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eth2.0-specs/specs/phase1/fraud-proofs.md
vbuterin 2a91b43eaf
Remove shard block chunking 
Only store a 32 byte root for every shard block

Rationale: originally, I added shard block chunking (store 4 chunks for every shard block instead of one root) to facilitate construction of data availability roots. However, it turns out that there is an easier technique. Set the width of the data availability rectangle's rows to be 1/4 the max size of a shard block, so each block would fill multiple rows. Then, non-full blocks will generally create lots of zero rows. For example if the block bodies are `31415926535` and `897932` with a max size of 24 bytes, the rows might look like this:

```
31415926
53500000
00000000
89793200
00000000
00000000
```
Zero rows would extend rightward to complete zero rows, and when extending downward we can count the number of zero rows, and reduce the number of extra rows that we make, so we only make a new row for every nonzero row in the original data. This way we get only a close-to-optimal ~4-5x blowup in the data even if the data has zero rows in the middle.
2020-01-28 17:31:51 -07:00

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Table of Contents generated with DocToc

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

TODO

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 i of a particular position to focus on
  3. The ShardTransition itself
  4. The full body of the block
  5. A Merkle proof to the shard_states in the parent block the attestation is referencing

The proof verifies that one of the two conditions is false:

  1. custody_bits[i][j] != generate_custody_bit(subkey, block_contents) for any j
  2. execute_state_transition(shard, slot, transition.shard_states[i-1].data, hash_tree_root(parent), get_shard_proposer_index(state, shard, slot), block_contents) != transition.shard_states[i].data (if i=0 then instead use parent.shard_states[shard][-1].data)

Shard state transition function

def shard_state_transition(shard: Shard,
                           slot: Slot,
                           pre_state: Root,
                           previous_beacon_root: Root,
                           proposer_pubkey: BLSPubkey,
                           block_data: ByteList[MAX_SHARD_BLOCK_SIZE]) -> Root:
    # We will add something more substantive in phase 2
    return hash(pre_state + hash_tree_root(previous_beacon_root) + hash_tree_root(block_data))

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 Bytes proposal where shard_state_transition(shard, slot, shard_state, get_block_root_at_slot(state, state.slot - 1), get_shard_proposer_index(state, shard, slot), 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[-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, shard, slot), 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) for proposal in proposals] and shard_state_roots = shard_states.