eth2.0-specs/specs/phase1/fraud-proofs.md

<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*

- [Ethereum 2.0 Phase 1 -- Shard Transition and Fraud Proofs](#ethereum-20-phase-1----shard-transition-and-fraud-proofs)
  - [Table of contents](#table-of-contents)
  - [Introduction](#introduction)
  - [Fraud proofs](#fraud-proofs)
    - [Shard state transition function](#shard-state-transition-function)
    - [Verifying the proof](#verifying-the-proof)
  - [Honest committee member behavior](#honest-committee-member-behavior)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->

# Ethereum 2.0 Phase 1 -- Shard Transition and Fraud Proofs

**Notice**: This document is a work-in-progress for researchers and implementers.

## Table of contents

<!-- TOC -->

 TODO

<!-- /TOC -->

## Introduction

This document describes the shard transition function and fraud proofs as part of Phase 1 of Ethereum 2.0.

## Fraud proofs

TODO. The intent is to have a single universal fraud proof type, which contains the following parts:

1. An on-time attestation on some `shard` signing a `ShardTransition`
2. An index `index` of a particular position to focus on
3. The `ShardTransition` itself
4. The full body of the block `ShardBlock`
5. A Merkle proof to the `shard_states` in the parent block `parent_block` the attestation is referencing

### Shard state transition function

```python
def shard_state_transition(beacon_state: BeaconState,
                           shard: Shard,
                           slot: Slot,
                           pre_state: Root,
                           previous_beacon_root: Root,
                           proposer_index: ValidatorIndex,
                           signed_block: SignedShardBlock,
                           validate_result: bool=True) -> Root:
    # We will add something more substantive in phase 2

    # Verify the proposer_index and signature
    assert proposer_index == signed_block.message.proposer_index
    if validate_result:
        assert verify_shard_block_signature(beacon_state, signed_block)

    return hash(pre_state + hash_tree_root(previous_beacon_root) + hash_tree_root(signed_block.message.data))
```

```python
def verify_shard_block_signature(beacon_state: BeaconState,
                                 signed_block: SignedShardBlock) -> bool:
    proposer = beacon_state.validators[signed_block.message.proposer_index]
    signing_root = compute_signing_root(signed_block.message, get_domain(beacon_state, DOMAIN_SHARD_PROPOSAL))
    return bls.Verify(proposer.pubkey, signing_root, signed_block.signature)
```

### Verifying the proof

```python
def verify_fraud_proof(beacon_state: BeaconState,
                       subkey: BLSPubkey,
                       attestation: Attestation,
                       index: uint64,
                       transition: ShardTransition,
                       signed_block: SignedShardBlock,
                       parent_block: ShardBlock) -> bool:
    # 1. Check if `custody_bits[index][j] != generate_custody_bit(subkey, block_contents)` for any `j`
    shard = get_shard(beacon_state, attestation)
    slot = attestation.data.slot
    custody_bits = attestation.custody_bits_blocks
    for j in range(custody_bits[index]):
        if custody_bits[index][j] != generate_custody_bit(subkey, signed_block):
            return True

    # 2. Verify the shard state transition
    if index == 0:
        parent_data = parent_block.shard_states[shard][-1].data
    else:
        parent_data = parent_block.shard_states[shard][index].data

    if shard_state_transition(
        beacon_state,
        shard,
        slot,
        transition.shard_states[index - 1].data,
        hash_tree_root(parent_block),
        get_shard_proposer_index(beacon_state, slot, shard),
        signed_block,
    ) != parent_data:
        return True

    return False
```

```python
def generate_custody_bit(subkey: BLSPubkey, block: ShardBlock) -> bool:
    # TODO
    ...
```

## Honest committee member behavior

Suppose you are a committee member on shard `shard` at slot `current_slot`. Let `state` be the head beacon state you are building on, and let `QUARTER_PERIOD = SECONDS_PER_SLOT // 4`. `2 * QUARTER_PERIOD` seconds into slot `slot`, run the following procedure:

* Initialize `proposals = []`, `shard_states = []`, `shard_state = state.shard_states[shard][-1]`, `start_slot = shard_state.slot`.
* For `slot in get_offset_slots(state, start_slot)`, do the following:
    * Look for all valid proposals for `slot`; that is, a SignedShardBlock `proposal` where `shard_state_transition(shard, slot, shard_state, get_block_root_at_slot(state, state.slot - 1), get_shard_proposer_index(state, slot, shard), proposal)` returns a result and does not throw an exception. Let `choices` be the set of non-empty valid proposals you discover.
    * If `len(choices) == 0`, do `proposals.append(make_empty_proposal(shard_state, slot))`
    * If `len(choices) == 1`, do `proposals.append(choices[0])`
    * If `len(choices) > 1`, let `winning_proposal` be the proposal with the largest number of total attestations from slots in `state.shard_next_slots[shard]....slot-1` supporting it or any of its descendants, breaking ties by choosing the first proposal locally seen. Do `proposals.append(winning_proposal)`.
    * If `proposals.message.data[-1]` is NOT an empty proposal, set `shard_state = shard_state_transition(shard, slot, shard_state, get_block_root_at_slot(state, state.slot - 1), get_shard_proposer_index(state, slot, shard), proposals[-1])` and do `shard_states.append(shard_state)`. If it is an empty proposal, leave `shard_state` unchanged.

Make an attestation using `shard_data_roots = [hash_tree_root(proposal.message.data) for proposal in proposals]` and `shard_state_roots = shard_states`.