Verifiable logs

Abstract

This EIP defines a method to make the eth_getLogs JSON-RPC response verifiable.

Motivation

The eth_getLogs endpoint is used by wallets to obtain the transaction history pertaining to an account or a topic. To verify correctness and completeness of the logs, a wallet would also have to obtain all block headers and check against their logs bloom. However, that mechanism is inefficient due to its high false positive rate and also involves an unpractical amount of network round trips. This EIP defines a replacement mechanism to efficiently and incrementally verify correctness and completeness of eth_getLogs responses.

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.

Configuration

Log accumulation

After executing all transactions of a block, commitments of all emitted logs are accumulated into the storage of LOG_CONTRACT_ADDRESS. The contract has no code, and its storage layout consists of three slots of type mapping. However to prevent EIP-158 cleanup, the contract's nonce is set to 1 at the first write.

| Name | Value | Type | | LOG_ADDRESS_STORAGE_SLOT | 0 | mapping(address => bytes32) | | LOG_TOPICS_STORAGE_SLOT | 1 | mapping(bytes32 => bytes32) | | LOG_ADDRESS_TOPICS_STORAGE_SLOT | 2 | mapping(bytes32 => bytes32) |

Additional metadata about each log's origin is mixed in to each LogEntry. The definition uses the Log SSZ type as defined in EIP-6466.

class BlockMeta(Container):
    timestamp: uint64
    number: uint64

class LogMeta(Container):
    block: BlockMeta
    transaction_index: uint64

class Log(Container):
    address: ExecutionAddress
    topics: List[Bytes32, MAX_TOPICS_PER_LOG]
    data: ByteList[MAX_LOG_DATA_SIZE]

class LogEntry(Container):
    meta: LogMeta
    log: Log

The hash_tree_root(LogEntry) commitments are subsequently tracked as part of LOG_CONTRACT_ADDRESS.

def accumulate_log(evm: Evm, entry_root: Bytes32, key: Bytes32):
    root = hashlib.sha256()
    root.update(entry_root)
    root.update(sload(evm.env.state, LOG_CONTRACT_ADDRESS, key))
    sstore(evm.env.state, LOG_CONTRACT_ADDRESS, key, root.digest())

def track_log(evm: Evm, entry: LogEntry) -> None:
    entry_root = entry.hash_tree_root()

    # Allow verification via `address` filter
    key = keccak256(abi.encode(entry.log.address, LOG_ADDRESS_STORAGE_SLOT))
    accumulate_log(evm, entry_root, key)

    for topic in entry.log.topics:
        # Allow verification via `topics` filter
        key = keccak256(abi.encode(topic, LOG_TOPICS_STORAGE_SLOT))
        accumulate_log(evm, entry_root, key)

        # Allow verification via combined `address` + `topics` filter
        key = keccak256(abi.encode(entry.log.address, topic))
        key = keccak256(abi.encode(key, LOG_ADDRESS_TOPICS_STORAGE_SLOT))
        accumulate_log(evm, entry_root, key)

JSON-RPC API

The eth_getLogs response format is extended to include:

• blockTimestamp: QUANTITY - The timestamp field of the block referred to by blockHash

Verification

For eth_getLogs(address, topics, fromBlock, toBlock), the response data can be verified for correctness and completion by obtaining:

1. fromBlock and toBlock block headers (validated against their known hashes)
2. fromBlock's parentBlock header (validated against fromBlock.parentHash)
3. Historical log accumulator at parentBlock based on given filters (validated with eth_getProof)
4. Log accumulator at toBlock based on given filters (validated with eth_getProof)

Starting from the historical log accumulator from (3), each response entry is applied to it in a way compatible with accumulate_log above. If the log accumulator ends up matching the value from (4), the response data is correct and LogEntry derived from it can be trusted.

Rationale

Making the eth_getLogs response verifiable adds the necessary security attributes to enable wallets to transition away from relying on trusted data providers, ultimately improving the wallet's privacy guarantees as it is no longer subject to the privacy policy of any given provider.

Gas cost

The gas cost produced by this scheme is significantly higher than what LOG# opcodes produce as of Prague, primarily due to the additional SLOAD / SSTORE requirement and the double cost of SHA256 opcodes compared to KECCAK256 opcodes. The gas cost increases outweigh the savings from dropping logs blooms.

If the mechanism turns out to be prohibitively expensive even when optimized, it may be necessary to move the log accumulators to a separate optimized data structure (not in state_root), or to an out-of-protocol zk system. Even then, the gas cost for logs should still reflect the actual overall cost to update a typical out-of-protocol accumulator to deter against log spamming.

Block number / transaction index in meta instead of hashes

As long as the accumulators are stored in the state trie, they cannot refer to the block hash as the block hash hashes over the state trie, producing a cyclic dependency. If an external system is used, hashes may be included as in that scenario the state root is not affected by the IVC.

Backwards Compatibility

It is still possible to process eth_getLogs responses from trusted servers as is, without verifying them. Client applications with strict response validation may need to be updated to allow the additional blockTimestamp field.

Security Considerations

This scheme reuses existing eth_getProof and SSZ Merkle proofs; it does not introduce new security risks.

Copyright

Copyright and related rights waived via CC0.