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Address Validity

The AddressValidity attestation type validates whether a string represents a valid address on supported blockchain networks (BTC, DOGE, and XRP). This validation ensures addresses meet chain-specific formatting and checksum requirements before they're used in transactions or smart contracts. The full specification is available on the official specification repo.

The primary contract interface for this attestation type is IAddressValidity. Let's walk through validating a Bitcoin testnet address using the FDC protocol. We will use the address mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs as an example throughout this guide. You can swap this with any valid testnet address from the supported chains. You can follow this tutorial with any other valid address - just make sure it is a valid testnet address.

This validation process works identically for BTC, DOGE, and XRP addresses, with only minor chain-specific parameter adjustments which we'll highlight throughout the guide.

In this guide, we will follow the steps outlined in the FDC overview. We will define a scripts/fdcExample/AddressValidity.ts file that will encapsulate the whole process.

scripts/fdcExample/AddressValidity.s.sol
import { run, web3 } from "hardhat";
import { AddressRegistryInstance } from "../../typechain-types";
import {
  prepareAttestationRequestBase,
  submitAttestationRequest,
  retrieveDataAndProofBase,
} from "./Base";

const AddressRegistry = artifacts.require("AddressRegistry");

const { VERIFIER_URL_TESTNET, VERIFIER_API_KEY, COSTON2_DA_LAYER_URL } =
  process.env;

...

async function main() {
  const data = await prepareAttestationRequest(addressStr);
  console.log("Data:", data, "\n");

  const abiEncodedRequest = data.abiEncodedRequest;
  const roundId = await submitAttestationRequest(abiEncodedRequest);

  const proof = await retrieveDataAndProof(abiEncodedRequest, roundId);

  const addressRegistry: AddressRegistryInstance =
    await deployAndVerifyContract();

  await interactWithContract(addressRegistry, proof);
}

main().then((data) => {
  process.exit(0);
});

The function names mostly mirror the steps described in the FDC guide.

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​

  • attestationType: UTF8 hex string encoding of the attestation type name, zero-padded to 32 bytes.
  • sourceId: UTF8 hex string encoding of the data source identifier name, zero-padded to 32 bytes.
  • requestBody: Specific to each attestation type.

For AddressValidity, requestBody contains a single field:

  • addressString: The address to verify.

Reference Documentation​

Example Values​

  • addressString: mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs.
  • attestationType: UTF8 hex encoding of AddressValidity, zero-padded to 32 bytes.
  • sourceId: UTF8 hex encoding of testBTC, zero-padded to 32 bytes.
    • "test" prefix denotes Bitcoin testnet.
    • Supports deployment on Flare testchains (Coston or Coston2).
    • Replace testBTC with testDOGE or testXRP for other chains.
  • urlTypeBase: string btc
    • Replace btc with doge or xrp for other chains.
scripts/fdcExample/AddressValidity.ts
// Request data
const addressStr = "mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs";

// Configuration constants
const attestationTypeBase = "AddressValidity";
const sourceIdBase = "testBTC";
const verifierUrlBase = VERIFIER_URL_TESTNET;
const urlTypeBase = "btc";

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 within the example repository, but they can also be defined locally in your contract or script.

scripts/fdcExample/Base.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/fdcExample/Base.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 to check what the response is.

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/fdcExample/Base.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 library file.

We construct the URL by appending to the verifier address https://fdc-verifiers-testnet.flare.network/ the path verifier/btc/AddressValidity/prepareRequest. If we were using another source, we would replace the string btc with doge or xrp accordingly (we would also have to replace testBTC with testDOGE or testXRP). Thus, the function that prepares the verifier request looks like:

scripts/fdcExample/AddressValidity.ts
async function prepareAttestationRequest(addressStr: string) {
  const requestBody = {
    addressStr: addressStr,
  };

  const url = `${verifierUrlBase}verifier/${urlTypeBase}/AddressValidity/prepareRequest`;
  const apiKey = VERIFIER_API_KEY ?? "";

  return await prepareAttestationRequestBase(
    url,
    apiKey,
    attestationTypeBase,
    sourceIdBase,
    requestBody,
  );
}
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).

0x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Let's break it down line by line:

  • First line: toUtf8HexString("AddressValidity")
  • Second line: toUtf8HexString("testBTC")
  • 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 AddressValidity.RequestBody Solidity struct

What this demonstrates is that, with some effort, the abiEncodedRequest can be constructed manually.

Submit request to FDC​

This step transitions from off-chain request preparation to on-chain 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 is shipped with the ContractRegistry library.

The ContractRegistry library allows us to

We define a Helpers contract that will give us access to the following contracts:

  • 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
contracts/fdcExample/Helpers.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.25;

import {console} from "hardhat/console.sol";
import {ContractRegistry} from "@flarenetwork/flare-periphery-contracts/coston2/ContractRegistry.sol";
import {IFdcHub} from "@flarenetwork/flare-periphery-contracts/coston2/IFdcHub.sol";
import {IFdcRequestFeeConfigurations} from
    "@flarenetwork/flare-periphery-contracts/coston2/IFdcRequestFeeConfigurations.sol";
import {IFlareSystemsManager} from "@flarenetwork/flare-periphery-contracts/coston2/IFlareSystemsManager.sol";
import {IRelay} from "@flarenetwork/flare-periphery-contracts/coston2/IRelay.sol";

contract Helpers {
    function getFdcHub() public view returns (IFdcHub) {
        return ContractRegistry.getFdcHub();
    }

    function getFdcRequestFeeConfigurations() public view returns (IFdcRequestFeeConfigurations) {
        return ContractRegistry.getFdcRequestFeeConfigurations();
    }

    function getFlareSystemsManager() public view returns (IFlareSystemsManager) {
        return ContractRegistry.getFlareSystemsManager();
    }

    function getRelay() public view returns (IRelay) {
        return ContractRegistry.getRelay();
    }
}

We expose the Helpers contract through the following function.

scripts/fdcExample/Base.ts
async function getHelpers() {
  const helpers: HelpersInstance = await Helpers.new();
  return helpers;
}

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/fdcExample/Base.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://${hre.network.name}-systems-explorer.flare.rocks/voting-epoch/${roundId}?tab=fdc\n`,
  );
  return roundId;
}
scripts/fdcExample/Base.ts
async function getFdcHub() {
  const helpers: HelpersInstance = await getHelpers();
  const fdcHubAddress: string = await helpers.getFdcHub();
  return await FdcHub.at(fdcHubAddress);
}

The request fee is obtained from the fdcRequestFeeConfigurations contract. We once again connect to the fdcRequestFeeConfigurations contract through the ContractRegistry library.

scripts/fdcExample/Base.ts
async function getFdcRequestFee(abiEncodedRequest: string) {
  const helpers: HelpersInstance = await getHelpers();
  const fdcRequestFeeConfigurationsAddress: string =
    await helpers.getFdcRequestFeeConfigurations();
  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/fdcExample/Base.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 ContractRegistry library and the previously defined Helpers contract as well.

scripts/fdcExample/Base.ts
async function getFlareSystemsManager() {
  const helpers: HelpersInstance = await getHelpers();
  const flareSystemsManagerAddress: string =
    await helpers.getFlareSystemsManager();
  return await FlareSystemsManager.at(flareSystemsManagerAddress);
}

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 page on the Flare Systems Explorer website. To check if the round has been finalized, go to Finalizations page.

If you want to learn more about how the FDC protocol works, check here.

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/fdcExample/Base.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/fdcExample/AddressValidity.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();
  while (!(await relay.isFinalized(200, 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 contract with a helper function.

scripts/fdcExample/Base.ts
async function getRelay() {
  const helpers: HelpersInstance = await getHelpers();
  const relayAddress: string = await helpers.getRelay();
  return await IRelay.at(relayAddress);
}

The following function posts a proof request to the DA Layer.

scripts/fdcExample/Base.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 IAddressValidity.Response struct, and is thus unambiguous.

scripts/fdcExample/AddressValidity.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 0x4164647265737356616c69646974790000000000000000000000000000000000.
  • 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 IAddressValidity.Response struct.

We can ascertain the form of the proof request, as well as examine the response in advance, trough the interactive documentation of the DA Layer server.

An example complete proof response and decoded IAddressValidity.Response.

An example DA Layer response for a request using the data provided in this example is:

{
  response_hex: "0x
  0000000000000000000000000000000000000000000000000000000000000020
  4164647265737356616c69646974790000000000000000000000000000000000
  7465737442544300000000000000000000000000000000000000000000000000
  00000000000000000000000000000000000000000000000000000000000e6bda
  000000000000000000000000000000000000000000000000ffffffffffffffff
  00000000000000000000000000000000000000000000000000000000000000c0
  0000000000000000000000000000000000000000000000000000000000000140
  0000000000000000000000000000000000000000000000000000000000000020
  0000000000000000000000000000000000000000000000000000000000000022
  6d6739503966347772397737633173674665695443356f4d4c59584363326337
  6873000000000000000000000000000000000000000000000000000000000000
  0000000000000000000000000000000000000000000000000000000000000001
  0000000000000000000000000000000000000000000000000000000000000060
  6810e152510fe893f9cc8954c4dfaecd5c2be00e2732d6fe3e25922f30c5a3c5
  0000000000000000000000000000000000000000000000000000000000000022
  6d6739503966347772397737633173674665695443356f4d4c59584363326337
  6873000000000000000000000000000000000000000000000000000000000000",
  attestation_type: "0x4164647265737356616c69646974790000000000000000000000000000000000",
  proof: [
    "0x275dc338dd4e6a0a8749caa098c6749e0e77e22ba9db264f334b5dfb79aa6321",
    "0x084e002bbe12f4a163d82ddd17861d1d3131c816fe3b998d575d134043a6c8f1",
    "0xc30304c7d430e3d0f83d05017035f13ca19dec2799917745967f4c48685eab49",
    "0x4d622137c9e7c9a1fa3a5d2942a183a8e926ba8659fe606495ea994acbb6ec0f"
  ]
}

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 IAddressValidity.Response struct is:

[
  attestationType: "0x4164647265737356616c69646974790000000000000000000000000000000000",
  sourceId: "0x7465737442544300000000000000000000000000000000000000000000000000",
  votingRound: "945114",
  lowestUsedTimestamp: "18446744073709551615",
  requestBody: [
    "mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs",
    addressStr: "mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs"
  ],
  responseBody: [
    true,
    "mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs",
    "0x6810e152510fe893f9cc8954c4dfaecd5c2be00e2732d6fe3e25922f30c5a3c5",
    isValid: true,
    standardAddress: "mg9P9f4wr9w7c1sgFeiTC5oMLYXCc2c7hs",
    standardAddressHash: "0x6810e152510fe893f9cc8954c4dfaecd5c2be00e2732d6fe3e25922f30c5a3c5"
  ]
]

Use the data​

We will now define a simple contract, that will demonstrate how the data can be used onchain. The contract will receive an address and proof, and decide if the address is valid. If the address is valid, the contract will add it to an array of valid addresses. Otherwise, it will raise an error.

The code for this contract is as follows.

contracts/fdcExample/AddressValidity.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.25;

import {ContractRegistry} from "@flarenetwork/flare-periphery-contracts/coston2/ContractRegistry.sol";
import {IFdcVerification} from "@flarenetwork/flare-periphery-contracts/coston2/IFdcVerification.sol";
import {IAddressValidity} from "@flarenetwork/flare-periphery-contracts/coston2/IAddressValidity.sol";

interface IAddressRegistry {
    function registerAddress(IAddressValidity.Proof memory _transaction) external;
}

contract AddressRegistry is IAddressRegistry {
    string[] public verifiedAddresses;

    function isAddressValidityProofValid(IAddressValidity.Proof memory transaction) public view returns (bool) {
        // Use the library to get the verifier contract and verify that this transaction was proved by state connector
        IFdcVerification fdc = ContractRegistry.getFdcVerification();
        return fdc.verifyAddressValidity(transaction);
    }

    function registerAddress(IAddressValidity.Proof memory _transaction) public {
        // 1. FDC Logic
        // Check that this AddressValidity has indeed been confirmed by the FDC
        require(isAddressValidityProofValid(_transaction), "Invalid transaction proof");

        // 2. Business logic
        string memory provedAddress = _transaction.data.requestBody.addressStr;

        verifiedAddresses.push(provedAddress);
    }
}

Verify proof​

FDC optimizes on-chain storage costs by implementing a hybrid data verification system. Instead of storing complete datasets on-chain, it stores only Merkle proofs, while maintaining the actual data through trusted off-chain 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 on-chain 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 verifyAddressValidity will return true, otherwise false.

We deploy and verify this contract with the deployAndVerifyContract function in the scripts/fdcExample/AddressValidity.ts file.

scripts/fdcExample/AddressValidity.ts
async function deployAndVerifyContract() {
  const args: any[] = [];
  const addressRegistry: AddressRegistryInstance = await AddressRegistry.new(
    ...args,
  );
  try {
    await run("verify:verify", {
      address: addressRegistry.address,
      constructorArguments: args,
    });
  } catch (e: any) {
    console.log(e);
  }
  console.log("AddressRegistry deployed to", addressRegistry.address, "\n");
  return addressRegistry;
}

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 IAddressValidity.Response struct. From that and the array of proofs, it constructs an IAddressValidity.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/AddressValidity.ts
async function interactWithContract(
  addressRegistry: AddressRegistryInstance,
  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 IAddressValidityVerification = await artifacts.require(
    "IAddressValidityVerification",
  );
  const responseType =
    IAddressValidityVerification._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 addressRegistry.registerAddress({
    merkleProof: proof.proof,
    data: decodedResponse,
  });
  console.log("Transaction:", transaction.tx, "\n");
  console.log(
    "Verified address:",
    await addressRegistry.verifiedAddresses(0),
    "\n",
  );
}

We can run the whole script by calling the following console command.

yarn hardhat run scripts/fdcExample/AddressValidity.ts