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Added withdrawals with accumulator #354

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Added withdrawals with accumulator #354

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bb45425
Added withdrawals with accumulator
vbuterin Dec 22, 2018
1d0f440
Added min validator withdrawal time
vbuterin Dec 22, 2018
52f3a05
Added the general case for how accumulator values map to partial Merk…
vbuterin Dec 22, 2018
4b90656
Fixed as per Justin's comments
vbuterin Dec 22, 2018
14bb5c5
Fix ToC
hwwhww Dec 23, 2018
beed3a1
`acc` -> `accumulator`
hwwhww Dec 23, 2018
200aa86
Some refactoring
hwwhww Dec 23, 2018
5a26883
Fixed error: accumulator + 1 -> position + 1
vbuterin Dec 23, 2018
78a5c7e
Bugfix
hwwhww Dec 23, 2018
34bdf4e
Added WITHDRAWN check as per #353
vbuterin Dec 23, 2018
95dfab0
Fixes
vbuterin Dec 23, 2018
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96 changes: 77 additions & 19 deletions specs/core/0_beacon-chain.md
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Original file line number Diff line number Diff line change
Expand Up @@ -168,9 +168,11 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
| `MAX_CASPER_VOTES` | `2**10` (= 1,024) | votes |
| `LATEST_BLOCK_ROOTS_LENGTH` | `2**13` (= 8,192) | block roots |
| `LATEST_RANDAO_MIXES_LENGTH` | `2**13` (= 8,192) | randao mixes |
| `MAX_WITHDRAWALS_PER_EPOCH` | 4 | [validators](#dfn-validator) |
| `EMPTY_SIGNATURE` | `[bytes48(0), bytes48(0)]` | - |

* For the safety of crosslinks `TARGET_COMMITTEE_SIZE` exceeds [the recommended minimum committee size of 111](https://vitalik.ca/files/Ithaca201807_Sharding.pdf); with sufficient active validators (at least `EPOCH_LENGTH * TARGET_COMMITTEE_SIZE`), the shuffling algorithm ensures committee sizes at least `TARGET_COMMITTEE_SIZE`. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)
* 4 withdrawals per epoch means that eg. if a 1/3 subset of 10 million ETH equivocate to cause a safety failure and all withdraw immediately, they will be able to leave in `3333333 / 32 / 4 = 26042` epochs, or 10 million seconds ~= 4 months.

### Deposit contract

Expand All @@ -197,6 +199,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
| `MIN_ATTESTATION_INCLUSION_DELAY` | `2**2` (= 4) | slots | 24 seconds |
| `EPOCH_LENGTH` | `2**6` (= 64) | slots | 6.4 minutes |
| `POW_RECEIPT_ROOT_VOTING_PERIOD` | `2**10` (= 1,024) | slots | ~1.7 hours |
| `MIN_VALIDATOR_WITHDRAWAL_TIME` | `2**14` (= 16,384) | slots | ~27 hours |
| `SHARD_PERSISTENT_COMMITTEE_CHANGE_PERIOD` | `2**17` (= 131,072) | slots | ~9 days |
| `COLLECTIVE_PENALTY_CALCULATION_PERIOD` | `2**20` (= 1,048,576) | slots | ~73 days |
| `ZERO_BALANCE_VALIDATOR_TTL` | `2**22` (= 4,194,304) | slots | ~291 days |
Expand All @@ -222,6 +225,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
| `ACTIVE_PENDING_EXIT` | `2` |
| `EXITED_WITHOUT_PENALTY` | `3` |
| `EXITED_WITH_PENALTY` | `4` |
| `WITHDRAWN` | `6` |

### Max operations per block

Expand Down Expand Up @@ -400,6 +404,19 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
}
```

#### `WithdrawalData`
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Rename to Withdrawal?


```python
{
# Withdrawal credentials
'credentials': 'hash32',
# Slot created
'slot': 'uint64',
# ETH to withdraw
'value': 'uint64',
}
```

### Beacon chain blocks

#### `BeaconBlock`
Expand Down Expand Up @@ -466,6 +483,9 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
'validator_registry_latest_change_slot': 'uint64',
'validator_registry_exit_count': 'uint64',
'validator_registry_delta_chain_tip': 'hash32', # For light clients to track deltas
'validator_withdrawal_accumulator': ['hash32'],
'validator_registry_withdrawal_count': 'uint64',


# Randomness and committees
'latest_randao_mixes': ['hash32'],
Expand All @@ -488,7 +508,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
# Recent state
'latest_crosslinks': [CrosslinkRecord],
'latest_block_roots': ['hash32'], # Needed to process attestations, older to newer
'latest_penalized_exit_balances': ['uint64'], # Balances penalized at every withdrawal period
'total_penalty_history': ['uint64'], # Balances penalized at every withdrawal period
'latest_attestations': [PendingAttestationRecord],
'batched_block_roots': ['hash32'],

Expand Down Expand Up @@ -950,6 +970,22 @@ def merkle_root(values):
return o[1]
```

#### `update_merkle_accumulator`

```python
def update_merkle_accumulator(acc, position, value):
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rename acc => accumulator

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h = value
j = 0
while (acc+1) % 2**(j+1) == 0:
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Spacing: +1 => + 1

h = hash(acc[j] + h)
j += 1
return acc[:j] + [h] + acc[j+1:]
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spacing

```

**INVARIANT**: if `len(values) = 2**k` for integer `k < len(acc)`, then if you apply `for i, v in enumerate(values): acc = update_merkle_accumulator(acc, i, v)`, at the end `acc[k] == merkle_root(values)`.

**INVARIANT**: in the above case but where `len(values)` is not an exact power of 2, any value in `values` will be a leaf of at least one Merkle tree whose root is in `acc`. For example, if `len(values) = 329` (note: 333 = 256 + 64 + 8 + 1), then `acc[8] == merkle_root(values[0:256])`, `acc[6] == merkle_root(values[256:320])`, `acc[3] == merkle_root(values[320:328])` and `acc[0] == merkle_root(values[328:329])`.

#### `get_attestation_participants`

```python
Expand Down Expand Up @@ -1165,6 +1201,8 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
validator_registry_latest_change_slot=INITIAL_SLOT_NUMBER,
validator_registry_exit_count=0,
validator_registry_delta_chain_tip=ZERO_HASH,
validator_withdrawal_accumulator=[ZERO_HASH for _ in range(64)], # Max capacity 2**64 to match size of uint64
validator_registry_withdrawal_count=0,

# Randomness and committees
latest_randao_mixes=[ZERO_HASH for _ in range(LATEST_RANDAO_MIXES_LENGTH)],
Expand All @@ -1185,7 +1223,7 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
# Recent state
latest_crosslinks=[CrosslinkRecord(slot=INITIAL_SLOT_NUMBER, shard_block_root=ZERO_HASH) for _ in range(SHARD_COUNT)],
latest_block_roots=[ZERO_HASH for _ in range(LATEST_BLOCK_ROOTS_LENGTH)],
latest_penalized_exit_balances=[],
total_penalty_history=[],
latest_attestations=[],
batched_block_roots=[],

Expand Down Expand Up @@ -1332,6 +1370,8 @@ def update_validator_status(state: BeaconState,
initiate_validator_exit(state, index)
if new_status in [EXITED_WITH_PENALTY, EXITED_WITHOUT_PENALTY]:
exit_validator(state, index, new_status)
if new_status == WITHDRAWN:
withdraw_validator(state, index)
```

The following are helpers and should only be called via `update_validator_status`:
Expand Down Expand Up @@ -1390,7 +1430,7 @@ def exit_validator(state: BeaconState,
validator.latest_status_change_slot = state.slot

if new_status == EXITED_WITH_PENALTY:
state.latest_penalized_exit_balances[state.slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD] += get_effective_balance(state, index)
state.total_penalty_history[state.slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD] += get_effective_balance(state, index)

whistleblower_index = get_beacon_proposer_index(state, state.slot)
whistleblower_reward = get_effective_balance(state, index) // WHISTLEBLOWER_REWARD_QUOTIENT
Expand Down Expand Up @@ -1418,6 +1458,20 @@ def exit_validator(state: BeaconState,
break
```

```python
def withdraw_validator(state: BeaconState,
index: int):
new_accumulator_object = WithdrawalData(
credentials=validator.withdrawal_credentials,
slot=state.slot,
value=state.validator_balances[index],
)
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It seems the third parameter of update_merkle_accumulator should be a Hash32 object?

    new_accumulator_object = hash_tree_root(
        WithdrawalData(
            credentials=validator.withdrawal_credentials,
            slot=state.slot,
            value=state.validator_balances[index],
        )
    )

state.validator_withdrawal_accumulator = update_merkle_accumulator(
state.validator_withdrawal_accumulator, state.validator_registry_withdrawal_count, new_accumulator_object
)
state.validator_registry_withdrawal_count += 1
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```

## Per-slot processing

Below are the processing steps that happen at every slot.
Expand Down Expand Up @@ -1758,22 +1812,26 @@ def update_validator_registry(state: BeaconState) -> None:
update_validator_status(state, index, new_status=EXITED_WITHOUT_PENALTY)


# Calculate the total ETH that has been penalized in the last ~2-3 withdrawal periods
period_index = current_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD
total_penalties = (
(latest_penalized_exit_balances[period_index]) +
(latest_penalized_exit_balances[period_index - 1] if period_index >= 1 else 0) +
(latest_penalized_exit_balances[period_index - 2] if period_index >= 2 else 0)
)

# Calculate penalties for slashed validators
def to_penalize(index):
return state.validator_registry[index].status == EXITED_WITH_PENALTY
validators_to_penalize = filter(to_penalize, range(len(validator_registry)))
for index in validators_to_penalize:
state.validator_balances[index] -= get_effective_balance(state, index) * min(total_penalties * 3, total_balance) // total_balance

return validator_registry, latest_penalized_exit_balances, validator_registry_delta_chain_tip
# Withdraw validators
all_indices = range(len(state.validator_registry))
def eligible(index):
return state.validator_registry[x].status in (EXITED_WITHOUT_PENALTY, EXITED_WITH_PENALTY) and \
state.slot >= state.validator_registry[x].latest_status_change_slot >= MIN_VALIDATOR_WITHDRAWAL_TIME
eligible_indices = filter(eligible, all_indices)
sorted_indices = sorted(eligible_indices, filter=lambda index: state.validator_registry[index].exit_count)
withdrawn_so_far = 0
for index in sorted_indices:
validator = state.validator_registry[index]
if validator.status == EXITED_WITH_PENALTY:
# Calculate and apply penalties for slashed validators
start_period = max(validator.latest_status_change_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD - 1, 0)
current_period = current_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD
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total_penalties = state.total_penalty_history[current_period] - state.total_penalty_history[start_period]
state.validator_balances[index] -= get_effective_balance(state, index) * min(total_penalties * 3, total_balance) // total_balance
update_validator_status(state, index, new_status=WITHDRAWN)
withdrawn_so_far += 1
if withdrawn_so_far >= MAX_WITHDRAWALS_PER_EPOCH:
break
```

Also perform the following updates:
Expand Down