From the previous episode, we have learned the basic “terminologies” of VeChain VIP-191 fee delegation. To summarize, the process can be simply described as below:
- The “user” creates a transaction body.
- The “user” generates a “user signature”.
- The transaction is sent to a “sponsor” to get a “sponsor signature”.
- The two signatures are combined to forge a “final signature”.
- The final transaction is posted to the VeChain network.
To get an overview, the idea is depicting as below:
Our example will involve a user with an “empty wallet” tries to call a smart contract (which costs gas) then the sponsor pays for the operation. Let’s break it down step by step.
Deploy the Smart Contract
First, we need to deploy a simple contract on test-net for us to interact with. The source code of the smart contract is as simple as below. It contains a
counter variable which will be increased each time the function
increaseAmount is called by any user.
Many tools can be used to deploy the smart contract, we won’t cover the process in this article, but you can read it from this tutorial.
Also, I have deployed it on test-net already, on address: 0x6d48628bb5bf20e5b4e591c948e0394e0d5bb078
We will go with this address in the rest of the article.
Build a Transaction Body
We employ the
thor-devkit.js library to help us with the building of a transaction. To call the method, we build a simple transaction like this:
Some fields need clarification:
In the above,
to the field is pointing to the contract we want to call;
data field (data: ‘0x74f667c4’) is exactly the encoded method
increaseAmount that we want to call.
nonce field needs to be a random hex number, don’t repeat yourself!
reserved field is included to notify that this transaction is using the feature of
vip-191 fee delegation.
Generate a User Signature
Since the user wants to call the smart contract, a signature from the user is required. We import an empty wallet and generate a “user signature” of the transaction.
The user wallet is as below:
Public address: 0x881Ab2380017870C49a9A114806C05F3CFE406e2
Private key: 2a0cbfe49ea7c18e89b87be4237e1717823fc16b52dc02e91fb30af122fba9b3
From the above code, the program has obtained a“user” signature as
originSignature. Next step we need to obtain the “sponsor” signature. Usually, the sponsor exists over the internet as a service. So we will use the HTTP protocol to obtain it.
Get a Sponsor Signature
This step requires us to send the original transaction to the sponsoring service and let the sponsor generate a signature for us. On the user’s side, the network operation is can be wrapped into a
Pay attention to the following details:
http://localhost:3000/ is the destination we post our request;
We have posted two things:
sender which is the user’s address, and
txBody is the original transaction itself;
sponsorSignature to be sent back from the sponsor and the user can finally assemble the signature.
If you have followed the above steps faithfully, then the job on the user’s side is almost done. The remote sponsor needs to react to the request and return the signature or reject it. We will continue in the next tutorial about:
- Write a sponsor service to generate “sponsor signature”.
- Combine the user signature with the sponsor’s signature.
- Assemble the final transaction and post to the VeChain network.