86 lines
4.6 KiB
Markdown
86 lines
4.6 KiB
Markdown
# Custody Game -- Honest Validator
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**Notice**: This document is a work-in-progress for researchers and implementers.
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This is an accompanying document to the [Custody Game](./), which describes the expected actions of a "validator"
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participating in the shard data Custody Game.
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## Table of contents
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<!-- TOC -->
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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- [Introduction](#introduction)
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- [Prerequisites](#prerequisites)
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- [Becoming a validator](#becoming-a-validator)
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- [Beacon chain validator assignments](#beacon-chain-validator-assignments)
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- [Custody slashings](#custody-slashings)
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- [Custody key reveals](#custody-key-reveals)
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- [Early derived secret reveals](#early-derived-secret-reveals)
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- [Construct attestation](#construct-attestation)
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- [How to avoid slashing](#how-to-avoid-slashing)
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- [Custody slashing](#custody-slashing)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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<!-- /TOC -->
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## Introduction
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## Prerequisites
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This document is an extension of the [Sharding -- Validator](../sharding/validator.md). All behaviors and definitions defined in the Sharding doc carry over unless explicitly noted or overridden.
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All terminology, constants, functions, and protocol mechanics defined in the [Custody Game -- The Beacon Chain](./beacon-chain.md)
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docs are requisite for this document and used throughout. Please see the Custody Game docs before continuing and use them as a reference throughout.
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## Becoming a validator
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Becoming a validator in Custody Game is unchanged from Phase 0. See the [Phase 0 validator guide](../../phase0/validator.md#becoming-a-validator) for details.
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## Beacon chain validator assignments
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Beacon chain validator assignments to beacon committees and beacon block proposal are unchanged from Phase 0. See the [Phase 0 validator guide](../../phase0/validator.md#validator-assignments) for details.
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##### Custody slashings
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Up to `MAX_CUSTODY_SLASHINGS`, [`CustodySlashing`](./beacon-chain.md#custodyslashing) objects can be included in the `block`. The custody slashings must satisfy the verification conditions found in [custody slashings processing](beacon-chain.md#custody-slashings). The validator receives a small "whistleblower" reward for each custody slashing included (THIS IS NOT CURRENTLY THE CASE BUT PROBABLY SHOULD BE).
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##### Custody key reveals
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Up to `MAX_CUSTODY_KEY_REVEALS`, [`CustodyKeyReveal`](./beacon-chain.md#custodykeyreveal) objects can be included in the `block`. The custody key reveals must satisfy the verification conditions found in [custody key reveal processing](beacon-chain.md#custody-key-reveals). The validator receives a small reward for each custody key reveal included.
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##### Early derived secret reveals
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Up to `MAX_EARLY_DERIVED_SECRET_REVEALS`, [`EarlyDerivedSecretReveal`](./beacon-chain.md#earlyderivedsecretreveal) objects can be included in the `block`. The early derived secret reveals must satisfy the verification conditions found in [early derived secret reveal processing](beacon-chain.md#custody-key-reveals). The validator receives a small "whistleblower" reward for each early derived secret reveal included.
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#### Construct attestation
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`attestation.data`, `attestation.aggregation_bits`, and `attestation.signature` are unchanged from Phase 0. But safety/validity in signing the message is premised upon calculation of the "custody bit" [TODO].
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## How to avoid slashing
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Proposer and Attester slashings described in Phase 0 remain in place with the addition of the following.
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### Custody slashing
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To avoid custody slashings, the attester must never sign any shard transition for which the custody bit is one. The custody bit is computed using the custody secret:
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```python
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def get_custody_secret(state: BeaconState,
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validator_index: ValidatorIndex,
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privkey: int,
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epoch: Epoch=None) -> BLSSignature:
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if epoch is None:
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epoch = get_current_epoch(state)
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period = get_custody_period_for_validator(validator_index, epoch)
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epoch_to_sign = get_randao_epoch_for_custody_period(period, validator_index)
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domain = get_domain(state, DOMAIN_RANDAO, epoch_to_sign)
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signing_root = compute_signing_root(Epoch(epoch_to_sign), domain)
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return bls.Sign(privkey, signing_root)
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```
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Note that the valid custody secret is always the one for the **attestation target epoch**, not to be confused with the epoch in which the shard block was generated.
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While they are the same most of the time, getting this wrong at custody epoch boundaries would result in a custody slashing.
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