16 KiB
16 KiB
Ethereum 2.0 Phase 1 -- Custody Game
Notice: This document is a work-in-progress for researchers and implementers.
Table of contents
Introduction
This document details the beacon chain additions and changes in Phase 1 of Ethereum 2.0 to support the shard data custody game, building upon the Phase 0 specification.
Constants
Misc
| Name | Value | Unit |
| - | - |
| BLS12_381_Q | 4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787 |
| MINOR_REWARD_QUOTIENT | 2**8 (= 256) |
| BYTES_PER_CUSTODY_ATOM | 48 | bytes |
Time parameters
| Name | Value | Unit | Duration |
|---|---|---|---|
RANDAO_PENALTY_EPOCHS |
2**1 (= 2) |
epochs | 12.8 minutes |
EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS |
2**14 |
epochs | ~73 days |
EPOCHS_PER_CUSTODY_PERIOD |
2**11 (= 2,048) |
epochs | ~9 days |
CUSTODY_PERIOD_TO_RANDAO_PADDING |
2**11 (= 2,048) |
epochs | ~9 days |
MAX_REVEAL_LATENESS_DECREMENT |
2**7 (= 128) |
epochs | ~14 hours |
Max operations per block
| Name | Value |
|---|---|
MAX_CUSTODY_KEY_REVEALS |
2**8 (= 256) |
MAX_EARLY_DERIVED_SECRET_REVEALS |
1 |
MAX_CUSTODY_SLASHINGS |
1 |
Reward and penalty quotients
| Name | Value |
|---|---|
EARLY_DERIVED_SECRET_REVEAL_SLOT_REWARD_MULTIPLE |
2**1 (= 2) |
Signature domain types
The following types are defined, mapping into DomainType (little endian):
| Name | Value |
|---|---|
DOMAIN_CUSTODY_BIT_SLASHING |
DomainType('0x83000000') |
Data structures
New Beacon Chain operations
CustodySlashing
class CustodySlashing(Container):
# Attestation.custody_bits_blocks[data_index][committee.index(malefactor_index)] is the target custody bit to check.
# (Attestation.data.shard_transition_root as ShardTransition).shard_data_roots[data_index] is the root of the data.
data_index: uint64
malefactor_index: ValidatorIndex
malefactor_secret: BLSSignature
whistleblower_index: ValidatorIndex
shard_transition: ShardTransition
attestation: Attestation
data: ByteList[MAX_SHARD_BLOCK_CHUNKS * SHARD_BLOCK_CHUNK_SIZE]
SignedCustodySlashing
class SignedCustodySlashing(Container):
message: CustodySlashing
signature: BLSSignature
CustodyKeyReveal
class CustodyKeyReveal(Container):
# Index of the validator whose key is being revealed
revealer_index: ValidatorIndex
# Reveal (masked signature)
reveal: BLSSignature
EarlyDerivedSecretReveal
Represents an early (punishable) reveal of one of the derived secrets, where derived secrets are RANDAO reveals and custody reveals (both are part of the same domain).
class EarlyDerivedSecretReveal(Container):
# Index of the validator whose key is being revealed
revealed_index: ValidatorIndex
# RANDAO epoch of the key that is being revealed
epoch: Epoch
# Reveal (masked signature)
reveal: BLSSignature
# Index of the validator who revealed (whistleblower)
masker_index: ValidatorIndex
# Mask used to hide the actual reveal signature (prevent reveal from being stolen)
mask: Bytes32
Helpers
legendre_bit
Returns the Legendre symbol (a/q) normalizes as a bit (i.e. ((a/q) + 1) // 2). In a production implementation, a well-optimized library (e.g. GMP) should be used for this.
def legendre_bit(a: int, q: int) -> int:
if a >= q:
return legendre_bit(a % q, q)
if a == 0:
return 0
assert(q > a > 0 and q % 2 == 1)
t = 1
n = q
while a != 0:
while a % 2 == 0:
a //= 2
r = n % 8
if r == 3 or r == 5:
t = -t
a, n = n, a
if a % 4 == n % 4 == 3:
t = -t
a %= n
if n == 1:
return (t + 1) // 2
else:
return 0
custody_atoms
Given one set of data, return the custody atoms: each atom will be combined with one legendre bit.
def get_custody_atoms(bytez: bytes) -> Sequence[bytes]:
bytez += b'\x00' * (-len(bytez) % BYTES_PER_CUSTODY_ATOM) # right-padding
return [bytez[i:i + BYTES_PER_CUSTODY_ATOM]
for i in range(0, len(bytez), BYTES_PER_CUSTODY_ATOM)]
compute_custody_bit
def compute_custody_bit(key: BLSSignature, data: bytes) -> bit:
full_G2_element = bls.signature_to_G2(key)
s = full_G2_element[0].coeffs
bits = [legendre_bit(sum(s[i % 2]**i * int.from_bytes(atom, "little")), BLS12_381_Q)
for i, atom in enumerate(get_custody_atoms(data))]
# XOR all atom bits
return bit(sum(bits) % 2)
get_randao_epoch_for_custody_period
def get_randao_epoch_for_custody_period(period: uint64, validator_index: ValidatorIndex) -> Epoch:
next_period_start = (period + 1) * EPOCHS_PER_CUSTODY_PERIOD - validator_index % EPOCHS_PER_CUSTODY_PERIOD
return Epoch(next_period_start + CUSTODY_PERIOD_TO_RANDAO_PADDING)
get_custody_period_for_validator
def get_custody_period_for_validator(validator_index: ValidatorIndex, epoch: Epoch) -> int:
'''
Return the reveal period for a given validator.
'''
return (epoch + validator_index % EPOCHS_PER_CUSTODY_PERIOD) // EPOCHS_PER_CUSTODY_PERIOD
Per-block processing
Custody Game Operations
def process_custody_game_operations(state: BeaconState, body: BeaconBlockBody) -> None:
def for_ops(operations: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
for operation in operations:
fn(state, operation)
for_ops(body.custody_key_reveals, process_custody_key_reveal)
for_ops(body.early_derived_secret_reveals, process_early_derived_secret_reveal)
for_ops(body.custody_slashings, process_custody_slashing)
Custody key reveals
def process_custody_key_reveal(state: BeaconState, reveal: CustodyKeyReveal) -> None:
"""
Process ``CustodyKeyReveal`` operation.
Note that this function mutates ``state``.
"""
revealer = state.validators[reveal.revealer_index]
epoch_to_sign = get_randao_epoch_for_custody_period(revealer.next_custody_secret_to_reveal, reveal.revealer_index)
custody_reveal_period = get_custody_period_for_validator(reveal.revealer_index, get_current_epoch(state))
assert revealer.next_custody_secret_to_reveal < custody_reveal_period
# Revealed validator is active or exited, but not withdrawn
assert is_slashable_validator(revealer, get_current_epoch(state))
# Verify signature
domain = get_domain(state, DOMAIN_RANDAO, epoch_to_sign)
signing_root = compute_signing_root(epoch_to_sign, domain)
assert bls.Verify(revealer.pubkey, signing_root, reveal.reveal)
# Decrement max reveal lateness if response is timely
if epoch_to_sign + EPOCHS_PER_CUSTODY_PERIOD >= get_current_epoch(state):
if revealer.max_reveal_lateness >= MAX_REVEAL_LATENESS_DECREMENT:
revealer.max_reveal_lateness -= MAX_REVEAL_LATENESS_DECREMENT
else:
revealer.max_reveal_lateness = 0
else:
revealer.max_reveal_lateness = max(
revealer.max_reveal_lateness,
get_current_epoch(state) - epoch_to_sign - EPOCHS_PER_CUSTODY_PERIOD
)
# Process reveal
revealer.next_custody_secret_to_reveal += 1
# Reward Block Proposer
proposer_index = get_beacon_proposer_index(state)
increase_balance(
state,
proposer_index,
Gwei(get_base_reward(state, reveal.revealer_index) // MINOR_REWARD_QUOTIENT)
)
Early derived secret reveals
def process_early_derived_secret_reveal(state: BeaconState, reveal: EarlyDerivedSecretReveal) -> None:
"""
Process ``EarlyDerivedSecretReveal`` operation.
Note that this function mutates ``state``.
"""
revealed_validator = state.validators[reveal.revealed_index]
derived_secret_location = reveal.epoch % EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS
assert reveal.epoch >= get_current_epoch(state) + RANDAO_PENALTY_EPOCHS
assert reveal.epoch < get_current_epoch(state) + EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS
assert not revealed_validator.slashed
assert reveal.revealed_index not in state.exposed_derived_secrets[derived_secret_location]
# Verify signature correctness
masker = state.validators[reveal.masker_index]
pubkeys = [revealed_validator.pubkey, masker.pubkey]
domain = get_domain(state, DOMAIN_RANDAO, reveal.epoch)
signing_roots = [compute_signing_root(root, domain) for root in [hash_tree_root(reveal.epoch), reveal.mask]]
assert bls.AggregateVerify(zip(pubkeys, signing_roots), reveal.reveal)
if reveal.epoch >= get_current_epoch(state) + CUSTODY_PERIOD_TO_RANDAO_PADDING:
# Full slashing when the secret was revealed so early it may be a valid custody
# round key
slash_validator(state, reveal.revealed_index, reveal.masker_index)
else:
# Only a small penalty proportional to proposer slot reward for RANDAO reveal
# that does not interfere with the custody period
# The penalty is proportional to the max proposer reward
# Calculate penalty
max_proposer_slot_reward = (
get_base_reward(state, reveal.revealed_index)
* SLOTS_PER_EPOCH
// len(get_active_validator_indices(state, get_current_epoch(state)))
// PROPOSER_REWARD_QUOTIENT
)
penalty = Gwei(
max_proposer_slot_reward
* EARLY_DERIVED_SECRET_REVEAL_SLOT_REWARD_MULTIPLE
* (len(state.exposed_derived_secrets[derived_secret_location]) + 1)
)
# Apply penalty
proposer_index = get_beacon_proposer_index(state)
whistleblower_index = reveal.masker_index
whistleblowing_reward = Gwei(penalty // WHISTLEBLOWER_REWARD_QUOTIENT)
proposer_reward = Gwei(whistleblowing_reward // PROPOSER_REWARD_QUOTIENT)
increase_balance(state, proposer_index, proposer_reward)
increase_balance(state, whistleblower_index, whistleblowing_reward - proposer_reward)
decrease_balance(state, reveal.revealed_index, penalty)
# Mark this derived secret as exposed so validator cannot be punished repeatedly
state.exposed_derived_secrets[derived_secret_location].append(reveal.revealed_index)
Custody Slashings
def process_custody_slashing(state: BeaconState, signed_custody_slashing: SignedCustodySlashing) -> None:
custody_slashing = signed_custody_slashing.message
attestation = custody_slashing.attestation
# Any signed custody-slashing should result in at least one slashing.
# If the custody bits are valid, then the claim itself is slashed.
malefactor = state.validators[custody_slashing.malefactor_index]
whistleblower = state.validators[custody_slashing.whistleblower_index]
domain = get_domain(state, DOMAIN_CUSTODY_BIT_SLASHING, get_current_epoch(state))
signing_root = compute_signing_root(custody_slashing, domain)
assert bls.Verify(whistleblower.pubkey, signing_root, signed_custody_slashing.signature)
# Verify that the whistleblower is slashable
assert is_slashable_validator(whistleblower, get_current_epoch(state))
# Verify that the claimed malefactor is slashable
assert is_slashable_validator(malefactor, get_current_epoch(state))
# Verify the attestation
assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))
# TODO: custody_slashing.data is not chunked like shard blocks yet, result is lots of padding.
# TODO: can do a single combined merkle proof of data being attested.
# Verify the shard transition is indeed attested by the attestation
shard_transition = custody_slashing.shard_transition
assert hash_tree_root(shard_transition) == attestation.shard_transition_root
# Verify that the provided data matches the shard-transition
shard_chunk_roots = shard_transition.shard_data_roots[custody_slashing.data_index]
assert hash_tree_root(custody_slashing.data) == chunks_to_body_root(shard_chunk_roots)
# Verify existence and participation of claimed malefactor
attesters = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
assert custody_slashing.malefactor_index in attesters
# Verify the malefactor custody key
epoch_to_sign = get_randao_epoch_for_custody_period(
get_custody_period_for_validator(custody_slashing.malefactor_index, attestation.data.target.epoch),
custody_slashing.malefactor_index,
)
domain = get_domain(state, DOMAIN_RANDAO, epoch_to_sign)
signing_root = compute_signing_root(epoch_to_sign, domain)
assert bls.Verify(malefactor.pubkey, signing_root, custody_slashing.malefactor_secret)
# Get the custody bit
custody_bits = attestation.custody_bits_blocks[custody_slashing.data_index]
committee = get_beacon_committee(state, attestation.data.slot, attestation.data.index)
claimed_custody_bit = custody_bits[committee.index(custody_slashing.malefactor_index)]
# Compute the custody bit
computed_custody_bit = compute_custody_bit(custody_slashing.malefactor_secret, custody_slashing.data)
# Verify the claim
if claimed_custody_bit != computed_custody_bit:
# Slash the malefactor, reward the other committee members
slash_validator(state, custody_slashing.malefactor_index)
others_count = len(committee) - 1
whistleblower_reward = Gwei(malefactor.effective_balance // WHISTLEBLOWER_REWARD_QUOTIENT // others_count)
for attester_index in attesters:
if attester_index != custody_slashing.malefactor_index:
increase_balance(state, attester_index, whistleblower_reward)
# No special whisteblower reward: it is expected to be an attester. Others are free to slash too however.
else:
# The claim was false, the custody bit was correct. Slash the whistleblower that induced this work.
slash_validator(state, custody_slashing.whistleblower_index)
Per-epoch processing
Handling of reveal deadlines
Run process_reveal_deadlines(state) after process_registry_updates(state):
def process_reveal_deadlines(state: BeaconState) -> None:
epoch = get_current_epoch(state)
for index, validator in enumerate(state.validators):
if get_custody_period_for_validator(ValidatorIndex(index), epoch) > validator.next_custody_secret_to_reveal:
slash_validator(state, ValidatorIndex(index))
Final updates
After process_final_updates(state), additional updates are made for the custody game:
def process_custody_final_updates(state: BeaconState) -> None:
# Clean up exposed RANDAO key reveals
state.exposed_derived_secrets[get_current_epoch(state) % EARLY_DERIVED_SECRET_PENALTY_MAX_FUTURE_EPOCHS] = []