The QBFT consensus protocol implementation in the Quorum clients (GoQuorum and Besu) allows you to use a smart contract to specify the validators that are used to propose and validate blocks. You can create your own smart contract based on your organisation's requirements.

These smart contracts must implement the ValidatorSmartContractInterface.sol interface contract, specifically the function:

function getValidators() external view returns (address[] memory)

This repository contains an example smart contract that implements the interface.

General information about QBFT can be found in the Hyperledger Besu documentation.

The example smart contract can be found in the contracts/allowlist directory.

The contract holds a list of accounts (the allowlist) that are each allowed to nominate one QBFT validator using an API.

Accounts on the allowlist can use the API of this contract to:

• Activate a validator
• Deactivate a validator
• Vote to add an account to the allowlist
• Vote to remove an account from the allowlist
• Remove votes they have cast to add or remove an account
• Execute the vote count for an account to be added or removed

For an election to be successful more than 50% of the current members of the allowlist must vote. Use the countVotes function to count the votes, and if successful, add or remove the specified account.

Events are emitted to enable users to get information about changes to the validators, allowed accounts, and voting.

To use this contract from the genesis block of a blockchain, the initial state of this contract must be set in the genesis file. The scripts/allowlist/genesisContent directory of this repository contains a javascript script that creates the storage section for this contract. Refer to the "Genesis file content" section.

Information: See the web3-js based script in the scripts/allowlist/cli directory for an example CLI script that calls the allowlist smart contract functions.

To create the contract code to add to the genesis file, compile the contract with the --bin-runtime option:

Copy the interface contract into this directory

cp ../ValidatorSmartContractInterface.sol .

Then compile the contract using solc 0.8.7

solc --optimize --bin-runtime --evm-version=byzantium -o . ./ValidatorSmartContractAllowList.sol

Unit tests are executed via Truffle:

yarn install
yarn truffle compile
yarn truffle test

The script/allowlist/genesisContent directory contains the createContent.js script that generates the content required for the genesis file of a network that uses the allowlist smart contracts (contracts/allowlist).

The script reads the file allowedAccountsAndValidators.txt that defines in each line the address of an allowed account and (optionally) the validator for that account. The account and the validator are specified using their address as a hexadecimal string. If the account has a nominated validator, the account and the validator hexadecimal strings need to be separated by a comma:

0x5B38Da6a701c568545dCfcB03FcB875f56beddC4, 0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2
0xCA35b7d915458EF540aDe6068dFe2F44E8fa733c
0x4b20993Bc481177ec7E8f571ceCaE8A9e22C02db, 0x79731D3Ca6b7E34aC0F824c42a7cC18A495cabaB

The three lines above specify three accounts on the allowlist, and the validators for the first and third account.

You can export the node's address using the following Besu command:

besu public-key export-address

Prerequisites:

• Install the script dependencies by running run yarn install in the scripts/allowlist/genesisContent directory.

Run the script in the scripts/allowlist/genesisContent directory:

node createContent.js

The script creates a file named Storage.txt. The content of this file for the above example looks as follows:

"<Address of Contract>": {
    "comment": "validator smart contract",
    "balance": "0x00",
    "code": "0x<Contract Code>",
    "storage": {
        "0000000000000000000000000000000000000000000000000000000000000000": "0000000000000000000000000000000000000000000000000000000000000002",
        "290decd9548b62a8d60345a988386fc84ba6bc95484008f6362f93160ef3e563": "000000000000000000000000ab8483f64d9c6d1ecf9b849ae677dd3315835cb2",
        "290decd9548b62a8d60345a988386fc84ba6bc95484008f6362f93160ef3e564": "00000000000000000000000079731d3ca6b7e34ac0f824c42a7cc18a495cabab",
        "36306db541fd1551fd93a60031e8a8c89d69ddef41d6249f5fdc265dbc8fffa2": "0000000000000000000000000000000000000000000000000000000000000101",
        "58d9a93947083dcdedec58d43912ce0326f251a85b7701c5de5bc7d7a150676e": "0000000000000000000000000000000000000000000000000000000000000001",
        "e20f19dc6931eb9e42fe3f21abe1a9ef59942d8e586871d88564d0d0b63a5e5c": "0000000000000000000000000000000000000000000000000000000000010101",
        "f4c32baaad9a468f8a07690e6d59a45329a58ffaa2080ee4ccc1c4e2d7249e78": "0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4",
        "5dbbb5c5a02cb5b882ed6d78dcc49118067d39a359082b7fe270d1949d2ca44d": "0000000000000000000000004b20993bc481177ec7e8f571cecae8a9e22c02db",
        "0000000000000000000000000000000000000000000000000000000000000003": "0000000000000000000000000000000000000000000000000000000000000003"
    },
    "version": "0x01"
}

The content of the file needs to be placed in the genesis file for the network. In addition the <Address of Contract> and <Contract Code> must be filled in.

An example of a genesis file using QBFT can be found in the genesis.json file in this directory.

• <Address of Contract> must be identical to validatorcontractaddress located in the qbft section of the genesis file.
• <Contract Code> must contain the binary runtime code for the ValidatorSmartContractAllowList.sol contract in contracts/allowlist. The binary can be found in the example genesis.json file in this directory. For this binary the ValidatorSmartContractAllowList.sol contract was compiled using --bin-runtime, --evm-version byzantium, and --optimize options of the solidity compiler.

General information about the genesis file can be found in the Besu documentation.

The storage section defines the state of the contract in the genesis block of the blockchain. For general information on the layout of state variables in storage see https://docs.soliditylang.org/en/v0.8.7/internals/layout_in_storage.html.

The storage section created by the createContent.js script defines

• the initial validators array (line 1 - 3 in the storage section)
• the initial allowedAccounts mapping (line 4 - 6 in the storage section)
• the initial validatorToAccounts mapping (line 7 and 8 in the storage section)
• the initial numAllowedAccounts uint (line 9 in the storage section)

For more detail please see the script.