 seconds if necessary. Then, it prepares a proof request, and posts it to the DA Layer server. Because it might take a few seconds for the server to generate the proof, the function ensures that the response actually contains a sufficient response, and retries otherwise. scripts/utils/fdc.ts ``` async function retrieveDataAndProofBase( url: string, abiEncodedRequest: string, roundId: number,) { console.log("Waiting for the round to finalize..."); // We check every 10 seconds if the round is finalized const relay: IRelayInstance = await getRelay(); const fdcVerification: IFdcVerificationInstance = await getFdcVerification(); const protocolId = await fdcVerification.fdcProtocolId(); while (!(await relay.isFinalized(protocolId, roundId))) { await sleep(10000); } console.log("Round finalized!\n"); const request = { votingRoundId: roundId, requestBytes: abiEncodedRequest, }; console.log("Prepared request:\n", request, "\n"); await sleep(10000); var proof = await postRequestToDALayer(url, request, true); console.log("Waiting for the DA Layer to generate the proof..."); while (proof.response_hex == undefined) { await sleep(5000); proof = await postRequestToDALayer(url, request, false); } console.log("Proof generated!\n"); console.log("Proof:", proof, "\n"); return proof;} ``` We access the Flare's official `Relay` and `FdcVerification` contracts with helper functions. scripts/utils/getters.ts ``` async function getRelay() { const FlareContractRegistry = artifacts.require("IFlareContractRegistry"); const registry: IFlareContractRegistryInstance = await FlareContractRegistry.at( "0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019", ); const relayAddress: string = await registry.getContractAddressByName("Relay"); return await IRelay.at(relayAddress);}async function getFdcVerification() { const FlareContractRegistry = artifacts.require("IFlareContractRegistry"); const registry: IFlareContractRegistryInstance = await FlareContractRegistry.at( "0xaD67FE66660Fb8dFE9d6b1b4240d8650e30F6019", ); const fdcVerificationAddress: string = await registry.getContractAddressByName("FdcVerification"); return await IFdcVerification.at(fdcVerificationAddress);} ``` The following function posts a proof request to the DA Layer. scripts/utils/fdc.ts ``` async function postRequestToDALayer( url: string, request: any, watchStatus: boolean = false,) { const response = await fetch(url, { method: "POST", headers: { // "X-API-KEY": "", "Content-Type": "application/json", }, body: JSON.stringify(request), }); if (watchStatus && response.status != 200) { throw new Error( `Response status is not OK, status ${response.status} ${response.statusText}\n`, ); } else if (watchStatus) { console.log("Response status is OK\n"); } return await response.json();} ``` The main prepare the URL of the DA Layer's `proof-by-request-raw` endpoint. We contact this specific endpoint, because it return the abi encoded `IEVMTransaction.Response` struct, and is thus unambiguous. scripts/fdcExample/EVMTransaction.ts ``` async function retrieveDataAndProof( abiEncodedRequest: string, roundId: number,) { const url = `${COSTON2_DA_LAYER_URL}/api/v1/fdc/proof-by-request-round-raw`; console.log("Url:", url, "\n"); return await retrieveDataAndProofBase(url, abiEncodedRequest, roundId);} ``` The response the DA Layer server returns has the following fields: - The field `attestationType` holds the UTF8 encoded hex string of the attestation type name, padded to 32 bytes. Thus, it should match the value of the `attestationType` parameter in the Prepare the request step. In our case, that value is `0x45564d5472616e73616374696f6e000000000000000000000000000000000000`. - The array `proofs` holds the Merkle proofs of our attestation request. - Lastly, `responseHex` is the ABI encoding of the chosen attestation type response struct. In this case, it is the `IEVMTransaction.Response` struct. We can ascertain the form of the proof request, as well as examine the response in advance, trough the [interactive documentation](https://ctn2-data-availability.flare.network/api-doc#/) of the DA Layer server.
An example complete proof response and decoded IEVMTransaction.Response. An example complete proof response and decoded `IEVMTransaction.Response`. An example DA Layer response for a request using the data provided in this example is: ``` { response_hex: '0x 0000000000000000000000000000000000000000000000000000000000000020 45564d5472616e73616374696f6e000000000000000000000000000000000000 7465737445544800000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000000000000e6c2c 0000000000000000000000000000000000000000000000000000000067724b20 00000000000000000000000000000000000000000000000000000000000000c0 0000000000000000000000000000000000000000000000000000000000000180 4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000001 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000000 000000000000000000000000000000000000000000000000000000000070acc6 0000000000000000000000000000000000000000000000000000000067724b20 00000000000000000000000070ad32b82b4fe2821c798e628d93645218e2a806 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 00000000000000000000000000000000000000000000000000d8b72d434c8000 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000520 00000000000000000000000000000000000000000000000000000000000003c5 3593564c00000000000000000000000000000000000000000000000000000000 0000006000000000000000000000000000000000000000000000000000000000 000000a000000000000000000000000000000000000000000000000000000000 6772521a00000000000000000000000000000000000000000000000000000000 000000040b000604000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000 0000000400000000000000000000000000000000000000000000000000000000 0000008000000000000000000000000000000000000000000000000000000000 000000e000000000000000000000000000000000000000000000000000000000 0000020000000000000000000000000000000000000000000000000000000000 0000028000000000000000000000000000000000000000000000000000000000 0000004000000000000000000000000000000000000000000000000000000000 0000000200000000000000000000000000000000000000000000000000d8b72d 434c800000000000000000000000000000000000000000000000000000000000 0000010000000000000000000000000000000000000000000000000000000000 0000000200000000000000000000000000000000000000000000000000d8b72d 434c800000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000 000000a000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000 0000002bfff9976782d46cc05630d1f6ebab18b2324d6b140001f41c7d4b196c b0c7b01d743fbc6116a902379c72380000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000 000000600000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902 379c7238000000000000000000000000e49acc3b16c097ec88dc9352ce4cd57a b7e35b9500000000000000000000000000000000000000000000000000000000 0000001900000000000000000000000000000000000000000000000000000000 000000600000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902 379c723800000000000000000000000070ad32b82b4fe2821c798e628d936452 18e2a80600000000000000000000000000000000000000000000000000000000 ad2090e40c000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000006 00000000000000000000000000000000000000000000000000000000000000c0 0000000000000000000000000000000000000000000000000000000000000200 0000000000000000000000000000000000000000000000000000000000000360 00000000000000000000000000000000000000000000000000000000000004c0 00000000000000000000000000000000000000000000000000000000000006a0 0000000000000000000000000000000000000000000000000000000000000800 000000000000000000000000000000000000000000000000000000000000003f 000000000000000000000000fff9976782d46cc05630d1f6ebab18b2324d6b14 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000100 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000002 e1fffcc4923d04b559f4d29a8bfc6cda04eb5b0d3c460751c2402c5c5cc9109c 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 0000000000000000000000000000000000000000000000000000000000000020 00000000000000000000000000000000000000000000000000d8b72d434c8000 0000000000000000000000000000000000000000000000000000000000000040 0000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902379c7238 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000003 ddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef 0000000000000000000000003289680dd4d6c10bb19b899729cda5eef58aeff1 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 0000000000000000000000000000000000000000000000000000000000000020 00000000000000000000000000000000000000000000000000000000ae6dcda8 0000000000000000000000000000000000000000000000000000000000000041 000000000000000000000000fff9976782d46cc05630d1f6ebab18b2324d6b14 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000003 ddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 0000000000000000000000003289680dd4d6c10bb19b899729cda5eef58aeff1 0000000000000000000000000000000000000000000000000000000000000020 00000000000000000000000000000000000000000000000000d8b72d434c8000 0000000000000000000000000000000000000000000000000000000000000042 0000000000000000000000003289680dd4d6c10bb19b899729cda5eef58aeff1 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000003 c42079f94a6350d7e6235f29174924f928cc2ac818eb64fed8004e115fbcca67 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 00000000000000000000000000000000000000000000000000000000000000a0 ffffffffffffffffffffffffffffffffffffffffffffffffffffffff51923258 00000000000000000000000000000000000000000000000000d8b72d434c8000 00000000000000000000000000000000000011d79ac448fce087b0605d7423c8 000000000000000000000000000000000000000000000000002231596d817570 000000000000000000000000000000000000000000000000000000000002925f 0000000000000000000000000000000000000000000000000000000000000043 0000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902379c7238 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000003 ddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 000000000000000000000000e49acc3b16c097ec88dc9352ce4cd57ab7e35b95 0000000000000000000000000000000000000000000000000000000000000020 00000000000000000000000000000000000000000000000000000000006fa26f 0000000000000000000000000000000000000000000000000000000000000044 0000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902379c7238 00000000000000000000000000000000000000000000000000000000000000a0 0000000000000000000000000000000000000000000000000000000000000120 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000003 ddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef 0000000000000000000000003fc91a3afd70395cd496c647d5a6cc9d4b2b7fad 00000000000000000000000070ad32b82b4fe2821c798e628d93645218e2a806 0000000000000000000000000000000000000000000000000000000000000020 00000000000000000000000000000000000000000000000000000000adfe2b39', attestation_type: '0x45564d5472616e73616374696f6e000000000000000000000000000000000000', proof: [ '0x9251c0e3047688af1305daf61f2b757527b731e7d1fad3c71d08734772fbebeb', '0xad5fdf0f8cb6bc42cdab5affb8f03a1fadaf1ef60875af76344b7ca3ab694c9b', '0xead06bac3be86604034e138784c86f0b14f2481c001e31e17d03c185488033dc' ]} ``` The `proof` field is dependent on the round in which the attestation request was submitted; it contains proofs for all of the requests submitted in that round. In the case of a single attestation request it is an empty list `[]` (the proof is the merkle root itself). The decoded `IEVMTransaction.Response` struct is: ``` [ attestationType: '0x45564d5472616e73616374696f6e000000000000000000000000000000000000', sourceId: '0x7465737445544800000000000000000000000000000000000000000000000000', votingRound: '945196', lowestUsedTimestamp: '1735543584', requestBody: [ '0x4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c', '1', true, true, [], transactionHash: '0x4e636c6590b22d8dcdade7ee3b5ae5572f42edb1878f09b3034b2f7c3362ef3c', requiredConfirmations: '1', provideInput: true, listEvents: true, logIndices: [] ], responseBody: [ '7384262', '1735543584', '0x70Ad32B82B4FE2821C798e628d93645218E2A806', false, '0x3fC91A3afd70395Cd496C647d5a6CC9D4B2b7FAD', '61000000000000000', '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', '1', [ ... ], blockNumber: '7384262', timestamp: '1735543584', sourceAddress: '0x70Ad32B82B4FE2821C798e628d93645218E2A806', isDeployment: false, receivingAddress: '0x3fC91A3afd70395Cd496C647d5a6CC9D4B2b7FAD', value: '61000000000000000', input: '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', status: '1', events: [ ... ] ]] ```
## Use the data[​](#use-the-data "Direct link to Use the data") We will now define a simple contract, that will demonstrate how the data can be used onchain. The contract will receive data and proof of an Ethereum transaction, and store all token transfers contained into an array of `TokenTransfer` structs. It will do so only if the transaction is valid. src/fdcExample/EVMTransaction.sol ``` struct TokenTransfer { address from; address to; uint256 value;} ``` The code of the contract is as follows. src/fdcExample/EVMTransaction.sol ``` contract TransferEventListener is ITransferEventListener { TokenTransfer[] public tokenTransfers; address public USDC_CONTRACT = 0x1c7D4B196Cb0C7B01d743Fbc6116a902379C7238; // USDC contract address on sepolia function isEVMTransactionProofValid( IEVMTransaction.Proof calldata transaction ) public view returns (bool) { // Use the library to get the verifier contract and verify that this transaction was proved by the FDC IFdcVerification fdc = ContractRegistry.getFdcVerification(); return fdc.verifyEVMTransaction(transaction); } function collectTransferEvents( IEVMTransaction.Proof calldata _transaction ) external { // 1. FDC Logic // Check that this EVMTransaction has indeed been confirmed by the FDC require( isEVMTransactionProofValid(_transaction), "Invalid transaction proof" ); // 2. Business logic // Go through all events for ( uint256 i = 0; i < _transaction.data.responseBody.events.length; i++ ) { // Get current event IEVMTransaction.Event memory _event = _transaction .data .responseBody .events[i]; // Disregard events that are not from the USDC contract if (_event.emitterAddress != USDC_CONTRACT) { continue; } // Disregard non Transfer events if ( _event.topics.length == 0 || // No topics // The topic0 doesn't match the Transfer event _event.topics[0] != keccak256(abi.encodePacked("Transfer(address,address,uint256)")) ) { continue; } // We now know that this is a Transfer event from the USDC contract - and therefore know how to decode topics and data // Topic 1 is the sender address sender = address(uint160(uint256(_event.topics[1]))); // Topic 2 is the receiver address receiver = address(uint160(uint256(_event.topics[2]))); // Data is the amount uint256 value = abi.decode(_event.data, (uint256)); // Add the transfer to the list tokenTransfers.push( TokenTransfer({from: sender, to: receiver, value: value}) ); } } function getTokenTransfers() external view returns (TokenTransfer[] memory) { TokenTransfer[] memory result = new TokenTransfer[]( tokenTransfers.length ); for (uint256 i = 0; i < tokenTransfers.length; i++) { result[i] = tokenTransfers[i]; } return result; }} ``` ### Verify proof[​](#verify-proof "Direct link to Verify proof") FDC optimizes onchain storage costs by implementing a hybrid data verification system. Instead of storing complete datasets onchain, it stores only Merkle proofs, while maintaining the actual data through trusted offchain providers. This approach significantly reduces gas costs while preserving data integrity. When requested, data providers supply the original data along with its corresponding Merkle proof. The protocol verifies data authenticity by comparing the provided Merkle proof against the onchain Merkle root. A successful match confirms the data's integrity and authenticity within the FDC system. While data verification is optional if you trust your data provider, FDC ensures transparency by making verification possible at any time. This capability is crucial for maintaining system integrity and allowing users to independently verify data when needed, particularly in production environments. FDC provides verification functionality through the `FdcVerification` contract. We then access the `FdcVerification` contract through the `ContractRegistry`, and feed it the proof. If we proof is valid, the function `verifyEVMTransaction` will return `true`, otherwise `false`. We deploy and verify this contract with the `deployAndVerifyContract` function in the `scripts/fdcExample/EVMTransaction.ts` file. scripts/fdcExample/EVMTransaction.ts ``` async function deployAndVerifyContract() { const args: any[] = []; const eventListener: TransferEventListenerInstance = await EVMTransaction.new( ...args, ); try { await run("verify:verify", { address: eventListener.address, constructorArguments: args, }); } catch (e: any) { console.log(e); } console.log("EVMTransaction deployed to", eventListener.address, "\n"); return eventListener;} ``` ## Interact with contract[​](#interact-with-contract "Direct link to Interact with contract") We define an additional function that allows us to interact with the just deployed contract. The `interactWithContract` function also takes the proof retrieved in the previous step as an argument. It abi decodes the `response_hex` value to an `IEVMTransaction.Response` struct. From that and the array of proofs, it constructs an `IEVMTransaction.Proof` object, on which it call the `registerAddress` function of the `AddressRegistry` contract deployed above. The contract verifies the address, and the script prints it to the console. scripts/fdcExample/EVMTransaction.ts ``` async function interactWithContract( eventListener: TransferEventListenerInstance, proof: any,) { console.log("Proof hex:", proof.response_hex, "\n"); // A piece of black magic that allows us to read the response type from an artifact const IEVMTransactionVerification = await artifacts.require( "IEVMTransactionVerification", ); const responseType = IEVMTransactionVerification._json.abi[0].inputs[0].components[1]; console.log("Response type:", responseType, "\n"); const decodedResponse = web3.eth.abi.decodeParameter( responseType, proof.response_hex, ); console.log("Decoded proof:", decodedResponse, "\n"); const transaction = await eventListener.collectTransferEvents({ merkleProof: proof.proof, data: decodedResponse, }); console.log("Transaction:", transaction.tx, "\n"); console.log("Token transfer:", await eventListener.tokenTransfers(0), "\n");} ``` We can run the whole script by calling the following console command. ``` yarn hardhat run scripts/fdcExample/EVMTransaction.ts --network coston2 ```