EIP-6465 - SSZ withdrawals root

Created	2023-02-08
Status	Draft
Category	Core
Type	Standards Track
Authors
Requires	EIP-2718 EIP-4895 EIP-6404 EIP-7495 EIP-7916

Abstract

This EIP defines a migration process of the existing Merkle-Patricia Trie (MPT) commitment for withdrawals to Simple Serialize (SSZ).

Motivation

While the consensus ExecutionPayloadHeader and the execution block header map to each other conceptually, they are encoded differently. This EIP aims to align the encoding of the withdrawals_root, taking advantage of the more modern SSZ format. This brings several advantages:

Reducing complexity: The proposed design reduces the number of use cases that require support for Merkle-Patricia Trie (MPT).
Reducing ambiguity: The name withdrawals_root is currently used to refer to different roots. While the execution block header refers to a Merkle Patricia Trie (MPT) root, the consensus ExecutionPayloadHeader instead refers to an SSZ root. With these changes, withdrawals_root consistently refers to the same SSZ root.

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.

Existing definitions

Definitions from existing specifications that are used throughout this document are replicated here for reference.

Name	SSZ equivalent
`ValidatorIndex`	`uint64`
`Gwei`	`uint64`
`ExecutionAddress`	`Bytes20`
`WithdrawalIndex`	`uint64`

Withdrawals

New withdrawals use a normalized SSZ representation. The existing consensus Withdrawal SSZ container is migrated to EIP-7495 ProgressiveContainer.

class Withdrawal(ProgressiveContainer[active_fields=[1, 1, 1, 1]]):
    index: WithdrawalIndex
    validator_index: ValidatorIndex
    address: ExecutionAddress
    amount: Gwei

Execution block header changes

The execution block header's withdrawals-root is transitioned from MPT to SSZ.

withdrawals = ProgressiveList[Withdrawal](
    withdrawal_0, withdrawal_1, withdrawal_2, ...)

block_header.withdrawals_root == withdrawals.hash_tree_root()

Consensus `ExecutionPayload` changes

When building a consensus ExecutionPayload, the withdrawals list is based on the new progressive container type.

class ExecutionPayload(...):
    ...
    withdrawals: ProgressiveList[Withdrawal]
    ...

The state transition function is updated to limit withdrawals to MAX_WITHDRAWALS_PER_PAYLOAD.

Rationale

This change was originally a candidate for inclusion in Shanghai, but was postponed to accelerate the rollout of withdrawals.

Backwards Compatibility

Applications that rely on the replaced MPT withdrawals_root in the block header require migration to the SSZ withdrawals_root.

RLP and SSZ withdrawals may clash when encoded. It is essential to use only a single format within one channel. The block header corresponding to the withdrawals can be consulted to identify the underlying fork.

Security Considerations

None