signData.js

async function signData(data) {
    
    // deletes old sig attribute, if existing
    delete data.sig;
    
    // convert obj to json
    const json = JSON.stringify(data);
    
    // load file with private key 
    const file = await util.promisify(fs.readFile)(path.join(privKeyFile));
    
    // Creates and returns a Verify object that uses RSA-SHA256
    //https://en.wikipedia.org/wiki/RSA_(cryptosystem)
    
    // https://en.wikipedia.org/wiki/RSA_(cryptosystem)
    // https://en.wikipedia.org/wiki/Secure_Hash_Algorithms
    const signer = crypto.createSign("RSA-SHA256");

    // Updates the Verify content with the given data
    signer.update(json);

    // Calculates the signature on the json data with the private key and returns the signature in hex as a string
    // https://nodejs.org/api/crypto.html#crypto_sign_sign_privatekey_outputformat
    return signer.sign(file.toString(), "hex");
  }