**Notice**: This document is a work-in-progress for researchers and implementers. This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](../core/0_beacon-chain.md), which describes the expected actions of a "validator" participating in the Ethereum 2.0 protocol.
This document represents the expected behavior of an "honest validator" with respect to Phase 0 of the Ethereum 2.0 protocol. This document does not distinguish between a "node" (i.e. the functionality of following and reading the beacon chain) and a "validator client" (i.e. the functionality of actively participating in consensus). The separation of concerns between these (potentially) two pieces of software is left as a design decision that is out of scope.
A validator is an entity that participates in the consensus of the Ethereum 2.0 protocol. This is an optional role for users in which they can post ETH as collateral and verify and attest to the validity of blocks to seek financial returns in exchange for building and securing the protocol. This is similar to proof-of-work networks in which miners provide collateral in the form of hardware/hash-power to seek returns in exchange for building and securing the protocol.
All terminology, constants, functions, and protocol mechanics defined in the [Phase 0 -- The Beacon Chain](../core/0_beacon-chain.md) and [Phase 0 -- Deposit Contract](../core/0_deposit-contract.md) doc are requisite for this document and used throughout. Please see the Phase 0 doc before continuing and use as a reference throughout.
Validator public keys are [G1 points](../bls_signature.md#g1-points) on the [BLS12-381 curve](https://z.cash/blog/new-snark-curve). A private key, `privkey`, must be securely generated along with the resultant `pubkey`. This `privkey` must be "hot", that is, constantly available to sign data throughout the lifetime of the validator.
A secondary withdrawal private key, `withdrawal_privkey`, must also be securely generated along with the resultant `withdrawal_pubkey`. This `withdrawal_privkey` does not have to be available for signing during the normal lifetime of a validator and can live in "cold storage".
In Phase 0, all incoming validator deposits originate from the Ethereum 1.0 proof-of-work chain. Deposits are made to the [deposit contract](../core/0_deposit-contract.md) located at `DEPOSIT_CONTRACT_ADDRESS`.
- Pack the validator's [initialization parameters](#initialization) into `deposit_data`, a [`DepositData`](../core/0_beacon-chain.md#depositdata) SSZ object.
- Let `amount` be the amount in Gwei to be deposited by the validator where `MIN_DEPOSIT_AMOUNT <= amount <= MAX_EFFECTIVE_BALANCE`.
- Let `signature` be the result of `bls_sign` of the `signing_root(deposit_data)` with `domain=compute_domain(DOMAIN_DEPOSIT)`. (Deposits are valid regardless of fork version, `compute_domain` will default to zeroes there).
- Send a transaction on the Ethereum 1.0 chain to `DEPOSIT_CONTRACT_ADDRESS` executing `def deposit(pubkey: bytes[48], withdrawal_credentials: bytes[32], signature: bytes[96])` along with a deposit of `amount` Gwei.
*Note*: Deposits made for the same `pubkey` are treated as for the same validator. A singular `Validator` will be added to `state.validators` with each additional deposit amount added to the validator's balance. A validator can only be activated when total deposits for the validator pubkey meet or exceed `MAX_EFFECTIVE_BALANCE`.
Deposits cannot be processed into the beacon chain until the Eth 1.0 block in which they were deposited or any of its descendants is added to the beacon chain `state.eth1_data`. This takes _a minimum_ of `ETH1_FOLLOW_DISTANCE` Eth 1.0 blocks (~4 hours) plus `ETH1_DATA_VOTING_PERIOD` epochs (~1.7 hours). Once the requisite Eth 1.0 data is added, the deposit will normally be added to a beacon chain block and processed into the `state.validators` within an epoch or two. The validator is then in a queue to be activated.
Once a validator has been processed and added to the beacon state's `validators`, the validator's `validator_index` is defined by the index into the registry at which the [`ValidatorRecord`](../core/0_beacon-chain.md#validator) contains the `pubkey` specified in the validator's deposit. A validator's `validator_index` is guaranteed to not change from the time of initial deposit until the validator exits and fully withdraws. This `validator_index` is used throughout the specification to dictate validator roles and responsibilities at any point and should be stored locally.
In normal operation, the validator is quickly activated, at which point the validator is added to the shuffling and begins validation after an additional `MAX_SEED_LOOKAHEAD` epochs (25.6 minutes).
The function [`is_active_validator`](../core/0_beacon-chain.md#is_active_validator) can be used to check if a validator is active during a given epoch. Usage is as follows:
*Note*: There is a maximum validator churn per finalized epoch, so the delay until activation is variable depending upon finality, total active validator balance, and the number of validators in the queue to be activated.
A validator can get committee assignments for a given epoch using the following helper via `get_committee_assignment(state, epoch, validator_index)` where `epoch <= next_epoch`.
A validator can use the following function to see if they are supposed to propose during a slot. This function can only be run with a `state` of the slot in question. Proposer selection is only stable within the context of the current epoch.
*Note*: To see if a validator is assigned to propose during the slot, the beacon state must be in the epoch in question. At the epoch boundaries, the validator must run an epoch transition into the epoch to successfully check the proposal assignment of the first slot.
*Note*: `BeaconBlock` proposal is distinct from beacon committee assignment, and in a given epoch each responsibility might occur at different a different slot.
The beacon chain shufflings are designed to provide a minimum of 1 epoch lookahead on the validator's upcoming committee assignments for attesting dictated by the shuffling and slot. Note that this lookahead does not apply to proposing, which must be checked during the epoch in question.
`get_committee_assignment` should be called at the start of each epoch to get the assignment for the next epoch (`current_epoch + 1`). A validator should plan for future assignments by noting at which future slot they will have to attest.
A validator has two primary responsibilities to the beacon chain: [proposing blocks](#block-proposal) and [creating attestations](#attestations-1). Proposals happen infrequently, whereas attestations should be created once per epoch.
A validator is expected to propose a [`BeaconBlock`](../core/0_beacon-chain.md#beaconblock) at the beginning of any slot during which `is_proposer(state, validator_index)` returns `True`. To propose, the validator selects the `BeaconBlock`, `parent`, that in their view of the fork choice is the head of the chain during `slot - 1`. The validator creates, signs, and broadcasts a `block` that is a child of `parent` that satisfies a valid [beacon chain state transition](../core/0_beacon-chain.md#beacon-chain-state-transition-function).
There is one proposer per slot, so if there are N active validators any individual validator will on average be assigned to propose once per N slots (e.g. at 312,500 validators = 10 million ETH, that's once per ~3 weeks).
Set `block.slot = slot` where `slot` is the current slot at which the validator has been selected to propose. The `parent` selected must satisfy that `parent.slot < block.slot`.
*Note*: There might be "skipped" slots between the `parent` and `block`. These skipped slots are processed in the state transition function without per-block processing.
*Note*: To calculate `state_root`, the validator should first run the state transition function on an unsigned `block` containing a stub for the `state_root`. It is useful to be able to run a state transition function that does _not_ validate signatures or state root for this purpose.
The `block.eth1_data` field is for block proposers to vote on recent Eth 1.0 data. This recent data contains an Eth 1.0 block hash as well as the associated deposit root (as calculated by the `get_deposit_root()` method of the deposit contract) and deposit count after execution of the corresponding Eth 1.0 block. If over half of the block proposers in the current Eth 1.0 voting period vote for the same `eth1_data` then `state.eth1_data` updates at the end of the voting period. Each deposit in `block.body.deposits` must verify against `state.eth1_data.eth1_deposit_root`.
Let `get_eth1_data(distance: uint64) -> Eth1Data` be the (subjective) function that returns the Eth 1.0 data at distance `distance` relative to the Eth 1.0 head at the start of the current Eth 1.0 voting period. Let `previous_eth1_distance` be the distance relative to the Eth 1.0 block corresponding to `state.eth1_data.block_hash` at the start of the current Eth 1.0 voting period. An honest block proposer sets `block.eth1_data = get_eth1_vote(state, previous_eth1_distance)` where:
Up to `MAX_PROPOSER_SLASHINGS`, [`ProposerSlashing`](../core/0_beacon-chain.md#proposerslashing) objects can be included in the `block`. The proposer slashings must satisfy the verification conditions found in [proposer slashings processing](../core/0_beacon-chain.md#proposer-slashings). The validator receives a small "whistleblower" reward for each proposer slashing found and included.
Up to `MAX_ATTESTER_SLASHINGS`, [`AttesterSlashing`](../core/0_beacon-chain.md#attesterslashing) objects can be included in the `block`. The attester slashings must satisfy the verification conditions found in [attester slashings processing](../core/0_beacon-chain.md#attester-slashings). The validator receives a small "whistleblower" reward for each attester slashing found and included.
Up to `MAX_ATTESTATIONS`, aggregate attestations can be included in the `block`. The attestations added must satisfy the verification conditions found in [attestation processing](../core/0_beacon-chain.md#attestations). To maximize profit, the validator should attempt to gather aggregate attestations that include singular attestations from the largest number of validators whose signatures from the same epoch have not previously been added on chain.
If there are any unprocessed deposits for the existing `state.eth1_data` (i.e. `state.eth1_data.deposit_count > state.eth1_deposit_index`), then pending deposits _must_ be added to the block. The expected number of deposits is exactly `min(MAX_DEPOSITS, eth1_data.deposit_count - state.eth1_deposit_index)`. These [`deposits`](../core/0_beacon-chain.md#deposit) are constructed from the `Deposit` logs from the [Eth 1.0 deposit contract](../core/0_deposit-contract.md) and must be processed in sequential order. The deposits included in the `block` must satisfy the verification conditions found in [deposits processing](../core/0_beacon-chain.md#deposits).
The `proof` for each deposit must be constructed against the deposit root contained in `state.eth1_data` rather than the deposit root at the time the deposit was initially logged from the 1.0 chain. This entails storing a full deposit merkle tree locally and computing updated proofs against the `eth1_data.deposit_root` as needed. See [`minimal_merkle.py`](https://github.com/ethereum/research/blob/master/spec_pythonizer/utils/merkle_minimal.py) for a sample implementation.
Up to `MAX_VOLUNTARY_EXITS`, [`VoluntaryExit`](../core/0_beacon-chain.md#voluntaryexit) objects can be included in the `block`. The exits must satisfy the verification conditions found in [exits processing](../core/0_beacon-chain.md#voluntary-exits).
A validator is expected to create, sign, and broadcast an attestation during each epoch. The `committee`, assigned `index`, and assigned `slot` for which the validator performs this role during an epoch are defined by `get_committee_assignment(state, epoch, validator_index)`.
A validator should create and broadcast the `attestation` to the associated attestation subnet one-third of the way through the `slot` during which the validator is assigned―that is, `SECONDS_PER_SLOT / 3` seconds after the start of `slot`.
First, the validator should construct `attestation_data`, an [`AttestationData`](../core/0_beacon-chain.md#attestationdata) object based upon the state at the assigned slot.
- Set `attestation_data.source = head_state.current_justified_checkpoint`.
- Set `attestation_data.target = Checkpoint(epoch=get_current_epoch(head_state), root=epoch_boundary_block_root)` where `epoch_boundary_block_root` is the root of block at the most recent epoch boundary.
Set `attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object defined in the previous section, [attestation data](#attestation-data).
- Let `attestation.aggregation_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` of length `len(committee)`, where the bit of the index of the validator in the `committee` is set to `0b1`.
*Note*: Calling `get_attesting_indices(state, attestation.data, attestation.aggregation_bits)` should return a list of length equal to 1, containing `validator_index`.
Finally, the validator broadcasts `attestation` to the associated attestation subnet -- the `index{attestation.data.index % ATTESTATION_SUBNET_COUNT}_beacon_attestation` pubsub topic.
## Attestation aggregation
The validator is expected to locally aggregate attestations with a similar `attestation_data` to their constructed `attestation` for the assigned `slot`.
The validator then _might_ broadcast their best aggregate to a global attestation channel two-thirds of the way through the `slot`-that is, `SECONDS_PER_SLOT * 2 / 3` seconds after the start of `slot`.
#### Construct aggregate
Collect `attestations` seen via gossip during the `slot` that have an equivalent `attestation_data` to that constructed by the validator.
The validator should create an `aggregate_attestation` with the following fields.
##### Data
Set `aggregate_attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object that is the same for each individual attestation being aggregated.
##### Aggregation bits
Let `aggregate_attestation.aggregation_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` of length `len(committee)`, where each bit set from each individual attestation is set to `0b1`.
##### Custody bits
- Let `aggregate_attestation.custody_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` filled with zeros of length `len(committee)`.
*Note*: This is a stub for Phase 0.
##### Aggregate signature
Set `aggregate_attestation.signature = aggregate_signature` where `aggregate_signature` is obtained from:
Starting two-thirds of the way into the slot, the validator begins running the following routine to decide if their best aggregate should be broadcast to the global attestation channel (`beacon_attestation`). `seen()` is defined as having seen an aggregate
"Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed: [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.
*Note*: Signed data must be within a sequential `Fork` context to conflict. Messages cannot be slashed across diverging forks. If the previous fork version is 1 and the chain splits into fork 2 and 102, messages from 1 can slashable against messages in forks 1, 2, and 102. Messages in 2 cannot be slashable against messages in 102, and vice versa.
To avoid "proposer slashings", a validator must not sign two conflicting [`BeaconBlock`](../core/0_beacon-chain.md#beaconblock) where conflicting is defined as two distinct blocks within the same epoch.
If the software crashes at some point within this routine, then when the validator comes back online, the hard disk has the record of the *potentially* signed/broadcast block and can effectively avoid slashing.
To avoid "attester slashings", a validator must not sign two conflicting [`AttestationData`](../core/0_beacon-chain.md#attestationdata) objects, i.e. two attestations that satisfy [`is_slashable_attestation_data`](../core/0_beacon-chain.md#is_slashable_attestation_data).
1. Save a record to hard disk that an attestation has been signed for source (i.e. `attestation_data.source.epoch`) and target (i.e. `attestation_data.target.epoch`).
If the software crashes at some point within this routine, then when the validator comes back online, the hard disk has the record of the *potentially* signed/broadcast attestation and can effectively avoid slashing.
