Gasless Transaction

[wenyuanhust]

Background

As is known to all, to send trasactions in Blockchains, users need to pay some transaction gas fee. This requires users holding asset of the Blockchain beforehand. This extra requirement may block lots of potential people interested in the Blockchain or dApp from engaging in it. For dApp developers, they definitely wish to have as many participants as possible. To help them, we proposed a way to let users sending trasactions, participate in dApp for example, without paying gas fee.

Solution

The general idea is to let the developer pre-payes the gas fee for their autheticated users. We can describe the solution as follows:

1. The developer transfers some token to a system contract. Meanwhile, it must specify which developer contract will these token pay for.
2. As the owner of the developer contract, the developer must set the whitelist. Only account in the whitelist can send trasactions freely. This mechenism prevents malicious attacks on the developer or the contract.
3. The account in the whitelist can call the specific contract(sending transaction) without acquring tokens beforehand.

Design

1. SponsorWhitelistControl

This is a system contract which should be deployed in the genesis block. It offers many interfaces to manage the elements needed to realize gas-less transaction. We call the account which will pay the gas fee for the contract callers sponsor.

2. Developer deployed contract

To enable users calling contract freely, the contract must offer the interface to add users to the whitelist.

3. User End

There is no need to do extra work.

4. Miner End

Instead of checking transaction sender’s account balance to pay gas fee, miners will firstly check the SponsorWhitelistControl to see wheather the gas fee can be payed by others(the sponsor stored in the system contract). If not, the miner will check the sender’s account balance aferwards.

Implementation

1. SponsorWhitelistControl

setSponsorForGas: set the gas fee sponsor account, the amount and the upper limit of the amount for each transaction.

It has four parameters:

• contract_addr: Set the contract address for sponsor.
• upper_bound: set the upper limit of the sponsorship for each transaction.
• Value: set the sponsor amount.
• Signer: sponsor account address.

Generally, the contract(dApp) developer will call this function to pre-pay the gas fee. The sponsor account is in control of the developer.

2. Developer’s contract

To enable gas-less transaction, the contract must have the addPrivilege method:

• account: add the address to the whitelist.
• Value: leave it blank.
• Signer: The signature address of this transaction.

Generally, only the contract owner can call this method to add address to the whitelist.

3. Transaction assembler

No need to change.

4. Miner

Add extra mechanism as follows: