Smart Bitcoin Cash adopts tendermint as its consensus engine. The quorum of validators are elected only by Bitcoin Cash mainnet's hash power, and they take on duties in epochs. In the future, both hash power and locked BCH can be used in election.

Epoch-based Validator Election

An epoch contains 2,016 blocks (takes about two weeks). During an epoch, mining pools use coinbase transactions to vote. This voting process is performed on Bitcoin Cash's mainnet and is totally permissionless because a new validator only needs endorsements from miners.

The voting protocol is simple: the first output of the coinbase transaction which meets the following requirement is regarded as a vote:

1. It begins with the "OP_RETURN" opcode

2. The pushed value after OP_RETURN is a 37-byte string, in which the starting 5 bytes are 0x73, 0x42, 0x43, 0x48 and 0x0, and the ending 32 bytes are an ed25519 public key, which is used to identify a node in tendermint

There may be multiple outputs meeting the above two requirements, but only the first one is regarded as a vote, and the others are ignored.

An epoch's end time is the largest timestamp of its blocks, and its duration time is the difference between the end times of adjacent epochs. The quorum elected during an epoch will stay in a stand-by state for 5% of the epoch's duration time. Then it takes its turn to be on duty, until the next quorum leaves its stand-by state, which is necessary because any Bitcoin Cash reorganization may alter the blocks in an epoch.

Register as a Validator

Before elected as a validator on duty, you must first register as a candidate validator, by sending transactions on smartBCH.

A special smart contract at the address 0x2710 handles transactions related to staking. It has several functions which can only be called from EOAs (externally owned accounts) and one function which can only be called by other smart contracts. It is implemented using native code and can do jobs which normal EVM contracts cannot do.

It's interface is as below:

interface StakingContract {
    function createValidator(address rewardTo, bytes32 introduction, bytes32 pubkey) external;
    function editValidator(address rewardTo, bytes32 introduction) external;
    function retire() external;
    function increaseMinGasPrice() external;
    function decreaseMinGasPrice() external;
    function sumVotingPower(address[] calldata addrList) external override returns (uint summedPower, uint totalPower) // this function can only be called by other contracts
}

An EOA can call createValidator to register itself as a candidate validator and set the following parameters:

1. rewardTo, the account who receives the validator's reward

2. introduction, a short UTF8 string (no longer than 32 bytes) describing the validator, whose tailing bytes are zeros.

3. pubkey, an ed25519 pubkey for which the coinbase transactions' outputs can vote.

After becoming a candidate validator, an EOA cannot call createValidator again. Instead, it can use editValidator to change rewardTo and introduction, but the pubkey cannot be changed.

A validator must pledge some BCH as collateral, which would be slashed if it misbehaves during its duty. Once its pledged collateral is less than a lower bound because of slashing, its voting power is reduced to zero until enough collateral is replenished. A validator can pledge collateral by sending BCH when calling createValidator and editValidator.

If a validator no longer wants to act as validator, it can call retire to mark its status as "retiring". The votes to a retiring validator is not counted when electing the next quorum.

Query a Address Set's Voting Power

A smart contract can call the sumVotingPower function to sum all the voting power owned by the address set specified in addrList. It also returns the total voting power owned by all the active validators. These two parameters are returned as summedPower and totalPower, respectively.

A smart contract can record incoming votes from validators and use sumVotingPower to decide whether the votes are enough. In such a way, people can make the validator set act as Supreme Court to decide some cases finally.

Adjust Gas Fee

Each validator can set its minimum acceptable gas price, which is respected in relaying transactions and receiving transactions into mempool. This parameter is not a consensus parameter. There is another consensus parameter: chain-wide minimum gas price, which is the lower bound of the minimum gas prices set by different validators.

This chain-wide minimum gas price is modified by the validators. To increase (decrease) it by a predefined percentage, a qualified EOA call the increaseMinGasPrice (decreaseMinGasPrice) function, respectively. A qualified EOA is a validator, or a validator's rewardTo account.

Distribute Rewards

For each executed block, the gathered gas fee is distributed to the validators who proposed or voted for it.

The proposer can get two kinds of rewards:

1. Reward for being a proposer, i.e., not missing its duty, which is 15% of all the gas fee.

2. Reward for collecting signatures for the last block, which is at most 15% of all the gas fee. This reward is in direct proportion to the voting power the proposer collects. When all the voting power is collected, it can enjoy 15%

After the proposer gets its rewards, the remained gas fee is distributed to the voters. The proposer is also a voter by nature, so it can also enjoy some reward as a voter. A voter's reward is in direct proportion to its voting power.

The distributed rewards are not immediately given to validators' rewardTo accounts. Instead, they are pending till the next epoch's end. That is, the rewards gained in epoch N will be given to a validator's rewardTo account at the end of epoch N+1.

A validator's life cycle ends when its voting power is zero and all the pending rewards are given to its rewardTo account.

Slashing

When a validator's mis-behavior is discovered, its pending rewards will be all confiscated and a fixed amount of collateral will be slashed.

Currently, only double-signing is considered as a mis-behavior.