Get Digital Certificate from Node.js Server + Return to PSPDFKit's `signDocument` method

digitally-sign-document.js

 async function digitallySignDocument({fileContents}) {
         const blob = new Blob([fileContents], {type: 'application/pdf'});
         const formData = new FormData();
         formData.append("file", blob);
         
         let {data} = await axios.post('node-server-url.com', formData, {
                'Content-Type': 'multipart/form-data'
            })

            return this.stringToArrayBuffer(data)
        }

node-server.js

const express = require('express')
const busboy = require('connect-busboy')
const forge = require('node-forge')
const fs = require('fs')

const app = express()

let signed;

app.use(busboy())

app.post('/', function (req, res) {
    req.pipe(req.busboy);
    req.busboy.on('file', function (field, file) {
        file.on('data', (pdfBuffer) => {
            signed = generatePKCS7(pdfBuffer)
        })
            .on('close', () => {
                res.send(signed)
            })
    })
})
app.listen(3000, function () {
    console.log('Server is listening on port 3000');
});

// Takes a PDF ArrayBuffer and returns a digital signature
function generatePKCS7(buffer) {
    const certificatePem = fs.readFileSync('./certs/cert.pem')
    const privateKeyPem = fs.readFileSync('./certs/private-key.pem')

    const certificate = forge.pki.certificateFromPem(
        certificatePem
    );
    const privateKey = forge.pki.privateKeyFromPem(privateKeyPem);

    const p7 = forge.pkcs7.createSignedData();
    p7.content = new forge.util.ByteBuffer(buffer)
    p7.addCertificate(certificate);
    p7.addSigner({
        key: privateKey,
        certificate: certificate,
        digestAlgorithm: forge.pki.oids.sha256,
        authenticatedAttributes: [
            {
                type: forge.pki.oids.contentType,
                value: forge.pki.oids.data
            },
            {
                type: forge.pki.oids.messageDigest
            },
            {
                type: forge.pki.oids.signingTime,
                value: new Date()
            }
        ]
    });

    p7.sign({detached: true});

    return forge.asn1.toDer(p7.toAsn1()).getBytes()
}

pspdfkit-sign-call.js

  try {
     await instance.signDocument(null, digitallySignDocument)
  } catch (e) {
    console.error('Unable to digitally sign');
    return;
  }

string-to-array-buffer.js

// Converts binary string returned from Node.js to array buffer
function stringToArrayBuffer(binaryString) {
            const buffer = new ArrayBuffer(binaryString.length);
            let bufferView = new Uint8Array(buffer);
            for (let i = 0, len = binaryString.length; i < len; i++) {
                bufferView[i] = binaryString.charCodeAt(i);
            }
            
         return buffer;
  }