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# Ethereum 2.0 Phase 0 -- Honest Validator
**Notice**: This document is a work-in-progress for researchers and implementers. This is an accompanying document to [Ethereum 2.0 Phase 1](./), which describes the expected actions of a "validator" participating in the Ethereum 2.0 Phase 1 protocol.
## Table of contents
<!-- TOC -->
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
- [Introduction](#introduction)
- [Prerequisites](#prerequisites)
- [Constants](#constants)
- [Misc](#misc)
- [Becoming a validator](#becoming-a-validator)
- [Beacon chain validator assignments](#beacon-chain-validator-assignments)
- [Lookahead](#lookahead)
- [Beacon chain responsibilities](#beacon-chain-responsibilities)
- [Block proposal](#block-proposal)
- [Preparing for a `BeaconBlock`](#preparing-for-a-beaconblock)
- [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
- [Custody slashings](#custody-slashings)
- [Custody key reveals](#custody-key-reveals)
- [Early derived secret reveals](#early-derived-secret-reveals)
- [Shard transitions](#shard-transitions)
- [Light client fields](#light-client-fields)
- [Packaging into a `SignedBeaconBlock`](#packaging-into-a-signedbeaconblock)
- [Attesting](#attesting)
- [`FullAttestationData`](#fullattestationdata)
- [`FullAttestation`](#fullattestation)
- [Timing](#timing)
- [Attestation data](#attestation-data)
- [Head shard root](#head-shard-root)
- [Shard transition](#shard-transition)
- [Construct attestation](#construct-attestation)
- [Attestation Aggregation](#attestation-aggregation)
- [Broadcast aggregate](#broadcast-aggregate)
- [`AggregateAndProof`](#aggregateandproof)
- [`SignedAggregateAndProof`](#signedaggregateandproof)
- [Light client committee](#light-client-committee)
- [Preparation](#preparation)
- [Light clent vote](#light-clent-vote)
- [Light client vote data](#light-client-vote-data)
- [`LightClientVoteData`](#lightclientvotedata)
- [Construct vote](#construct-vote)
- [`LightClientVote`](#lightclientvote)
- [Broadcast](#broadcast)
- [Light client vote aggregation](#light-client-vote-aggregation)
- [Aggregation selection](#aggregation-selection)
- [Construct aggregate](#construct-aggregate)
- [Broadcast aggregate](#broadcast-aggregate-1)
- [`LightAggregateAndProof`](#lightaggregateandproof)
- [`SignedLightAggregateAndProof`](#signedlightaggregateandproof)
- [How to avoid slashing](#how-to-avoid-slashing)
- [Custody slashing](#custody-slashing)
<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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## Introduction
This document represents the expected behavior of an "honest validator" with respect to Phase 1 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.
## Prerequisites
This document is an extension of the [Phase 0 -- Validator](../phase0/validator.md). All behaviors and definitions defined in the Phase 0 doc carry over unless explicitly noted or overridden.
All terminology, constants, functions, and protocol mechanics defined in the [Phase 1 -- The Beacon Chain](./beacon-chain.md) and [Phase 1 -- Custody Game](./custody-game.md) docs are requisite for this document and used throughout. Please see the Phase 1 docs before continuing and use as a reference throughout.
## Constants
See constants from [Phase 0 validator guide](../phase0/validator.md#constants).
### Misc
| Name | Value | Unit | Duration |
| - | - | :-: | :-: |
| `TARGET_LIGHT_CLIENT_AGGREGATORS_PER_SLOT` | `2**3` (= 8) | validators | |
| `LIGHT_CLIENT_PREPARATION_EPOCHS` | `2**2` (= 4) | epochs | |
## Becoming a validator
Becoming a validator in Phase 1 is unchanged from Phase 0. See the [Phase 0 validator guide](../phase0/validator.md#becoming-a-validator) for details.
## Beacon chain validator assignments
Beacon chain validator assignments to beacon committees and beacon block proposal are unchanged from Phase 0. See the [Phase 0 validator guide](../phase0/validator.md#validator-assignments) for details.
### Lookahead
Lookahead for beacon committee assignments operates in the same manner as Phase 0, but committee members must join a shard block pubsub topic in addition to the committee attestation topic.
Specifically _after_ finding stable peers of attestation subnets (see Phase 0) a validator should:
* Let `shard = compute_shard_from_committee_index(committe_index)`
* Subscribe to the pubsub topic `shard_{shard}_block` (attestation subnet peers should have this topic available).
## Beacon chain responsibilities
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.
These responsibilities are largely unchanged from Phase 0, but utilize the updated `SignedBeaconBlock`, `BeaconBlock`, `BeaconBlockBody`, `Attestation`, and `AttestationData` definitions found in Phase 1. Below notes only the additional and modified behavior with respect to Phase 0.
Phase 1 adds light client committees and associated responsibilities, discussed [below](#light-client-committee).
### Block proposal
#### Preparing for a `BeaconBlock`
`slot`, `proposer_index`, `parent_root` fields are unchanged.
#### Constructing the `BeaconBlockBody`
`randao_reveal`, `eth1_data`, and `graffiti` are unchanged.
`proposer_slashings`, `deposits`, and `voluntary_exits` are unchanged.
`attester_slashings` and `attestations` operate exactly as in Phase 0, but with new definitations of `AttesterSlashing` and `Attestation`, along with modified validation conditions found in `process_attester_slashing` and `process_attestation`.
##### Custody slashings
Up to `MAX_CUSTODY_SLASHINGS`, [`CustodySlashing`](./custody-game.md#custodyslashing) objects can be included in the `block`. The custody slashings must satisfy the verification conditions found in [custody slashings processing](./custody-game.md#custody-slashings). The validator receives a small "whistleblower" reward for each custody slashing included (THIS IS NOT CURRENTLY THE CASE BUT PROBABLY SHOULD BE).
##### Custody key reveals
Up to `MAX_CUSTODY_KEY_REVEALS`, [`CustodyKeyReveal`](./custody-game.md#custodykeyreveal) objects can be included in the `block`. The custody key reveals must satisfy the verification conditions found in [custody key reveal processing](./custody-game.md#custody-key-reveals). The validator receives a small reward for each custody key reveal included.
##### Early derived secret reveals
Up to `MAX_EARLY_DERIVED_SECRET_REVEALS`, [`EarlyDerivedSecretReveal`](./custody-game.md#earlyderivedsecretreveal) objects can be included in the `block`. The early derived secret reveals must satisfy the verification conditions found in [early derived secret reveal processing](./custody-game.md#custody-key-reveals). The validator receives a small "whistleblower" reward for each early derived secrete reveal included.
##### Shard transitions
Exactly `MAX_SHARDS` [`ShardTransition`](./beacon-chain#shardtransition) objects are included in the block. Default each to an empty `ShardTransition()`. Then for each committee assigned to the slot with an associated `committee_index` and `shard`, set `shard_transitions[shard] = full_transitions[winning_root]` if the committee had enough weight to form a crosslink this slot.
Specifically:
* Call `shards, winning_roots = get_shard_winning_roots(state, block.slot, block.body.attestations)`
* Let `full_transitions` be a dictionary mapping from the `shard_transition_root`s found in `attestations` to the corresponding full `ShardTransition`
* Then for each `shard` and `winning_root` in `zip(shards, winning_roots)` set `shard_transitions[shard] = full_transitions[winning_root]`
*Note*: The `state` passed into `get_shard_winning_roots` must be transitioned the slot of `block.slot` to run accurately due to the internal use of `get_online_validator_indices` and `is_on_time_attestation`.
```python
def get_shard_winning_roots(state: BeaconState,
attestations: Sequence[Attestation]) -> Tuple[Sequence[Shard], Sequence[Root]]:
shards = []
winning_roots = []
online_indices = get_online_validator_indices(state)
committee_count = get_committee_count_at_slot(state, state.slot)
for committee_index in map(CommitteeIndex, range(committee_count)):
shard = compute_shard_from_committee_index(state, committee_index, state.slot)
# All attestations in the block for this committee/shard and are "on time"
shard_attestations = [
attestation for attestation in attestations
if is_on_time_attestation(state, attestation) and attestation.data.index == committee_index
]
committee = get_beacon_committee(state, state.slot, committee_index)
# Loop over all shard transition roots, looking for a winning root
shard_transition_roots = set([a.data.shard_transition_root for a in shard_attestations])
for shard_transition_root in sorted(shard_transition_roots):
transition_attestations = [
a for a in shard_attestations
if a.data.shard_transition_root == shard_transition_root
]
transition_participants: Set[ValidatorIndex] = set()
for attestation in transition_attestations:
participants = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
transition_participants = transition_participants.union(participants)
enough_online_stake = (
get_total_balance(state, online_indices.intersection(transition_participants)) * 3 >=
get_total_balance(state, online_indices.intersection(committee)) * 2
)
if enough_online_stake:
shards.append(shard)
winning_roots.append(shard_transition_root)
break
return shards, winning_roots
```
##### Light client fields
First retrieve `best_aggregate` from `get_best_light_client_aggregate` where `aggregates` is a list of valid aggregated `LightClientVote`s for the previous slot.
Then:
* Set `light_client_bits = best_aggregate.aggregation_bits`
* Set `light_client_signature = best_aggregate.signature`
```python
def get_best_light_client_aggregate(block: BeaconBlock,
aggregates: Sequence[LightClientVote]) -> LightClientVote:
viable_aggregates = [
aggregate for aggregate in aggregates
if (
aggregate.data.slot == compute_previous_slot(block.slot)
and aggregate.data.beacon_block_root == block.parent_root
)
]
return max(
viable_aggregates,
# Ties broken by lexicographically by hash_tree_root
key=lambda a: (len([i for i in a.aggregation_bits if i == 1]), hash_tree_root(a)),
default=LightClientVote(),
)
```
#### Packaging into a `SignedBeaconBlock`
Packaging into a `SignedBeaconBlock` is unchanged from Phase 0.
### Attesting
A validator is expected to create, sign, and broadcast an attestation during each epoch.
Assignments and the core of this duty are unchanged from Phase 0. There are a few additional fields related to the assigned shard chain.
The `Attestation` and `AttestationData` defined in the [Phase 1 Beacon Chain spec]() utilizes `shard_transition_root: Root` rather than a full `ShardTransition`. For the purposes of the validator and p2p layer, a modified `FullAttestationData` and containing `FullAttestation` are used to send the accompanying `ShardTransition` in its entirety. Note that due to the properties of SSZ `hash_tree_root`, the root and signatures of `AttestationData` and `FullAttestationData` are equivalent.
#### `FullAttestationData`
```python
class FullAttestationData(Container):
slot: Slot
index: CommitteeIndex
# LMD GHOST vote
beacon_block_root: Root
# FFG vote
source: Checkpoint
target: Checkpoint
# Current-slot shard block root
shard_head_root: Root
# Full shard transition
shard_transition: ShardTransition
```
#### `FullAttestation`
```python
class FullAttestation(Container):
aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
data: FullAttestationData
signature: BLSSignature
```
#### Timing
Note the timing of when to create/broadcast is altered from Phase 1.
A validator should create and broadcast the `attestation` to the associated attestation subnet when either (a) the validator has received a valid `BeaconBlock` from the expected beacon block proposer and a valid `ShardBlock` for the expected shard block proposer for the assigned `slot` or (b) one-half of the `slot` has transpired (`SECONDS_PER_SLOT / 2` seconds after the start of `slot`) -- whichever comes _first_.
#### Attestation data
`attestation_data` is constructed in the same manner as Phase 0 but uses `FullAttestationData` with the addition of two fields -- `shard_head_root` and `shard_transition`.
- Let `head_block` be the result of running the fork choice during the assigned slot.
- Let `head_state` be the state of `head_block` processed through any empty slots up to the assigned slot using `process_slots(state, slot)`.
- Let `shard_head_block` be the result of running the fork choice on the assigned shard chain during the assigned slot.
- Let `shard_blocks` be the shard blocks in the chain starting immediately _after_ the most recent crosslink (`head_state.shard_transitions[shard].latest_block_root`) up to the `shard_head_block` (i.e. the value of the shard fork choice store of `get_pending_shard_blocks(store, shard_store)`).
*Note*: We assume that the fork choice only follows branches with valid `offset_slots` with respect to the most recent beacon state shard transition for the queried shard.
##### Head shard root
Set `attestation_data.shard_head_root = hash_tree_root(shard_head_block)`.
##### Shard transition
Set `shard_transition` to the value returned by `get_shard_transition(head_state, shard, shard_blocks)`.
```python
def get_shard_transition_fields(
beacon_state: BeaconState,
shard: Shard,
shard_blocks: Sequence[SignedShardBlock],
validate_signature: bool=True,
) -> Tuple[Sequence[uint64], Sequence[Root], Sequence[ShardState]]:
shard_states = []
shard_data_roots = []
shard_block_lengths = []
shard_state = beacon_state.shard_states[shard]
shard_block_slots = [shard_block.message.slot for shard_block in shard_blocks]
offset_slots = compute_offset_slots(
get_latest_slot_for_shard(beacon_state, shard),
Slot(beacon_state.slot + 1),
)
for slot in offset_slots:
if slot in shard_block_slots:
shard_block = shard_blocks[shard_block_slots.index(slot)]
shard_data_roots.append(hash_tree_root(shard_block.message.body))
else:
shard_block = SignedShardBlock(message=ShardBlock(slot=slot, shard=shard))
shard_data_roots.append(Root())
shard_state = get_post_shard_state(shard_state, shard_block.message)
shard_states.append(shard_state)
shard_block_lengths.append(len(shard_block.message.body))
return shard_block_lengths, shard_data_roots, shard_states
```
```python
def get_shard_transition(beacon_state: BeaconState,
shard: Shard,
shard_blocks: Sequence[SignedShardBlock]) -> ShardTransition:
offset_slots = compute_offset_slots(
get_latest_slot_for_shard(beacon_state, shard),
Slot(beacon_state.slot + 1),
)
shard_block_lengths, shard_data_roots, shard_states = (
get_shard_transition_fields(beacon_state, shard, shard_blocks)
)
if len(shard_blocks) > 0:
proposer_signatures = [shard_block.signature for shard_block in shard_blocks]
proposer_signature_aggregate = bls.Aggregate(proposer_signatures)
else:
proposer_signature_aggregate = NO_SIGNATURE
return ShardTransition(
start_slot=offset_slots[0],
shard_block_lengths=shard_block_lengths,
shard_data_roots=shard_data_roots,
shard_states=shard_states,
proposer_signature_aggregate=proposer_signature_aggregate,
)
```
#### Construct attestation
Next, the validator creates `attestation`, a `FullAttestation` as defined above.
`attestation.data`, `attestation.aggregation_bits`, and `attestation.signature` are unchanged from Phase 0. But safety/validity in signing the message is premised upon calculation of the "custody bit" [TODO].
### Attestation Aggregation
Some validators are selected to locally aggregate attestations with a similar `attestation_data` to their constructed `attestation` for the assigned `slot`.
Aggregation selection and the core of this duty are largely unchanged from Phase 0. Any additional components or changes are noted.
#### Broadcast aggregate
Note the timing of when to broadcast aggregates is altered in Phase 1+.
If the validator is selected to aggregate (`is_aggregator`), then they broadcast their best aggregate as a `SignedAggregateAndProof` to the global aggregate channel (`beacon_aggregate_and_proof`) three-fourths of the way through the `slot`-that is, `SECONDS_PER_SLOT * 3 / 4` seconds after the start of `slot`.
##### `AggregateAndProof`
`AggregateAndProof` is unchanged other than the contained `Attestation`.
```python
class AggregateAndProof(Container):
aggregator_index: ValidatorIndex
aggregate: Attestation
selection_proof: BLSSignature
```
##### `SignedAggregateAndProof`
`AggregateAndProof` is unchanged other than the contained `AggregateAndProof`.
```python
class SignedAggregateAndProof(Container):
message: AggregateAndProof
signature: BLSSignature
```
### Light client committee
In addition to the core beacon chain responsibilities, Phase 1 adds an additional role -- the Light Client Committee -- to aid in light client functionality.
Validators serve on the light client committee for `LIGHT_CLIENT_COMMITTEE_PERIOD` epochs and the assignment to be on a committee is known `LIGHT_CLIENT_COMMITTEE_PERIOD` epochs in advance.
#### Preparation
When `get_current_epoch(state) % LIGHT_CLIENT_COMMITTEE_PERIOD == LIGHT_CLIENT_COMMITTEE_PERIOD - LIGHT_CLIENT_PREPARATION_EPOCHS` each validator must check if they are in the next period light client committee by calling `is_in_next_light_client_committee()`.
If the validator is in the next light client committee, they must join the `light_client_votes` pubsub topic to begin duties at the start of the next period.
```python
def is_in_next_light_client_committee(state: BeaconState, index: ValidatorIndex) -> bool:
next_committee = get_light_client_committee(state, get_current_epoch(state) + LIGHT_CLIENT_COMMITTEE_PERIOD)
return index in next_committee
```
#### Light clent vote
During a period of epochs that the validator is a part of the light client committee (`validator_index in get_light_client_committee(state, epoch)`), the validator creates and broadcasts a `LightClientVote` at each slot.
A validator should create and broadcast the `light_client_vote` to the `light_client_votes` pubsub topic when either (a) the validator has received a valid block from the expected block proposer for the current `slot` or (b) two-thirds of the `slot` have transpired (`SECONDS_PER_SLOT / 3` seconds after the start of `slot`) -- whichever comes _first_.
- Let `light_client_committee = get_light_client_committee(state, compute_epoch_at_slot(slot))`
##### Light client vote data
First the validator constructs `light_client_vote_data`, a [`LightClientVoteData`](#lightclientvotedata) object.
* Let `head_block` be the result of running the fork choice during the assigned slot.
* Set `light_client_vote.slot = slot`.
* Set `light_client_vote.beacon_block_root = hash_tree_root(head_block)`.
###### `LightClientVoteData`
```python
class LightClientVoteData(Container):
slot: Slot
beacon_block_root: Root
```
##### Construct vote
Then the validator constructs `light_client_vote`, a [`LightClientVote`](#lightclientvote) object.
* Set `light_client_vote.data = light_client_vote_data`.
* Set `light_client_vote.aggregation_bits` to be a `Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE]`, where the bit of the index of the validator in the `light_client_committee` is set to `0b1` and all other bits are are set to `0b0`.
* Set `light_client_vote.signature = vote_signature` where `vote_signature` is obtained from:
```python
def get_light_client_vote_signature(state: BeaconState,
light_client_vote_data: LightClientVoteData,
privkey: int) -> BLSSignature:
domain = get_domain(state, DOMAIN_LIGHT_CLIENT, compute_epoch_at_slot(light_client_vote_data.slot))
signing_root = compute_signing_root(light_client_vote_data, domain)
return bls.Sign(privkey, signing_root)
```
###### `LightClientVote`
```python
class LightClientVote(Container):
data: LightClientVoteData
aggregation_bits: Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE]
signature: BLSSignature
```
##### Broadcast
Finally, the validator broadcasts `light_client_vote` to the `light_client_votes` pubsub topic.
#### Light client vote aggregation
Some validators in the light client committee are selected to locally aggregate light client votes with a similar `light_client_vote_data` to their constructed `light_client_vote` for the assigned `slot`.
#### Aggregation selection
A validator is selected to aggregate based upon the return value of `is_light_client_aggregator()`.
```python
def get_light_client_slot_signature(state: BeaconState, slot: Slot, privkey: int) -> BLSSignature:
domain = get_domain(state, DOMAIN_LIGHT_SELECTION_PROOF, compute_epoch_at_slot(slot))
signing_root = compute_signing_root(slot, domain)
return bls.Sign(privkey, signing_root)
```
```python
def is_light_client_aggregator(state: BeaconState, slot: Slot, slot_signature: BLSSignature) -> bool:
committee = get_light_client_committee(state, compute_epoch_at_slot(slot))
modulo = max(1, len(committee) // TARGET_LIGHT_CLIENT_AGGREGATORS_PER_SLOT)
return bytes_to_int(hash(slot_signature)[0:8]) % modulo == 0
```
#### Construct aggregate
If the validator is selected to aggregate (`is_light_client_aggregator()`), they construct an aggregate light client vote via the following.
Collect `light_client_votes` seen via gossip during the `slot` that have an equivalent `light_client_vote_data` to that constructed by the validator, and create a `aggregate_light_client_vote: LightClientVote` with the following fields.
* Set `aggregate_light_client_vote.data = light_client_vote_data` where `light_client_vote_data` is the `LightClientVoteData` object that is the same for each individual light client vote being aggregated.
* Set `aggregate_light_client_vote.aggregation_bits` to be a `Bitvector[LIGHT_CLIENT_COMMITTEE_SIZE]`, where each bit set from each individual light client vote is set to `0b1`.
* Set `aggregate_light_client_vote.signature = aggregate_light_client_signature` where `aggregate_light_client_signature` is obtained from `get_aggregate_light_client_signature`.
```python
def get_aggregate_light_client_signature(light_client_votes: Sequence[LightClientVote]) -> BLSSignature:
signatures = [light_client_vote.signature for light_client_vote in light_client_votes]
return bls.Aggregate(signatures)
```
#### Broadcast aggregate
If the validator is selected to aggregate (`is_light_client_aggregator`), then they broadcast their best aggregate light client vote as a `SignedLightAggregateAndProof` to the global aggregate light client vote channel (`aggregate_light_client_votes`) two-thirds of the way through the `slot`-that is, `SECONDS_PER_SLOT * 2 / 3` seconds after the start of `slot`.
Selection proofs are provided in `LightAggregateAndProof` to prove to the gossip channel that the validator has been selected as an aggregator.
`LightAggregateAndProof` messages are signed by the aggregator and broadcast inside of `SignedLightAggregateAndProof` objects to prevent a class of DoS attacks and message forgeries.
First, `light_aggregate_and_proof = get_light_aggregate_and_proof(state, validator_index, aggregate_light_client_vote, privkey)` is constructed.
```python
def get_light_aggregate_and_proof(state: BeaconState,
aggregator_index: ValidatorIndex,
aggregate: LightClientVote,
privkey: int) -> LightAggregateAndProof:
return LightAggregateAndProof(
aggregator_index=aggregator_index,
aggregate=aggregate,
selection_proof=get_light_client_slot_signature(state, aggregate.data.slot, privkey),
)
```
Then `signed_light_aggregate_and_proof = SignedLightAggregateAndProof(message=light_aggregate_and_proof, signature=signature)` is constructed and broadast. Where `signature` is obtained from:
```python
def get_light_aggregate_and_proof_signature(state: BeaconState,
aggregate_and_proof: LightAggregateAndProof,
privkey: int) -> BLSSignature:
aggregate = aggregate_and_proof.aggregate
domain = get_domain(state, DOMAIN_LIGHT_AGGREGATE_AND_PROOF, compute_epoch_at_slot(aggregate.data.slot))
signing_root = compute_signing_root(aggregate_and_proof, domain)
return bls.Sign(privkey, signing_root)
```
##### `LightAggregateAndProof`
```python
class LightAggregateAndProof(Container):
aggregator_index: ValidatorIndex
aggregate: LightClientVote
selection_proof: BLSSignature
```
##### `SignedLightAggregateAndProof`
```python
class SignedLightAggregateAndProof(Container):
message: LightAggregateAndProof
signature: BLSSignature
```
## How to avoid slashing
Proposer and Attester slashings described in Phase 0 remain in place with the
addition of the following.
### Custody slashing
To avoid custody slashings, the attester must never sign any shard transition for which the custody bit is one. The custody bit is computed using the custody secret:
```python
def get_custody_secret(state: BeaconState,
validator_index: ValidatorIndex,
privkey: int,
epoch: Epoch=None) -> BLSSignature:
if epoch is None:
epoch = get_current_epoch(state)
period = get_custody_period_for_validator(validator_index, epoch)
epoch_to_sign = get_randao_epoch_for_custody_period(period, validator_index)
domain = get_domain(state, DOMAIN_RANDAO, epoch_to_sign)
signing_root = compute_signing_root(Epoch(epoch_to_sign), domain)
return bls.Sign(privkey, signing_root)
```
Note that the valid custody secret is always the one for the **attestation target epoch**, not to be confused with the epoch in which the shard block was generated. While they are the same most of the time, getting this wrong at custody epoch boundaries would result in a custody slashing.
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