# Payment
> Retrieve a Payment transaction data from Bitcoin, Dogecoin, or XRPL.
> 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/foundry/payment
The `Payment` attestation type enables data collection about a transaction, classified as payment on the native chain. The currently supported chain are: `BTC`, `DOGE`, and `XRP`. You can learn more about it in the official [specification repo](/fdc/attestation-types/payment).
We will now demonstrate how the FDC protocol can be used to collect the data of a given XRLP payment transaction. The transaction we will be observing has the id `2A3E7C7F6077B4D12207A9F063515EACE70FBBF3C55514CD8BD659D4AB721447`; this is an arbitrary transaction that we acquired from the XRPL testnet [explorer](https://testnet.xrpl.org/).
The same procedure works for all supported sources, `BTC`, `DOGE`, and `XRP`. 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 follow the steps outlined in the [FDC overview](/fdc/overview).
Our implementation requires handling the FDC voting round finalization process. To manage this, we will create separate scripts in `script/fdcExample/Payment.s.sol` that handle different stages of the validation process:
script/fdcExample/Payment.s.sol
```
// SPDX-License-Identifier: MITpragma solidity ^0.8.25;import {Script} from "dependencies/forge-std-1.9.5/src/Script.sol";...string constant attestationTypeName = "Payment";string constant dirPath = "data/";contract PrepareAttestationRequest is Script { ...}contract SubmitAttestationRequest is Script { ...}contract RetrieveDataAndProof is Script { ...}contract Deploy is Script { ...}...
```
The names of included contracts mostly mirror the steps described in the [FDC guide](/fdc/overview).
To bridge the separate executions of the scripts, we will save the relevant data of each script to a file in the `dirPath` folder. Each succeeding script will then read that file to load the data.
## Prepare request[](#prepare-request "Direct link to Prepare request")
The JSON request to the verifier is the same form for all attestation types, but the values of the fields differ between them. It contains the following fields.
### 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 `Payment`, `requestBody` is a JSON containing the fields:
- `transactionId`: id of the transaction; as `bytes32`
- `inUtxo`: UTXO chains support multiple source addresses, so this is the index of the address considered, as `uint256`; for non-UTXO chains this should always be `0`
- `utxo`: UTXO chains support multiple receiving addresses, so this is the index of the address considered, as `uint256`; for non-UTXO chains this should always be `0`
### Reference Documentation[](#reference-documentation "Direct link to Reference Documentation")
- [Payment Specification](/fdc/attestation-types/payment)
- [Verifier Interactive Docs](https://fdc-verifiers-testnet.flare.network/verifier/xrp/api-doc#/)
- API available for [DOGE](https://fdc-verifiers-testnet.flare.network/verifier/doge/api-doc#/) and [BTC](https://fdc-verifiers-testnet.flare.network/verifier/btc_testnet4/api-doc#/).
### Example Values[](#example-values "Direct link to Example Values")
- `transactionId`: the above address `9421cbb7f195df66d16703442a408261fa973514a0bd9dfc680f10eb3942d11f`
- `inUtxo`: non-default `0`
- `utxo`: non-default `0`
### Encoding Functions[](#encoding-functions "Direct link to Encoding Functions")
To encode values into UTF8 hex:
- `toUtf8HexString`: Converts a string to UTF8 hex.
- `toHexString`: Zero-right-pads the string to 32 bytes.
These functions are included in the [Base library](https://github.com/flare-foundation/flare-foundry-starter/blob/master/script/fdcExample/Base.s.sol) within the [example repository](https://github.com/flare-foundation/flare-foundry-starter), but they can also be defined locally in your contract or script.
script/fdcExample/Base.s.sol
```
function toHexString( bytes memory data) public pure returns (string memory) { bytes memory alphabet = "0123456789abcdef"; bytes memory str = new bytes(2 + data.length * 2); str[0] = "0"; str[1] = "x"; for (uint i = 0; i < data.length; i++) { str[2 + i * 2] = alphabet[uint(uint8(data[i] >> 4))]; str[3 + i * 2] = alphabet[uint(uint8(data[i] & 0x0f))]; } return string(str);}
```
script/fdcExample/Base.s.sol
```
function toUtf8HexString( string memory _string) internal pure returns (string memory) { string memory encodedString = toHexString( abi.encodePacked(_string) ); uint256 stringLength = bytes(encodedString).length; require(stringLength <= 64, "String too long"); uint256 paddingLength = 64 - stringLength + 2; for (uint256 i = 0; i < paddingLength; i++) { encodedString = string.concat(encodedString, "0"); } return encodedString;}
```
We also define a helper function for formatting data into a JSON string.
script/fdcExample/Base.s.sol
```
function prepareAttestationRequest( string memory attestationType, string memory sourceId, string memory requestBody) internal view returns (string[] memory, string memory) { // We read the API key from the .env file string memory apiKey = vm.envString("VERIFIER_API_KEY_TESTNET"); // Preparing headers string[] memory headers = prepareHeaders(apiKey); // Preparing body string memory body = prepareBody( attestationType, sourceId, requestBody ); console.log( "headers: %s", string.concat("{", headers[0], ", ", headers[1]), "}\n" ); console.log("body: %s\n", body); return (headers, body);}function prepareHeaders( string memory apiKey) internal pure returns (string[] memory) { string[] memory headers = new string[](2); headers[0] = string.concat('"X-API-KEY": ', apiKey); headers[1] = '"Content-Type": "application/json"'; return headers;}function prepareBody( string memory attestationType, string memory sourceId, string memory body) internal pure returns (string memory) { return string.concat( '{"attestationType": ', '"', attestationType, '"', ', "sourceId": ', '"', sourceId, '"', ', "requestBody": ', body, "}" );}
```
In the example repository, these are once again included within the [Base](https://github.com/flare-foundation/flare-foundry-starter/blob/master/script/fdcExample/Base.s.sol) library file.
Thus, the part of the script that prepares the verifier request looks like:
script/fdcExample/Payment.s.sol
```
// SPDX-License-Identifier: MITpragma solidity ^0.8.25;import {console} from "dependencies/forge-std-1.9.5/src/console.sol";import {Script} from "dependencies/forge-std-1.9.5/src/Script.sol";import {Base} from "./Base.s.sol";...string constant attestationTypeName = "Payment";string constant dirPath = "data/";contract PrepareAttestationRequest is Script { using Surl for *; // Setting request data string public transactionId = "2A3E7C7F6077B4D12207A9F063515EACE70FBBF3C55514CD8BD659D4AB721447"; string public inUtxo = "0"; string public utxo = "0"; string public baseSourceName = "xrp"; // Part of verifier URL string public sourceName = "testXRP"; // XRLP test chain ID function prepareRequestBody( string memory transactionId, string memory inUtxo, string memory utxo ) private pure returns (string memory) { return string.concat( '{"transactionId": "', transactionId, '", "inUtxo": "', inUtxo, '", "utxo": "', utxo, '"}' ); } function run() external { // Preparing request data string memory attestationType = Base.toUtf8HexString( attestationTypeName ); string memory sourceId = Base.toUtf8HexString(sourceName); string memory requestBody = prepareRequestBody( transactionId, inUtxo, utxo ); (string[] memory headers, string memory body) = prepareAttestationRequest(attestationType, sourceId, requestBody); ... }}...
```
If you are accessing a different chain, replace the `baseSourceName` with an appropriate value, `doge` or `btc`.
The code above differs slightly from the [starter example](https://github.com/flare-foundation/flare-foundry-starter). But, if we remove the ellipses `...` signifying missing code, we can still run the script.
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/btc_testnet4/api-doc#/Payment/BTCPaymentVerifierController_prepareRequest) to check what the response will be.
We can run the script by calling the following commands in the console.
```
source .env
```
```
forge script script/fdcExample/Payment.s.sol:PrepareAttestationRequest --private-key $PRIVATE_KEY --rpc-url $COSTON2_RPC_URL --etherscan-api-key $FLARE_RPC_API_KEY --broadcast --ffi
```
The prerequisite for this is that the `.env` file is not missing the `PRIVATE KEY` and `COSTON2_RPC_URL` values. The script can also access other chains; that can be achieved by replacing the `--rpc-url` value with `COSTON_RPC_URL`, `FLARE_RPC_URL`, or `SONGBIRD_RPC_URL`.
## Post request to verifier.[](#post-request-to-verifier "Direct link to Post request to verifier.")
To post a request to a verifier server, we use the `surl` package. We place `using Surl for *;` at the start of our `PostRequest` contract, and then call its `post` method on the verifier URL.
script/fdcExample/Payment.s.sol
```
(, bytes memory data) = url.post(headers, body);
```
We construct the URL by appending to the verifier address `https://fdc-verifiers-testnet.flare.network/` the path `verifier/btc_testnet4/Payment/prepareRequest`. We can do so dynamically with the following code.
script/fdcExample/Payment.s.sol
```
string memory baseUrl = "https://fdc-verifiers-testnet.flare.network/";string memory url = string.concat( baseUrl, "verifier/", baseSourceName, "/", attestationTypeName, "/prepareRequest");console.log("url: %s", url);string memory requestBody = string.concat( '{"addressStr": "', addressStr, '"}');
```
Lastly, we parse the return data from the verifier server. Using the Foundry `parseJson` shortcode, and a custom struct `AttestationResponse`, we decode the returned data and extract from it the ABI encoded request.
script/fdcExample/Base.s.sol
```
function parseAttestationRequest( bytes memory data) internal pure returns (AttestationResponse memory) { string memory dataString = string(data); bytes memory dataJson = vm.parseJson(dataString); AttestationResponse memory response = abi.decode( dataJson, (AttestationResponse) ); console.log("response status: %s\n", response.status); console.log("response abiEncodedRequest: "); console.logBytes(response.abiEncodedRequest); console.log("\n"); return response;}
```
Details
Details
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("Payment")`
- **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 `Payment.RequestBody` Solidity struct
What this demonstrates is that, with some effort, the `abiEncodedRequest` can be constructed manually.
We write the `abiEncodedRequest` to a file (`data/Payment_abiEncodedRequest.txt`) to it in the next step.
script/fdcExample/Payment.s.sol
```
Base.writeToFile( dirPath, string.concat(attestationTypeName, "_abiEncodedRequest"), StringsBase.toHexString(response.abiEncodedRequest), true);
```
## 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. Now, we submit the validated request to the blockchain using deployed smart contracts.
### Submit request[](#submit-request "Direct link to Submit request")
The entire submission process requires only five key steps:
script/fdcExample/Base.s.sol
```
function submitAttestationRequest( AttestationResponse memory response) internal { uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY"); vm.startBroadcast(deployerPrivateKey); IFdcRequestFeeConfigurations fdcRequestFeeConfigurations = ContractRegistry .getFdcRequestFeeConfigurations(); uint256 requestFee = fdcRequestFeeConfigurations.getRequestFee( response.abiEncodedRequest ); console.log("request fee: %s\n", requestFee); vm.stopBroadcast(); vm.startBroadcast(deployerPrivateKey); IFdcHub fdcHub = ContractRegistry.getFdcHub(); console.log("fcdHub address:"); console.log(address(fdcHub)); console.log("\n"); fdcHub.requestAttestation{value: requestFee}(response.abiEncodedRequest); vm.stopBroadcast();}
```
### Step-by-Step Breakdown[](#step-by-step-breakdown "Direct link to Step-by-Step Breakdown")
1. Load Private Key The private key is read from the `.env` file using Foundry's `envUint` function:
```
uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
```
2. Obtain Request Fee We retrieve the required requestFee from the `FdcRequestFeeConfigurations` contract:
```
IFdcRequestFeeConfigurations fdcRequestFeeConfigurations = ContractRegistry .getFdcRequestFeeConfigurations(); uint256 requestFee = fdcRequestFeeConfigurations.getRequestFee( response.abiEncodedRequest );
```
This is done in a separate broadcast to ensure `requestFee` is available before submitting the request.
3. Access `FdcHub` Contract Using the `ContractRegistry` library (from `flare-periphery`), we fetch the `FdcHub` contract:
```
IFdcHub fdcHub = ContractRegistry.getFdcHub(); console.log("fcdHub address:"); console.log(address(fdcHub)); console.log("\n");
```
4. Submit the Attestation Request We send the attestation request with the required fee:
```
fdcHub.requestAttestation{value: requestFee}(response.abiEncodedRequest);
```
5. Calculate Voting Round Number To determine the voting round in which the attestation request is processed, we query the `FlareSystemsManager` contract:
```
// Calculating roundId IFlareSystemsManager flareSystemsManager = ContractRegistry .getFlareSystemsManager(); uint32 roundId = flareSystemsManager.getCurrentVotingEpochId(); console.log("roundId: %s\n", Strings.toString(roundId));
```
This can be done within the existing broadcast or in a new one (as done in the demo repository for better code organization).
## Wait for response[](#wait-for-response "Direct link to Wait for response")
We wait for the round to finalize. This takes no more than 180 seconds.
You 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.
To learn more about how the FDC protocol works, check [here](/fdc/overview).
## Prepare proof request[](#prepare-proof-request "Direct link to Prepare proof request")
We prepare the proof request in a similar manner as in the step Prepare the request, by string concatenation. We import two new variables from the `.env` file; the URL of a verifier server and the corresponding API key.
script/fdcExample/Payment.s.sol
```
string memory daLayerUrl = vm.envString("COSTON2_DA_LAYER_URL");string memory apiKey = vm.envString("X_API_KEY");
```
Also, by repeatedly using the Foundry shortcode `vm.readLine`, we read the data, saved to a file in the previous step, to variables.
script/fdcExample/Payment.s.sol
```
string memory requestBytes = vm.readLine( string.concat( dirPath, attestationTypeName, "_abiEncodedRequest", ".txt" ));string memory votingRoundId = vm.readLine( string.concat( dirPath, attestationTypeName, "_votingRoundId", ".txt" ));
```
The code is as follows.
script/fdcExample/Payment.s.sol
```
contract RetrieveDataAndProof is Script { using Surl for *; function run() external { string memory daLayerUrl = vm.envString("COSTON2_DA_LAYER_URL"); string memory apiKey = vm.envString("X_API_KEY"); string memory fileName = string.concat(attestationTypeName, ".txt"); string memory filePath = string.concat(dirPath, fileName); // We import the roundId and abiEncodedRequest from the first file string memory votingRoundId = vm.readLine(filePath); string memory requestBytes = vm.readLine(filePath); console.log("votingRoundId: %s\n", votingRoundId); console.log("requestBytes: %s\n", requestBytes); // Preparing the proof request string[] memory headers = prepareHeaders(apiKey); string memory body = string.concat( '{"votingRoundId":', votingRoundId, ',"requestBytes":"', requestBytes, '"}' ); console.log("body: %s\n", body); console.log( "headers: %s", string.concat("{", headers[0], ", ", headers[1]), "}\n" ); ... }}
```
## Post proof request to DA Layer[](#post-proof-request-to-da-layer "Direct link to Post proof request to DA Layer")
We post the proof request to a chosen DA Layer provider server also with the same code as we did in the previous step.
script/fdcExample/Payment.s.sol
```
string memory url = string.concat( daLayerUrl, // "api/v0/fdc/get-proof-round-id-bytes" "api/v1/fdc/proof-by-request-round-raw");console.log("url: %s\n", url);(, bytes memory data) = Base.postAttestationRequest(url, headers, body);
```
Parsing the returned data requires the definition of an auxiliary `struct`.
script/fdcExample/Base.s.sol
```
struct ParsableProof { bytes32 attestationType; bytes32[] proofs; bytes responseHex;}
```
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 `IPayment.Response` struct. We retrieve this data as follows.
script/fdcExample/Payment.s.sol
```
bytes memory dataJson = parseData(data);ParsableProof memory proof = abi.decode(dataJson, (ParsableProof));IPayment.Response memory proofResponse = abi.decode( proof.responseHex, (IPayment.Response));
```
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 5061796d656e7400000000000000000000000000000000000000000000000000 7465737458525000000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000000000000e6c2d 00000000000000000000000000000000000000000000000000000000a019d806 2a3e7c7f6077b4d12207a9f063515eace70fbbf3c55514cd8bd659d4ab721447 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000 000000000000000000000000000000000000000000000000000000000048f822 0000000000000000000000000000000000000000000000000000000067ac9486 7f5b4967a9fbe9b447fed6d4e3699051516b6afe5f94db2e77ccf86470bfd74d a1475e9840d916c22f494c0dc25428d2affb5ae1f496efc82bbb59d46a336779 cd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82 cd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82 0000000000000000000000000000000000000000000000000000000005f5e10c 0000000000000000000000000000000000000000000000000000000005f5e10c 0000000000000000000000000000000000000000000000000000000005f5e100 0000000000000000000000000000000000000000000000000000000005f5e100 0000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000000', attestation_type: '0x5061796d656e7400000000000000000000000000000000000000000000000000', proof: [ '0xe1f98d39167eab17b2157c06efb80530b161d5eb15c439fc476e3242e30b3ac1', '0x23a8ffdb2cbaf0e2f3653923a159150f8d4c3ad5160f9e127cc9797ba233e6c2', '0xd756b90367b336e127f0759a1457825b4c2bf9011b71b56e15d9fcb7ff735ec8', '0xc881d1566868a986aef2bda47e9ab6dafeb8241bde5f5d53235837595829a5ea' ]}
```
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: '0x5061796d656e7400000000000000000000000000000000000000000000000000', sourceId: '0x7465737458525000000000000000000000000000000000000000000000000000', votingRound: '945197', lowestUsedTimestamp: '2686048262', requestBody: [ '0x2a3e7c7f6077b4d12207a9f063515eace70fbbf3c55514cd8bd659d4ab721447', '0', '0', transactionId: '0x2a3e7c7f6077b4d12207a9f063515eace70fbbf3c55514cd8bd659d4ab721447', inUtxo: '0', utxo: '0' ], responseBody: [ '4782114', '1739363462', '0x7f5b4967a9fbe9b447fed6d4e3699051516b6afe5f94db2e77ccf86470bfd74d', '0xa1475e9840d916c22f494c0dc25428d2affb5ae1f496efc82bbb59d46a336779', '0xcd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82', '0xcd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82', '100000012', '100000012', '100000000', '100000000', '0x0000000000000000000000000000000000000000000000000000000000000000', true, '0', blockNumber: '4782114', blockTimestamp: '1739363462', sourceAddressHash: '0x7f5b4967a9fbe9b447fed6d4e3699051516b6afe5f94db2e77ccf86470bfd74d', sourceAddressesRoot: '0xa1475e9840d916c22f494c0dc25428d2affb5ae1f496efc82bbb59d46a336779', receivingAddressHash: '0xcd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82', intendedReceivingAddressHash: '0xcd582d251987f15ecb29b69c2e02051479e84c176e39cbbdf04a4d0ef89bcf82', spentAmount: '100000012', intendedSpentAmount: '100000012', receivedAmount: '100000000', intendedReceivedAmount: '100000000', standardPaymentReference: '0x0000000000000000000000000000000000000000000000000000000000000000', oneToOne: true, status: '0' ]]
```
## Verify proof[](#verify-proof "Direct link to Verify proof")
Because every node holds a copy of the whole chain, storing data on the blockchain is expensive. For the sake of efficiency, FDC keeps only the Merkle proof onchain, while the data itself can be obtained from outside data providers. Per our request, they supply us with the specified data. That data then be encrypted, and its Merkle proof compared to the Merkle root stored onchain. If they match, the data can be trusted.
This step is not strictly necessary; if we trust our data provider, we can skip this step. And in practice, we do just that. But it is crucial, that should we want to verify the data, we can do so.
One way to do it is using the `FdcVerification` contract. We first store our data as an appropriate Solidity struct, namely `IPayment.Proof`.
script/fdcExample/Payment.s.sol
```
IPayment.Proof memory _proof = IPayment.Proof( proof.proofs, proofResponse);
```
We then access the `FdcVerification` contract through the `ContractRegistry`, and feed it the proof. If the proof is valid, the function `verifyPayment` will return `true`, otherwise `false`. As before, we wrap the whole thing into a broadcast environment, using the `PRIVATE_KEY` variable from our `.env` file.
script/fdcExample/Payment.s.sol
```
uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");vm.startBroadcast(deployerPrivateKey);bool isValid = ContractRegistry .getFdcVerification() .verifyPayment(proof);console.log("proof is valid: %s\n", StringsBase.toString(isValid));vm.stopBroadcast();
```
In actuality, we will only verify the proof within a deployed contract, which we will define in the next step. What we will do here instead is, we will save the proof to a file so that it can be later loaded into a variable. The code that does this is as follows.
script/fdcExample/EVMTransaction.s.sol
```
Base.writeToFile( dirPath, string.concat(attestationTypeName, "_proof"), StringsBase.toHexString(abi.encode(_proof)), true);
```
## 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 a Payment transaction, and store it into an array of special `Payment` structs. It will do so only if the transaction is valid.
src/fdcExample/Payment.sol
```
struct Payment { uint64 blockNumber; uint64 blockTimestamp; bytes32 sourceAddressHash; bytes32 receivingAddressHash; int256 spentAmount; bytes32 standardPaymentReference; uint8 status;}
```
First, we define an interface that the contract will inherit from. We do so, so that we may contact the contract later through a script.
src/fdcExample/Payment.sol
```
interface ITransferEventListener { function registerPayment( IPayment.Proof calldata _transaction ) external;}
```
The interface exposes the only function the script will call, `collectTransferEvents`. We now define the contract as follows.
src/fdcExample/Payment.sol
```
contract PaymentRegistry is IPaymentRegistry { Payment[] public verifiedPayments; function isPaymentProofValid( IPayment.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(); console.log("transaction: %s\n", FdcStrings.toJsonString(transaction)); // return true; return fdc.verifyPayment(transaction); } function registerPayment(IPayment.Proof calldata _transaction) external { // 1. FDC Logic // Check that this Payment has indeed been confirmed by the FDC require(isPaymentProofValid(_transaction), "Invalid transaction proof"); // 2. Business logic Payment memory provedPayment = Payment( _transaction.data.responseBody.blockNumber, _transaction.data.responseBody.blockTimestamp, _transaction.data.responseBody.sourceAddressHash, _transaction.data.responseBody.receivingAddressHash, _transaction.data.responseBody.spentAmount, _transaction.data.responseBody.standardPaymentReference, _transaction.data.responseBody.status ); verifiedPayments.push(provedPayment); }}
```
We deploy the contract through a simple script. The script creates a new `PaymentRegistry` contract, and writes its address to a file (`data/Payment_listenerAddress.txt`).
script/fdcExample/DeployContract.s.sol
```
contract DeployContract is Script { function run() external { uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY"); vm.startBroadcast(deployerPrivateKey); PaymentRegistry paymentRegistry = new PaymentRegistry(); address _address = address(paymentRegistry); vm.stopBroadcast(); Base.writeToFile( dirPath, string.concat(attestationTypeName, "_address"), StringsBase.toHexString(abi.encodePacked(_address)), true ); }}
```
We deploy the contract with the following console command.
```
forge script script/fdcExample/Payment.s.sol:DeployContract --private-key $PRIVATE_KEY --rpc-url $COSTON2_RPC_URL --broadcast --verify --verifier blockscout --verifier-url $COSTON2_EXPLORER_API --ffi
```
Lastly, we define a script that interacts with the above contract. It first reads the ABI encoded proof data, and the contract address, from files. Then, it connects to the above contract at the saved address (this is why we require the interface). With that, it is able to call the `registerPayment` method of the contract.
script/fdcExample/Payment.s.sol
```
contract InteractWithContract is Script { function run() external { string memory addressString = vm.readLine( string.concat(dirPath, attestationTypeName, "_address", ".txt") ); address _address = vm.parseAddress(addressString); string memory proofString = vm.readLine( string.concat(dirPath, attestationTypeName, "_proof", ".txt") ); bytes memory proofBytes = vm.parseBytes(proofString); IPayment.Proof memory proof = abi.decode(proofBytes, (IPayment.Proof)); uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY"); vm.startBroadcast(deployerPrivateKey); IPaymentRegistry registry = IPaymentRegistry(_address); registry.registerPayment(proof); vm.stopBroadcast(); }}
```
We run this script with the console command:
```
forge script script/fdcExample/Payment.s.sol:InteractWithContract --private-key $PRIVATE_KEY --rpc-url $COSTON2_RPC_URL --etherscan-api-key $FLARE_RPC_API_KEY --broadcast --ffi
```