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Persistent committee refactors

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vbuterin 2019-02-08 04:08:52 -06:00 committed by GitHub
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1 changed files with 28 additions and 10 deletions
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38
specs/core/1_shard-data-chains.md
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@ -31,6 +31,17 @@ Phase 1 depends upon all of the constants defined in [Phase 0](0_beacon-chain.md
## Helper functions
#### get_split_offset
````python
def get_split_offset(list_size: int, chunks: int, index: int) -> int:
"""
Returns a value such that for a list L, chunk count k and index i,
split(L, k)[i] == L[get_split_offset(len(L), k, i): get_split_offset(len(L), k+1, i)]
"""
return (len(list_size) * index) // chunks
````
#### get_persistent_committee
```python
@ -40,20 +51,28 @@ def get_persistent_commmitee(seed: Bytes32,
epoch: EpochNumber) -> List[ValidatorIndex]:
earlier_committee_start = epoch - (epoch % PERSISTENT_COMMITTEE_PERIOD) - PERSISTENT_COMMITTEE_PERIOD * 2
earlier_committee = split(shuffle(
get_active_validator_indices(validators, earlier_committee_start),
generate_seed(state, earlier_committee_start)
), SHARD_COUNT)[shard]
earlier_validator_set = get_active_validator_indices(validators, earlier_committee_start)
earlier_seed = generate_seed(state, earlier_committee_start)
earlier_start_offset = get_split_offset(len(earlier_validator_set), SHARD_COUNT, shard)
earlier_end_offset = get_split_offset(len(earlier_validator_set), SHARD_COUNT, shard+1)
earlier_committee = [
earlier_validator_set[get_permuted_index(i, len(earlier_validator_set), earlier_seed)]
for i in range(earlier_start_offset, earlier_end_offset)
]
later_committee_start = epoch - (epoch % PERSISTENT_COMMITTEE_PERIOD) - PERSISTENT_COMMITTEE_PERIOD
later_committee = split(shuffle(
get_active_validator_indices(validators, later_committee_start),
generate_seed(state, later_committee_start)
), SHARD_COUNT)[shard]
later_validator_set = get_active_validator_indices(validators, later_committee_start)
later_seed = generate_seed(state, later_committee_start)
later_start_offset = get_split_offset(len(later_validator_set), SHARD_COUNT, shard)
later_end_offset = get_split_offset(len(later_validator_set), SHARD_COUNT, shard+1)
later_committee = [
later_validator_set[get_permuted_index(i, len(later_validator_set), later_seed)]
for i in range(later_start_offset, later_end_offset)
]
def get_switchover_epoch(index):
return (
bytes_to_int(hash(generate_seed(state, earlier_committee_start) + bytes3(index))[0:8]) %
bytes_to_int(hash(earlier_seed + bytes3(index))[0:8]) %
PERSISTENT_COMMITTEE_PERIOD
)
@ -108,7 +127,6 @@ To validate a block header on shard `shard_block.shard_id`, compute as follows:
* Let `proposer_index = hash(state.randao_mix + int_to_bytes8(shard_block.shard_id) + int_to_bytes8(shard_block.slot)) % len(validators)`. Let `msg` be the `shard_block` but with `shard_block.signature` set to `[0, 0]`. Verify that `bls_verify(pubkey=validators[proposer_index].pubkey, message_hash=hash(msg), signature=shard_block.signature, domain=get_domain(state, shard_block.slot, SHARD_PROPOSER_DOMAIN))` passes.
* Let `group_public_key = bls_aggregate_pubkeys([state.validators[index].pubkey for i, index in enumerate(persistent_committee) if get_bitfield_bit(shard_block.participation_bitfield, i) is True])`. Verify that `bls_verify(pubkey=group_public_key, message_hash=shard_block.parent_root, sig=shard_block.aggregate_signature, domain=get_domain(state, slot, SHARD_ATTESTER_DOMAIN))` passes.
### Verifying shard block data
At network layer, we expect a shard block header to be broadcast along with its `block_body`.