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Merge pull request #86 from ethereum/fix-justified-attestation 

clarify attestation justified slot validity conditions
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Danny Ryan 2018-10-17 11:34:33 -05:00 committed by GitHub
parent 0e86ab8206 9b9cfae117
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1 changed files with 9 additions and 5 deletions
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specs/beacon-chain.md
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@ -603,7 +603,10 @@ def exit_validator(index, crystallized_state, penalize, current_slot):
This procedure should be carried out every block.
First, set `recent_block_hashes` to the output of the following, where `parent_hash` is the hash of the immediate previous block (ie. must be equal to `ancestor_hashes[0]`):
* Let `parent_hash` be the hash of the immediate previous beacon block (ie. equal to `ancestor_hashes[0]`).
* Let `parent` be the beacon block with the hash `parent_hash`
First, set `recent_block_hashes` to the output of the following:
```python
def append_to_recent_block_hashes(old_block_hashes: List[Hash32],
@ -631,12 +634,13 @@ A block can have 0 or more `AttestationRecord` objects
For each one of these attestations:
* Verify that `slot <= parent.slot` and `slot >= max(parent.slot - CYCLE_LENGTH + 1, 0)`
* Verify that the `justified_slot` and `justified_block_hash` given are in the chain and are equal to or earlier than the `last_justified_slot` in the crystallized state.
* Verify that `slot <= parent.slot` and `slot >= max(parent.slot - CYCLE_LENGTH + 1, 0)`.
* Verify that `justified_slot` is equal to or earlier than `last_justified_slot`.
* Verify that `justified_block_hash` is the hash of the block in the current chain at the slot -- `justified_slot`.
* Compute `parent_hashes` = `[get_block_hash(active_state, block, slot - CYCLE_LENGTH + i) for i in range(1, CYCLE_LENGTH - len(oblique_parent_hashes) + 1)] + oblique_parent_hashes` (eg, if `CYCLE_LENGTH = 4`, `slot = 5`, the actual block hashes starting from slot 0 are `Z A B C D E F G H I J`, and `oblique_parent_hashes = [D', E']` then `parent_hashes = [B, C, D' E']`). Note that when *creating* an attestation for a block, the hash of that block itself won't yet be in the `active_state`, so you would need to add it explicitly.
* Let `attestation_indices` be `get_shards_and_committees_for_slot(crystallized_state, slot)[x]`, choosing `x` so that `attestation_indices.shard` equals the `shard` value provided to find the set of validators that is creating this attestation record.
* Verify that `len(attester_bitfield) == ceil_div8(len(attestation_indices))`, where `ceil_div8 = (x + 7) // 8`. Verify that bits `len(attestation_indices)....` and higher, if present (i.e. `len(attestation_indices)` is not a multiple of 8), are all zero
* Derive a group public key by adding the public keys of all of the attesters in `attestation_indices` for whom the corresponding bit in `attester_bitfield` (the ith bit is `(attester_bitfield[i // 8] >> (7 - (i %8))) % 2`) equals 1
* Verify that `len(attester_bitfield) == ceil_div8(len(attestation_indices))`, where `ceil_div8 = (x + 7) // 8`. Verify that bits `len(attestation_indices)....` and higher, if present (i.e. `len(attestation_indices)` is not a multiple of 8), are all zero.
* Derive a group public key by adding the public keys of all of the attesters in `attestation_indices` for whom the corresponding bit in `attester_bitfield` (the ith bit is `(attester_bitfield[i // 8] >> (7 - (i %8))) % 2`) equals 1.
* Let `fork_version = pre_fork_version if slot < fork_slot_number else post_fork_version`.
* Verify that `aggregate_sig` verifies using the group pubkey generated and the serialized form of `AttestationSignedData(fork_version, slot, shard, parent_hashes, shard_block_hash, justified_slot)` as the message.