500 lines
18 KiB
Markdown
500 lines
18 KiB
Markdown
# Ethereum 2.0 Phase 1 -- Custody Game
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**NOTICE**: This spec is a work-in-progress for researchers and implementers.
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## Table of contents
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<!-- TOC -->
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- [Ethereum 2.0 Phase 1 -- Custody Game](#ethereum-20-phase-1----custody-game)
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- [Table of contents](#table-of-contents)
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- [Introduction](#introduction)
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- [Terminology](#terminology)
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- [Constants](#constants)
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- [Misc](#misc)
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- [Time parameters](#time-parameters)
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- [Max operations per block](#max-operations-per-block)
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- [Signature domains](#signature-domains)
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- [Data structures](#data-structures)
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- [Custody objects](#custody-objects)
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- [`CustodyChunkChallenge`](#custodychunkchallenge)
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- [`CustodyBitChallenge`](#custodybitchallenge)
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- [`CustodyChunkChallengeRecord`](#custodychunkchallengerecord)
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- [`CustodyBitChallengeRecord`](#custodybitchallengerecord)
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- [`CustodyResponse`](#custodyresponse)
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- [`CustodyKeyReveal`](#custodykeyreveal)
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- [Phase 0 container updates](#phase-0-container-updates)
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- [`Validator`](#validator)
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- [`BeaconState`](#beaconstate)
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- [`BeaconBlockBody`](#beaconblockbody)
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- [Helpers](#helpers)
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- [`get_crosslink_chunk_count`](#get_crosslink_chunk_count)
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- [`get_custody_chunk_bit`](#get_custody_chunk_bit)
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- [`epoch_to_custody_period`](#epoch_to_custody_period)
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- [`verify_custody_key`](#verify_custody_key)
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- [Per-block processing](#per-block-processing)
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- [Operations](#operations)
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- [Custody reveals](#custody-reveals)
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- [Chunk challenges](#chunk-challenges)
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- [Bit challenges](#bit-challenges)
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- [Custody responses](#custody-responses)
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- [Per-epoch processing](#per-epoch-processing)
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<!-- /TOC -->
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## Introduction
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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](0_beacon-chain.md) specification.
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## Terminology
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* **Custody game**:
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* **Custody period**:
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* **Custody chunk**:
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* **Custody chunk bit**:
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* **Custody chunk challenge**:
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* **Custody bit**:
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* **Custody bit challenge**:
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* **Custody key**:
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* **Custody key reveal**:
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* **Custody key mask**:
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* **Custody response**:
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* **Custody response deadline**:
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## Constants
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### Misc
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| Name | Value |
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| `BYTES_PER_SHARD_BLOCK` | `2**14` (= 16,384) |
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| `BYTES_PER_CUSTODY_CHUNK` | `2**9` (= 512) |
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| `MINOR_REWARD_QUOTIENT` | `2**8` (= 256) |
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### Time parameters
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| Name | Value | Unit | Duration |
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| - | - | :-: | :-: |
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| `MAX_CHUNK_CHALLENGE_DELAY` | `2**11` (= 2,048) | epochs | ~9 days |
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| `EPOCHS_PER_CUSTODY_PERIOD` | `2**11` (= 2,048) | epochs | ~9 days |
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| `CUSTODY_RESPONSE_DEADLINE` | `2**14` (= 16,384) | epochs | ~73 days |
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### Max operations per block
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| Name | Value |
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| - | - |
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| `MAX_CUSTODY_KEY_REVEALS` | `2**4` (= 16) |
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| `MAX_CUSTODY_CHUNK_CHALLENGES` | `2**2` (= 4) |
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| `MAX_CUSTODY_BIT_CHALLENGES` | `2**2` (= 4) |
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| `MAX_CUSTODY_RESPONSES` | `2**5` (= 32) |
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### Signature domains
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| Name | Value |
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| - | - |
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| `DOMAIN_CUSTODY_KEY_REVEAL` | `6` |
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| `DOMAIN_CUSTODY_BIT_CHALLENGE` | `7` |
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## Data structures
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### Custody objects
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#### `CustodyChunkChallenge`
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```python
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{
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'responder_index': ValidatorIndex,
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'attestation': Attestation,
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'chunk_index': 'uint64',
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}
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```
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#### `CustodyBitChallenge`
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```python
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{
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'responder_index': ValidatorIndex,
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'attestation': Attestation,
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'challenger_index': ValidatorIndex,
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'responder_key': BLSSignature,
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'chunk_bits': Bitfield,
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'signature': BLSSignature,
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}
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```
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#### `CustodyChunkChallengeRecord`
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```python
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{
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'challenge_index': 'uint64',
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'challenger_index': ValidatorIndex,
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'responder_index': ValidatorIndex,
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'deadline': Epoch,
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'crosslink_data_root': Hash,
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'depth': 'uint64',
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'chunk_index': 'uint64',
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}
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```
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#### `CustodyBitChallengeRecord`
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```python
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{
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'challenge_index': 'uint64',
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'challenger_index': ValidatorIndex,
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'responder_index': ValidatorIndex,
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'deadline': Epoch,
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'crosslink_data_root': Hash,
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'chunk_bits': Bitfield,
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'responder_key': BLSSignature,
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}
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```
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#### `CustodyResponse`
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```python
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{
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'challenge_index': 'uint64',
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'chunk_index': 'uint64',
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'chunk': ['byte', BYTES_PER_CUSTODY_CHUNK],
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'branch': [Hash],
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}
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```
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#### `CustodyKeyReveal`
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```python
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{
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'revealer_index': ValidatorIndex,
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'period': 'uint64',
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'key': BLSSignature,
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'masker_index': ValidatorIndex,
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'mask': Hash,
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}
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```
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### Phase 0 container updates
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Add the following fields to the end of the specified container objects. Fields with underlying type `uint64` are initialized to `0` and list fields are initialized to `[]`.
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#### `Validator`
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```python
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'custody_reveal_index': 'uint64',
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'max_reveal_lateness': 'uint64',
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```
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#### `BeaconState`
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```python
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'custody_chunk_challenge_records': [CustodyChunkChallengeRecord],
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'custody_bit_challenge_records': [CustodyBitChallengeRecord],
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'custody_challenge_index': 'uint64',
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```
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#### `BeaconBlockBody`
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```python
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'custody_key_reveals': [CustodyKeyReveal],
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'custody_chunk_challenges': [CustodyChunkChallenge],
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'custody_bit_challenges': [CustodyBitChallenge],
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'custody_responses': [CustodyResponse],
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```
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## Helpers
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### `get_crosslink_chunk_count`
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```python
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def get_custody_chunk_count(attestation: Attestation) -> int:
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crosslink_start_epoch = attestation.data.latest_crosslink.epoch
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crosslink_end_epoch = slot_to_epoch(attestation.data.slot)
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crosslink_crosslink_length = min(MAX_CROSSLINK_EPOCHS, end_epoch - start_epoch)
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chunks_per_epoch = 2 * BYTES_PER_SHARD_BLOCK * SLOTS_PER_EPOCH // BYTES_PER_CUSTODY_CHUNK
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return crosslink_crosslink_length * chunks_per_epoch
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```
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### `get_custody_chunk_bit`
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```python
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def get_custody_chunk_bit(key: BLSSignature, chunk: bytes) -> bool:
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# TODO: Replace with something MPC-friendly, e.g. the Legendre symbol
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return get_bitfield_bit(hash(challenge.responder_key + chunk), 0)
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```
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### `epoch_to_custody_period`
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```python
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def epoch_to_custody_period(epoch: Epoch) -> int:
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return epoch // EPOCHS_PER_CUSTODY_PERIOD
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```
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### `verify_custody_key`
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```python
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def verify_custody_key(state: BeaconState, reveal: CustodyKeyReveal) -> bool:
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# Case 1: non-masked non-punitive non-early reveal
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pubkeys = [state.validator_registry[reveal.revealer_index].pubkey]
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message_hashes = [hash_tree_root(reveal.period)]
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# Case 2: masked punitive early reveal
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# Masking prevents proposer stealing the whistleblower reward
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# Secure under the aggregate extraction infeasibility assumption
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# See pages 11-12 of https://crypto.stanford.edu/~dabo/pubs/papers/aggreg.pdf
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if reveal.mask != ZERO_HASH:
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pubkeys.append(state.validator_registry[reveal.masker_index].pubkey)
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message_hashes.append(reveal.mask)
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return bls_verify_multiple(
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pubkeys=pubkeys,
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message_hashes=message_hashes,
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signature=reveal.key,
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domain=get_domain(
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fork=state.fork,
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epoch=reveal.period * EPOCHS_PER_CUSTODY_PERIOD,
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domain_type=DOMAIN_CUSTODY_KEY_REVEAL,
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),
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)
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```
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## Per-block processing
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### Operations
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Add the following operations to the per-block processing, in order the given below and after all other operations in phase 0.
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#### Custody reveals
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Verify that `len(block.body.custody_key_reveals) <= MAX_CUSTODY_KEY_REVEALS`.
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For each `reveal` in `block.body.custody_key_reveals`, run the following function:
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```python
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def process_custody_reveal(state: BeaconState,
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reveal: CustodyKeyReveal) -> None:
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assert verify_custody_key(state, reveal)
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revealer = state.validator_registry[reveal.revealer_index]
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current_custody_period = epoch_to_custody_period(get_current_epoch(state))
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# Case 1: non-masked non-punitive non-early reveal
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if reveal.mask == ZERO_HASH:
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assert reveal.period == epoch_to_custody_period(revealer.activation_epoch) + revealer.custody_reveal_index
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# Revealer is active or exited
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assert is_active_validator(revealer, get_current_epoch(state)) or revealer.exit_epoch > get_current_epoch(state)
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revealer.custody_reveal_index += 1
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revealer.max_reveal_lateness = max(revealer.max_reveal_lateness, current_custody_period - reveal.period)
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proposer_index = get_beacon_proposer_index(state, state.slot)
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increase_balance(state, proposer_index, base_reward(state, index) // MINOR_REWARD_QUOTIENT)
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# Case 2: masked punitive early reveal
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else:
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assert reveal.period > current_custody_period
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assert revealer.slashed is False
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slash_validator(state, reveal.revealer_index, reveal.masker_index)
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```
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#### Chunk challenges
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Verify that `len(block.body.custody_chunk_challenges) <= MAX_CUSTODY_CHUNK_CHALLENGES`.
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For each `challenge` in `block.body.custody_chunk_challenges`, run the following function:
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```python
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def process_chunk_challenge(state: BeaconState,
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challenge: CustodyChunkChallenge) -> None:
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# Verify the attestation
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assert verify_standalone_attestation(state, convert_to_standalone(state, challenge.attestation))
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# Verify it is not too late to challenge
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assert slot_to_epoch(challenge.attestation.data.slot) >= get_current_epoch(state) - MAX_CHUNK_CHALLENGE_DELAY
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responder = state.validator_registry[challenge.responder_index]
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assert responder.exit_epoch >= get_current_epoch(state) - MAX_CHUNK_CHALLENGE_DELAY
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# Verify the responder participated in the attestation
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attesters = get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)
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assert challenge.responder_index in attesters
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# Verify the challenge is not a duplicate
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for record in state.custody_chunk_challenge_records:
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assert (
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record.crosslink_data_root != challenge.attestation.data.crosslink_data_root or
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record.chunk_index != challenge.chunk_index
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)
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# Verify depth
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depth = math.log2(next_power_of_two(get_custody_chunk_count(challenge.attestation)))
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assert challenge.chunk_index < 2**depth
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# Add new chunk challenge record
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state.custody_chunk_challenge_records.append(CustodyChunkChallengeRecord(
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challenge_index=state.custody_challenge_index,
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challenger_index=get_beacon_proposer_index(state, state.slot),
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responder_index=challenge.responder_index
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deadline=get_current_epoch(state) + CUSTODY_RESPONSE_DEADLINE,
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crosslink_data_root=challenge.attestation.data.crosslink_data_root,
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depth=depth,
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chunk_index=challenge.chunk_index,
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))
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state.custody_challenge_index += 1
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# Postpone responder withdrawability
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responder.withdrawable_epoch = FAR_FUTURE_EPOCH
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```
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#### Bit challenges
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Verify that `len(block.body.custody_bit_challenges) <= MAX_CUSTODY_BIT_CHALLENGES`.
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For each `challenge` in `block.body.custody_bit_challenges`, run the following function:
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```python
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def process_bit_challenge(state: BeaconState,
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challenge: CustodyBitChallenge) -> None:
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# Verify challenge signature
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challenger = state.validator_registry[challenge.challenger_index]
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assert bls_verify(
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pubkey=challenger.pubkey,
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message_hash=signed_root(challenge),
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signature=challenge.signature,
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domain=get_domain(state, get_current_epoch(state), DOMAIN_CUSTODY_BIT_CHALLENGE),
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)
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# Verify the challenger is not slashed
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assert challenger.slashed is False
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# Verify the attestation
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assert verify_standalone_attestation(state, convert_to_standalone(state, challenge.attestation))
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# Verify the attestation is eligible for challenging
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responder = state.validator_registry[challenge.responder_index]
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min_challengeable_epoch = responder.exit_epoch - EPOCHS_PER_CUSTODY_PERIOD * (1 + responder.max_reveal_lateness)
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assert min_challengeable_epoch <= slot_to_epoch(challenge.attestation.data.slot)
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# Verify the responder participated in the attestation
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attesters = get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)
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assert challenge.responder_index in attesters
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# A validator can be the challenger or responder for at most one challenge at a time
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for record in state.custody_bit_challenge_records:
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assert record.challenger_index != challenge.challenger_index
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assert record.responder_index != challenge.responder_index
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# Verify the responder key
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assert verify_custody_key(state, CustodyKeyReveal(
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revealer_index=challenge.responder_index,
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period=epoch_to_custody_period(slot_to_epoch(attestation.data.slot)),
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key=challenge.responder_key,
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masker_index=0,
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mask=ZERO_HASH,
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))
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# Verify the chunk count
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chunk_count = get_custody_chunk_count(challenge.attestation)
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assert verify_bitfield(challenge.chunk_bits, chunk_count)
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# Verify the xor of the chunk bits does not equal the custody bit
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chunk_bits_xor = 0b0
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for i in range(chunk_count):
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chunk_bits_xor ^ get_bitfield_bit(challenge.chunk_bits, i)
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custody_bit = get_bitfield_bit(attestation.custody_bitfield, attesters.index(responder_index))
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assert custody_bit != chunk_bits_xor
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# Add new bit challenge record
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state.custody_bit_challenge_records.append(CustodyBitChallengeRecord(
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challenge_index=state.custody_challenge_index,
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challenger_index=challenge.challenger_index,
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responder_index=challenge.responder_index,
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deadline=get_current_epoch(state) + CUSTODY_RESPONSE_DEADLINE
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crosslink_data_root=challenge.attestation.crosslink_data_root,
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chunk_bits=challenge.chunk_bits,
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responder_key=challenge.responder_key,
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))
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state.custody_challenge_index += 1
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# Postpone responder withdrawability
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responder.withdrawable_epoch = FAR_FUTURE_EPOCH
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```
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#### Custody responses
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Verify that `len(block.body.custody_responses) <= MAX_CUSTODY_RESPONSES`.
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For each `response` in `block.body.custody_responses`, run the following function:
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```python
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def process_custody_response(state: BeaconState,
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response: CustodyResponse) -> None:
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chunk_challenge = next(record for record in state.custody_chunk_challenge_records if record.challenge_index == response.challenge_index, None)
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if chunk_challenge is not None:
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return process_chunk_challenge_response(state, response, chunk_challenge)
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bit_challenge = next(record for record in state.custody_bit_challenge_records if record.challenge_index == response.challenge_index, None)
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if bit_challenge is not None:
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return process_bit_challenge_response(state, response, bit_challenge)
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assert False
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```
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```python
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def process_chunk_challenge_response(state: BeaconState,
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response: CustodyResponse,
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challenge: CustodyChunkChallengeRecord) -> None:
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# Verify chunk index
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assert response.chunk_index == challenge.chunk_index
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# Verify the chunk matches the crosslink data root
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assert verify_merkle_branch(
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leaf=hash_tree_root(response.chunk),
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branch=response.branch,
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depth=challenge.depth,
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index=response.chunk_index,
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root=challenge.crosslink_data_root,
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)
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# Clear the challenge
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state.custody_chunk_challenge_records.remove(challenge)
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# Reward the proposer
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proposer_index = get_beacon_proposer_index(state, state.slot)
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increase_balance(state, proposer_index, base_reward(state, index) // MINOR_REWARD_QUOTIENT)
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```
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```python
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def process_bit_challenge_response(state: BeaconState,
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response: CustodyResponse,
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challenge: CustodyBitChallengeRecord) -> None:
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# Verify chunk index
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assert response.chunk_index < len(challenge.chunk_bits)
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# Verify the chunk matches the crosslink data root
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assert verify_merkle_branch(
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leaf=hash_tree_root(response.chunk),
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branch=response.branch,
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depth=math.log2(next_power_of_two(len(challenge.chunk_bits))),
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index=response.chunk_index,
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root=challenge.crosslink_data_root,
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)
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# Verify the chunk bit does not match the challenge chunk bit
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assert get_custody_chunk_bit(challenge.responder_key, response.chunk) != get_bitfield_bit(challenge.chunk_bits, response.chunk_index)
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# Clear the challenge
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|
state.custody_bit_challenge_records.remove(challenge)
|
|
# Slash challenger
|
|
slash_validator(state, challenge.challenger_index, challenge.responder_index)
|
|
```
|
|
|
|
## Per-epoch processing
|
|
|
|
Run `process_challenge_deadlines(state)` immediately after `process_ejections(state)`:
|
|
|
|
```python
|
|
def process_challenge_deadlines(state: BeaconState) -> None:
|
|
for challenge in state.custody_chunk_challenge_records:
|
|
if get_current_epoch(state) > challenge.deadline:
|
|
slash_validator(state, challenge.responder_index, challenge.challenger_index)
|
|
state.custody_chunk_challenge_records.remove(challenge)
|
|
|
|
for challenge in state.custody_bit_challenge_records:
|
|
if get_current_epoch(state) > challenge.deadline:
|
|
slash_validator(state, challenge.responder_index, challenge.challenger_index)
|
|
state.custody_bit_challenge_records.remove(challenge)
|
|
```
|
|
|
|
In `process_penalties_and_exits`, change the definition of `eligible` to the following (note that it is not a pure function because `state` is declared in the surrounding scope):
|
|
|
|
```python
|
|
def eligible(index):
|
|
validator = state.validator_registry[index]
|
|
# Cannot exit if there are still open chunk challenges
|
|
if len([record for record in state.custody_chunk_challenge_records if record.responder_index == index]) > 0:
|
|
return False
|
|
# Cannot exit if you have not revealed all of your custody keys
|
|
elif epoch_to_custody_period(revealer.activation_epoch) + validator.custody_reveal_index <= epoch_to_custody_period(validator.exit_epoch):
|
|
return False
|
|
# Cannot exit if you already have
|
|
elif validator.withdrawable_epoch < FAR_FUTURE_EPOCH:
|
|
return False
|
|
# Return minimum time
|
|
else:
|
|
return current_epoch >= validator.exit_epoch + MIN_VALIDATOR_WITHDRAWAL_EPOCHS
|
|
```
|