Forward compatible consensus data structures

Abstract

This EIP defines the changes needed to adopt ProgressiveContainer from EIP-7495 and ProgressiveList from EIP-7916 in consensus data structures.

Motivation

Ethereum's consensus data structures make heavy use of Simple Serialize (SSZ) Container, which defines how they are serialized and merkleized. The merkleization scheme allows application implementations to verify that individual fields (and partial fields) have not been tampered with. This is useful, for example, in smart contracts of decentralized staking pools that wish to verify that participating validators have not been slashed.

While SSZ Container defines how data structures are merkleized, the merkleization is prone to change across the different forks. When that happens, e.g., because new features are added or old features get removed, existing verifier implementations need to be updated to be able to continue processing proofs.

ProgressiveContainer, of EIP-7495, is a forward compatible alternative that guarantees a forward compatible merkleization scheme. By transitioning consensus data structures to use ProgressiveContainer, smart contracts that contain verifier logic no longer have to be maintained in lockstep with Ethereum's fork schedule as long as the underlying features that they verify don't change. For example, as long as the concept of slashing is represented using the boolean slashed field, existing verifiers will not break when unrelated features get added or removed. This is also true for off-chain verifiers, e.g., in hardware wallets or in operating systems for mobile devices that are on a different software update cadence than Ethereum.

Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Container conversion

Container types that are expected to evolve over forks SHALL be redefined as ProgressiveContainer(active_fields=[1] * len(type.fields())).

For example, given a type in the old fork:

class Foo(Container):
    a: uint8
    b: uint16

This type can be converted to support stable Merkleization in the new fork:

class Foo(ProgressiveContainer(active_fields=[1, 1])):
    a: uint8
    b: uint16

As part of the conversion, a stable generalized index (gindex) is assigned to each field that remains valid in future forks.

• If a fork appends a field, active_fields MUST be extended with a trailing 1.
• If a fork removes a field, the corresponding active_fields bit MUST be changed to 0.
• Compatibility rules SHOULD be enforced, e.g., by defining a CompatibleUnion[fork_1.Foo, fork_2.Foo, fork_3.Foo, ...] type in the unit test framework.

List[type, N] / Bitlist conversion

List types frequently have been defined with excessively large capacities N with the intention that N is never reached in practice. In other cases, the capacity itself has changed over time.

• List types with dynamic or unbounded capacity semantics SHALL be redefined as ProgressiveList[type], and the application logic SHALL be updated to check for an appropriate limit at runtime.
• Bitlist types with dynamic or unbounded capacity semantics SHALL be redefined as ProgressiveBitlist

As part of the conversion, a stable generalized index (gindex) is assigned to each list element that remains valid regardless of the number of added elements.

Converted types

The following types SHALL be converted to ProgressiveContainer:

• Attestation
  • The aggregation_bits field is redefined to use ProgressiveBitlist
• IndexedAttestation
  • The attesting_indices field is redefined to use ProgressiveList
• ExecutionPayload
  • The transactions and withdrawals fields are redefined to use ProgressiveList
  • The MAX_TRANSACTIONS_PER_PAYLOAD (1M) limit is no longer enforced
• Transaction is redefined as ProgressiveByteList
  • The MAX_BYTES_PER_TRANSACTION (1 GB) limit is no longer enforced
• ExecutionRequests
  • The deposits, withdrawals and consolidations fields are redefined to use ProgressiveList
• BeaconBlockBody
  • The proposer_slashings, attester_slashings, attestations, deposits, voluntary_exits and bls_to_execution_changes fields are redefined to use ProgressiveList
• BeaconState
  • The validators, balances, previous_epoch_participation, current_epoch_participation, inactivity_scores, pending_deposits, pending_partial_withdrawals and pending_consolidations fields are redefined to use ProgressiveList
• The blob_kzg_commitments, kzg_proofs and column fields are redefined to use ProgressiveList

Immutable types

These types are used as part of the ProgressiveContainer definitions, and, as they are not ProgressiveContainer themselves, are considered to have immutable Merkleization. If a future fork requires changing these types in an incompatible way, a new type SHALL be defined and assigned a new field name.

Type	Description
Slot	Slot number on the beacon chain
Epoch	Epoch number on the beacon chain, a group of slots
CommitteeIndex	Index of a committee within a slot
ValidatorIndex	Unique index of a beacon chain validator
Gwei	Amount in Gwei (1 ETH = 10^9 Gwei = 10^18 Wei)
Root	Byte vector containing an SSZ Merkle root
Hash32	Byte vector containing an opaque 32-byte hash
Version	Consensus fork version number
BLSPubkey	Cryptographic type representing a BLS12-381 public key
BLSSignature	Cryptographic type representing a BLS12-381 signature
KZGCommitment	G1 curve point for the KZG polynomial commitment scheme
Fork	Consensus fork information
Checkpoint	Tuple referring to the most recent beacon block up through an epoch's start slot
Validator	Information about a beacon chain validator
AttestationData	Vote that attests to the availability and validity of a particular consensus block
Eth1Data	Target tracker for importing deposits from transaction logs
DepositData	Log data emitted as part of a transaction's receipt when depositing to the beacon chain
BeaconBlockHeader	Consensus block header
ProposerSlashing	Tuple of two equivocating consensus block headers
Deposit	Tuple of deposit data and its inclusion proof
VoluntaryExit	Consensus originated request to exit a validator from the beacon chain
SignedVoluntaryExit	Tuple of voluntary exit request and its signature
SyncAggregate	Cryptographic type representing an aggregate sync committee signature
ExecutionAddress	Byte vector containing an account address on the execution layer
WithdrawalIndex	Unique index of a withdrawal from any validator's balance to the execution layer
Withdrawal	Withdrawal from a beacon chain validator's balance to the execution layer
DepositRequest	Tuple of flattened deposit data and its sequential index
WithdrawalRequest	Execution originated request to withdraw from a validator to the execution layer
ConsolidationRequest	Execution originated request to consolidate two beacon chain validators
BLSToExecutionChange	Request to register the withdrawal account address of a beacon chain validator
SignedBLSToExecutionChange	Tuple of withdrawal account address registration request and its signature
ParticipationFlags	Participation tracker of a beacon chain validator within an epoch
HistoricalSummary	Tuple combining a historical block root and historical state root
PendingDeposit	Pending operation for depositing to a beacon chain validator
PendingPartialWithdrawal	Pending operation for withdrawing from a beacon chain validator
PendingConsolidation	Pending operation for consolidating two beacon chain validators

Rationale

Best timing?

Applying this EIP breaks hash_tree_root and Merkle tree verifiers a single time, while promising forward compatibility from the fork going forward. It is best to apply it before merkleization would be broken by different changes. Merkleization is broken by a Container reaching a new power of 2 in its number of fields.

Can this be applied retroactively?

While ProgressiveContainer serializes in the same way as the legacy Container, the merkleization and hash_tree_root of affected data structures changes. Therefore, verifiers that wish to process Merkle proofs of legacy variants still need to support the corresponding legacy schemes.

Immutability

Once a field in a ProgressiveContainer has been published, its name can no longer be used to represent a different type in the future. This is in line with historical management of certain cases:

• Phase0: BeaconState contained previous_epoch_attestations / current_epoch_attestations
• Altair: BeaconState replaced these fields with previous_epoch_participation / current_epoch_participation

Furthermore, new fields have to be appended at the end of ProgressiveContainer. This is in line with historical management of other cases:

• Capella appended historical_summaries to BeaconState instead of squeezing the new field next to historical_roots

With ProgressiveContainer, stable Merkleization requires these rules to become strict.

Cleanup opportunities

BeaconState

• The eth1_data, eth1_data_votes, eth1_deposit_index and deposit_requests_start_index fields could be dropped as they are no longer needed after the EIP-6110 transition period finishes.
• historical_summaries could be redefined to use ProgressiveList and also integrate the historical historical_roots data by merging in full HistoricalSummary data from an archive (historical_root is frozen since Capella), simplifying access to historical block and state roots.

Attestation

• The committee_bits is defined as a Bitvector, but the top bits are forced to 0 based on get_committee_count_per_slot(state, data.target.epoch). It could be re-defined as a ProgressiveBitlist.

IndexedAttestation

The attesting_indices are limited to MAX_VALIDATORS_PER_COMMITTEE * MAX_COMMITTEES_PER_SLOT, which is insufficient when the IndexedAttestation is formed from SingleAttestation traffic. SingleAttestation allows validators that are not assigned to a slot to produce signatures that are not aggregatable into an Attestation (as such validators are not assigned), but that are still slashable.

Further, MAX_ATTESTER_SLASHINGS_ELECTRA at 1 limits inclusion efficiency of slashings in non-finality scenarios with a lot of forks where slashings happen across multiple different AttestationData values.

Limits could be rethought to be based on actual resource usage, e.g., by limiting:

• The total number of attesting_indices across all AttesterSlashing (shared total)
• The total number of signature checks across all AttesterSlashing, ProposerSlashing, VoluntaryExit, and SignedBLSToExecutionChange messages

Limiting totals rather than individual resources would allow extra attester slashings to be included at the cost of potentially delaying inclusion of a couple altruistic messages (if there even are any VoluntaryExit / SignedBLSToExecutionChange messages at that time), thus increasing security and block packing efficiency.

ExecutionPayload

The block_hash field could be moved to the top of ExecutionPayload, reducing the Merkle proof size.

ExecutionRequests

As deposits cannot be retried by the user (they pay the ETH upfront), deposit requests cannot fizzle like other requests; they are always included in the same block (since Pectra). For that reason, the current MAX_DEPOSIT_REQUESTS_PER_PAYLOAD (8192) is essentially unbounded at current gas limits, but may eventually become reachable (around ~192M gas, or earlier with gas repricings). The CL limit for deposit requests could be dropped to avoid scaling issues, instead solely relying on EL backpressure (gas fees, 1 ETH deposit minimum).

For other requests (withdrawal / consolidation requests), a shared total limit based on the added CL state size could provide more flexibility than the current per-operation limits. For example, in times without consolidation requests, space could be used to enqueue more withdrawal requests.

BeaconBlockHeader

The BeaconBlockHeader is currently proposed to be kept as is. Updating the BeaconBlockHeader to ProgressiveContainer is tricky as it breaks hash_tree_root(latest_block_header) in the BeaconState. One option could be to store latest_block_header_root separately, possibly also incorporating the block proposer signature into the hash to avoid proposer signature checks while backfilling historical blocks.

Validator

The Validator container is currently proposed to be kept as is. Updating the Validator to ProgressiveContainer would add an extra hash for each validator; validators are mostly immutable so rarely need to be re-hashed. With a conversion, implementations may have to incrementally construct the new Validator entries ahead of the fork when validator counts are high. It should be evaluated whether the hashing overhead is worth a clean transition to future fields, e.g., for holding postquantum keys. Also consider that such a transition may also involve a new hash function, which is a breaking change to the Merkle proofs, so generalized indices do not have to be stable across that transition.

Backwards Compatibility

Existing Merkle proof verifiers need to be updated to support the new Merkle tree shape. This includes verifiers in smart contracts on different blockchains and verifiers in hardware wallets, if applicable.

Security Considerations

Before this EIP, all List types had a fixed upper bound, enabling implementations to reject messages exceeding that size early. With ProgressiveList, that is no longer possible, as there is no more maximum message size. Instead, the length checks have to be implemented as part of P2P gossip validation, and as part of the state transition logic. However, many of the limits are not practically reachable (e.g., the gas limit is reached before the maximum payload size). Further note that SSZ is simple enough that clever implementations could still enforce those length checks on the serialized data, before fully decoding it.

All data inbound via libp2p is decrypted, then uncompressed with Snappy, then hashed with MESSAGE_DOMAIN_VALID_SNAPPY / MESSAGE_DOMAIN_INVALID_SNAPPY prefix depending on whether the decompression worked to compute the libp2p message ID, while honoring a global MAX_PAYLOAD_SIZE message size limit. This has to be done even if the underlying SSZ data ends up being invalid.

As SSZ does not use variable-length encoding, it does not have uncontrolled blowup (no 1 byte becomes 100 MB). Therefore, attempting to decode a MAX_PAYLOAD_SIZE message before checking dynamic List limits does not decrease security. Any intentional DoS attacks can already target a heavier portion of the processing pipeline (e.g., by sending invalid BLS signatures, or by sending invalid Snappy data that still needs to be hashed to compute the message ID). Therefore, the EIP does not notably impact security.

Copyright

Copyright and related rights waived via CC0.