User's side Snippet

user.js

/**
 * Attempt to commit a transaction to a contract.
 * 
 * With a hefty wallet as "sponsor" + an empty wallet as "sender".
 * 
 */
const fetch = require("node-fetch");
const cry = require('thor-devkit/dist/cry')
const Transaction = require('thor-devkit/dist/transaction').Transaction

async function getSponosrSignature(sender, txBody) {
    const url = 'http://localhost:3000/'
    const response = await fetch(url, {
        method: 'POST',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({
            'sender': sender,
            'txBody': txBody
        })
    })

    const r = await response.json()
    return r['sponsor_signature']
}

// run it!
(async function () {
    // User private key.
    const originPriv = Buffer.from(
        '2a0cbfe49ea7c18e89b87be4237e1717823fc16b52dc02e91fb30af122fba9b3',
        'hex'
    )
    // User public address: 0x881Ab2380017870C49a9A114806C05F3CFE406e2
    const originAddress = cry.publicKeyToAddress(cry.secp256k1.derivePublicKey(originPriv))

    // Construct transaction body.
    const txBody = {
        // Test-net: 0x27,  Main-net: 0x4a.
        chainTag: 0x27,
        // After which block this tx should happen?
        // 16 characters of block ID.
        blockRef: '0x004984e1064ed410',
        // Expires after 30 days since blockRef.
        expiration: 30 * 8640,
        // Call the contract method "increaseAmount"
        clauses: [{
            to: '0x6d48628bb5bf20e5b4e591c948e0394e0d5bb078',
            value: 0,
            data: '0x74f667c4'
        }],
        gasPriceCoef: 0,
        gas: 50000,
        dependsOn: null,
        nonce: '0xa3b6232f', // Random number
        // Must include this field to activate VIP-191.
        reserved: { 
            features: 1
        }
    }

    // Construct a transaction.
    const tx = new Transaction(txBody)

    // Construct the hash for signing.
    const originHash = tx.signingHash()

    // Construct the user signature.
    const originSignature = cry.secp256k1.sign(originHash, originPriv)

    // Fetch the sponsor signature.
    const sponsorSignature = await getSponosrSignature(
        '0x'+originAddress.toString('hex'),
        txBody
    )
    // Compose a combined signature of user + sponsor.
    const sig = Buffer.concat([
        originSignature,
        Buffer.from(sponsorSignature, 'hex')
    ])

    // Mount on the combined signature.
    tx.signature = sig

    // Convert the tx to raw format.
    const rawTx = '0x' + tx.encode().toString('hex')

    // Submit the raw transaction by hand to the test-net.
    const url = 'https://sync-testnet.vechain.org/transactions'
    fetch(url, {
        method: 'POST',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({'raw': rawTx})
    }).then(response => {
        response.text().then(r => {console.log(r)})
    }).catch(err => {
        console.log('err', err)
    })

}());