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RANDAO hash chain and no stale shuffles 

* Make RANDAO into a hash chain (this makes it easy for applications to prove the intermediate RANDAO reveals).
* Include `state.slot` when shuffle to avoid stale shuffles with skip slots
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Justin 2019-01-02 14:20:01 +00:00 committed by GitHub
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14
specs/core/0_beacon-chain.md
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@ -187,6 +187,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
| - | - |
| `INITIAL_FORK_VERSION` | `0` |
| `INITIAL_SLOT_NUMBER` | `0` |
| `GENESIS_START_SHARD` | `0` |
| `ZERO_HASH` | `bytes([0] * 32)` |
### Time parameters
@ -863,10 +864,11 @@ def split(values: List[Any], split_count: int) -> List[Any]:
```python
def get_new_shuffling(seed: Hash32,
slot: int,
validators: List[ValidatorRecord],
crosslinking_start_shard: int) -> List[List[ShardCommittee]]:
"""
Shuffles ``validators`` into shard committees using ``seed`` as entropy.
Shuffles ``validators`` into shard committees using ``seed`` and ``slot`` as entropy.
"""
active_validator_indices = get_active_validator_indices(validators)
@ -879,7 +881,7 @@ def get_new_shuffling(seed: Hash32,
)
# Shuffle with seed
shuffled_active_validator_indices = shuffle(active_validator_indices, seed)
shuffled_active_validator_indices = shuffle(active_validator_indices, xor(seed, Hash32(slot))
# Split the shuffled list into epoch_length pieces
validators_per_slot = split(shuffled_active_validator_indices, EPOCH_LENGTH)
@ -1221,7 +1223,7 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
update_validator_status(state, validator_index, ACTIVE)
# set initial committee shuffling
initial_shuffling = get_new_shuffling(ZERO_HASH, state.validator_registry, 0)
initial_shuffling = get_new_shuffling(ZERO_HASH, state.slot, state.validator_registry, GENESIS_START_SHARD)
state.shard_committees_at_slots = initial_shuffling + initial_shuffling
return state
@ -1458,7 +1460,7 @@ Below are the processing steps that happen at every `block`.
* Let `repeat_hash(x, n) = x if n == 0 else repeat_hash(hash(x), n-1)`.
* Let `proposer = state.validator_registry[get_beacon_proposer_index(state, state.slot)]`.
* Verify that `repeat_hash(block.randao_reveal, proposer.randao_layers) == proposer.randao_commitment`.
* Set `state.latest_randao_mixes[state.slot % LATEST_RANDAO_MIXES_LENGTH] = xor(state.latest_randao_mixes[state.slot % LATEST_RANDAO_MIXES_LENGTH], block.randao_reveal)`
* Set `state.latest_randao_mixes[state.slot % LATEST_RANDAO_MIXES_LENGTH] = hash(xor(state.latest_randao_mixes[state.slot % LATEST_RANDAO_MIXES_LENGTH], block.randao_reveal))`
* Set `proposer.randao_commitment = block.randao_reveal`.
* Set `proposer.randao_layers = 0`.
@ -1783,14 +1785,14 @@ Also perform the following updates:
* Set `state.validator_registry_latest_change_slot = state.slot`.
* Set `state.shard_committees_at_slots[:EPOCH_LENGTH] = state.shard_committees_at_slots[EPOCH_LENGTH:]`.
* Set `state.shard_committees_at_slots[EPOCH_LENGTH:] = get_new_shuffling(state.latest_randao_mixes[(state.slot - EPOCH_LENGTH) % LATEST_RANDAO_MIXES_LENGTH], state.validator_registry, next_start_shard)` where `next_start_shard = (state.shard_committees_at_slots[-1][-1].shard + 1) % SHARD_COUNT`.
* Set `state.shard_committees_at_slots[EPOCH_LENGTH:] = get_new_shuffling(state.latest_randao_mixes[(state.slot - EPOCH_LENGTH) % LATEST_RANDAO_MIXES_LENGTH], state.slot, state.validator_registry, next_start_shard)` where `next_start_shard = (state.shard_committees_at_slots[-1][-1].shard + 1) % SHARD_COUNT`.
If a validator registry update does _not_ happen do the following:
* Set `state.shard_committees_at_slots[:EPOCH_LENGTH] = state.shard_committees_at_slots[EPOCH_LENGTH:]`.
* Let `epochs_since_last_registry_change = (state.slot - state.validator_registry_latest_change_slot) // EPOCH_LENGTH`.
* Let `start_shard = state.shard_committees_at_slots[0][0].shard`.
* If `epochs_since_last_registry_change` is an exact power of 2, set `state.shard_committees_at_slots[EPOCH_LENGTH:] = get_new_shuffling(state.latest_randao_mixes[(state.slot - EPOCH_LENGTH) % LATEST_RANDAO_MIXES_LENGTH], state.validator_registry, start_shard)`. Note that `start_shard` is not changed from the last epoch.
* If `epochs_since_last_registry_change` is an exact power of 2, set `state.shard_committees_at_slots[EPOCH_LENGTH:] = get_new_shuffling(state.latest_randao_mixes[(state.slot - EPOCH_LENGTH) % LATEST_RANDAO_MIXES_LENGTH], state.slot, state.validator_registry, start_shard)`. Note that `start_shard` is not changed from the last epoch.
### Final updates