Bitcore - Payment Protocol Library Implementation

I recently attempted to implement the Bitcore Payment Protocol Library and ran into several pieces that took me a few hours of debugging to wrap my head around.

I've decided to share some sample code that I wrote to, hopefully, save others from this headache in case they also decide to implement using this library.

The notable "gotchas" were these:

• When setting the Payment Details from an output, use the ".message" field: e.g. details.set('outputs', output.message);

• Bitcore Payment Protocol Lib expects the Certificates in "der" format - not "pem". For the Private Key, ".pem" appears fine.

• Be careful when you are reading the above Certificates in. You must read these as binary files - not UTF8 files.

• If you're using a Certificate that is not derived direct from a Root Cert Authority (e.g. you're using LetsEncrypt), then you must include the chain of certs.

Code with GOTCHA lines is below (untested, but is mostly copy+paste of what I used with a few modifications).

This doesn't appear to be working with Bitcoin.com Wallet still. If I work out why, I'll post amended code - but BIP70 implementations seem to be inconsistent across the board.

const express = require('express')
const fs = require('fs');
const router = express.Router()
// GOTCHA - Not mine, but apparently order here matters!
const PaymentProtocol = require('bitcore-payment-protocol');
const bitcore = require('bitcore-lib-cash');

// BIP70 Payment Request Endpoint
router.get('/req', async (req, res) => { 
    // Setup certificates
    // GOTCHA: NOTE THAT THESE ARE IN .der FORMAT - DO NOT READ AS UTF8
    var certificate = fs.readFileSync('./cert.der');
    var chain = fs.readFileSync('./chain.der');
    // GOTCHA: NOTE THAT THIS IS IN .pem and that I'm therefore reading as "utf8"
    var privKey = fs.readFileSync('./privkey.pem', 'utf8');
    var now = Date.now() / 1000 | 0;
    
    // Create an output
    // GOTCHA: ADDRESS MUST BE A BITCOINCASH: ADDRESS!
    var address = bitcore.Address.fromString("bitcoincash:qz2fn6wwwxs2wcdf9cfdhv4ln0qvjadg6csjcjasuf");
    var script = bitcore.Script.buildPublicKeyHashOut(address);
    var rawScript = new Buffer(script.toString(), 'ucs2');
    var output = new PaymentProtocol().makeOutput();
    output.set('amount', 10000);
    output.set('script', rawScript);
    
    // Construct the payment details
    var details = new PaymentProtocol('BCH').makePaymentDetails();
    details.set('network', 'main');
    details.set('outputs', output.message);
    details.set('time', now);
    details.set('expires', now + 60 * 60 * 24);
    details.set('memo', "Message that the user will see");
    details.set('payment_url', `https://your-service.com/ack`);
    details.set('merchant_data', new Buffer("INVOICE_ID_AND_THINGS_OF_THAT_SORT")); // identify the request
    
    // Load the X509 certificate
    var certificates = new PaymentProtocol().makeX509Certificates();
    // GOTCHA: CHAIN THE CERTIFICATES - THIS IS WHY IT'S AN ARRAy
    certificates.set('certificate', [certificate, chain]);

    // Form the request
    var request = new PaymentProtocol().makePaymentRequest();
    request.set('payment_details_version', 1);
    request.set('pki_type', 'x509+sha256');
    request.set('pki_data', certificates.serialize());
    request.set('serialized_payment_details', details.serialize());
    request.sign(privKey);

    // Serialize the request
    var rawBody = request.serialize();
    
    // Set output headers
    res.set({
      'Content-Type': PaymentProtocol.LEGACY_PAYMENT.BCH.REQUEST_CONTENT_TYPE,
      'Content-Length': request.length,
      'Content-Transfer-Encoding': 'binary'
    });
    
    res.send(rawBody);
});

// GOTCHA
// In ExpressJS, due to the Content-Type header, req.body will be empty by default.
// You have to explicitly tell ExpressJS to process raw body as follows:
// app.use(bodyParser.raw({ type:'*/*' }));
// That said, I was lazy - you can probably just put this middleware straight
// into this endpoint and actually select based on Content Type.
router.post('/ack', async (req, res) => { 
    // Decode payment
    var body = PaymentProtocol.Payment.decode(req.body);
    var payment = new PaymentProtocol().makePayment(body);
    var merchantData = payment.get('merchant_data');
    var transactions = payment.get('transactions');
    var refundTo = payment.get('refund_to');
    var memo = payment.get('memo');
    
    // TODO Send payment
    
    // Make a payment acknowledgement
    var ack = new PaymentProtocol().makePaymentACK();
    ack.set('payment', payment.message);
    ack.set('memo', 'Thank you for your payment!');
    var rawBody = ack.serialize();
    
    res.set({
      'Content-Type': PaymentProtocol.LEGACY_PAYMENT.BCH.PAYMENT_ACK_CONTENT_TYPE,
      'Content-Length': rawBody.length,
      'Content-Transfer-Encoding': 'binary'
    });
    
    res.send(rawBody);
});

Useful links:

https://github.com/bitpay/bitcore-payment-protocol

https://github.com/bitpay/bitcore-payment-protocol/blob/master/docs/index.md

https://medium.com/@nusrath501khan/creating-a-bip-70-payment-request-183933c33259

Godspeed.