Establishing the format and endpoint for transmitting a snapshot of the deposit Merkle tree
Table of Contents
- Backwards Compatibility
- Test Cases
- Reference Implementation
- Security Considerations
This EIP defines a standard format for transmitting the deposit contract Merkle tree in a compressed form during weak subjectivity sync. This allows newly syncing consensus clients to reconstruct the deposit tree much faster than downloading all historical deposits. The format proposed also allows clients to prune deposits that are no longer needed to participate fully in consensus (see Deposit Finalization Flow).
Most client implementations require beacon nodes to download and store every deposit log since the launch of the deposit contract in order to reconstruct the deposit Merkle tree. This approach requires nodes to store far more deposits than necessary to fully participate in consensus. It also needlessly increases the time it takes for new nodes to fully sync, which is especially noticeable during weak subjectivity sync. Furthermore, if EIP-4444 is adopted, it will not always be possible to download all historical deposit logs from full nodes.
Consensus clients MAY continue to implement the deposit Merkle tree however they choose. However, when transmitting the tree to newly syncing nodes, clients MUST use the following format:
class DepositTreeSnapshot: finalized: List[Hash32, DEPOSIT_CONTRACT_DEPTH] deposit_root: Hash32 deposit_count: uint64 execution_block_hash: Hash32 execution_block_height: uint64
finalized is a variable-length list (of maximum size
DEPOSIT_CONTRACT_DEPTH) containing the hashes defined in the Deposit Finalization Flow section below. The fields
execution_block_hash store the same information as the
Eth1Data object that corresponds to the snapshot, and
execution_block_height is the height of the execution block with hash
execution_block_hash. Consensus clients MUST make this structure available via the Beacon Node API endpoint:
During deposit processing, the beacon chain requires deposits to be submitted along with a Merkle path to the deposit root. This is required exactly once for each deposit. When a deposit has been processed by the beacon chain and the deposit finalization conditions have been met, many of the hashes along the path to the deposit root will never be required again to construct Merkle proofs on chain. These unnecessary hashes MAY be pruned to save space. The image below illustrates the evolution of the deposit Merkle tree under this process alongside the corresponding
DepositTreeSnapshot as new deposits are added and older deposits become finalized:
The format in this specification was chosen to achieve several goals simultaneously:
- Enable reconstruction of the deposit contract Merkle tree under the adoption of EIP-4444
- Avoid requiring consensus nodes to retain more deposits than necessary to fully participate in consensus
- Simplicity of implementation (see Reference Implementation section)
- Increase speed of weak subjectivity sync
- Compatibility with existing implementations of this mechanism (see discussion)
DepositTreeSnapshot structure includes both
execution_block_height for convenience to consensus node implementors. While only one of these fields is strictly necessary, different clients may have already designed their block cache logic around one or the other. Sending only one of these would force some consensus clients to query the execution engine for the other information, but as this is happening in the context of a newly syncing consensus node, it is very likely that the execution engine will not be synced, especially post-merge. The
deposit_root field is also not strictly necessary, but by including it, newly syncing consensus nodes can cheaply validate any received snapshot against itself (see the
calculate_root() method in the Reference Implementation).
The deposit contract can only provide the tree at the head of the chain. Because the beacon chain’s view of the deposit contract lags behind the execution chain by
ETH1_FOLLOW_DISTANCE, there are almost always deposits which haven’t yet been included in the chain that need proofs constructed from an earlier version of the tree than exists at the head.
In principle, a node could scan backwards through the chain starting from the weak subjectivity checkpoint to locate a suitable
Deposit, and then extract the rightmost branch of the tree from that. The node would also need to extract the
execution_block_hash from which to start syncing new deposits from the
Eth1Data in the corresponding
BeaconState. This approach is less desirable for a few reasons:
- More difficult to implement due to the edge cases involved in finding a suitable deposit to anchor to (the rightmost branch of the latest not-yet-included deposit is required)
- This would make backfilling beacon blocks a requirement for reconstructing the deposit tree and therefore a requirement for block production
- This is inherently slower than getting this information from the weak subjectivity checkpoint
This proposal is fully backwards compatible.
Test cases are included in test_cases.yaml. Each case is structured as follows:
class DepositTestCase: deposit_data: DepositData # These are all the inputs to the deposit contract's deposit() function deposit_data_root: Hash32 # The tree hash root of this deposit (calculated for convenience) eth1_data: Eth1Data # An Eth1Data object that can be used to finalize the tree after pushing this deposit block_height: uint64 # The height of the execution block with this Eth1Data snapshot: DepositTreeSnapshot # The resulting DepositTreeSnapshot object if the tree were finalized after this deposit
This EIP also includes other files for testing:
- deposit_snapshot.py contains the same code as the Reference Implementation
- eip_4881.py contains boilerplate declarations
- test_deposit_snapshot.py includes code for running test cases against the reference implementation
If these files are downloaded to the same directory, the test cases can be run by executing
pytest in that directory.
This implementation lacks full error checking and is optimized for readability over efficiency. If
tree is a
DepositTree, then the
DepositTreeSnapshot can be obtained by calling
tree.get_snapshot() and a new instance of the tree can be recovered from the snapshot by calling
DepositTree.from_snapshot(). See the Deposit Finalization Conditions section for discussion on when the tree can be pruned by calling
Generating proofs for deposits against an earlier version of the tree is relatively fast in this implementation; just create a copy of the finalized tree with
copy = DepositTree.from_snapshot(tree.get_snapshot()) and then append the remaining deposits to the desired count with
copy.push_leaf(deposit). Proofs can then be obtained with
from __future__ import annotations from typing import List, Optional, Tuple from dataclasses import dataclass from abc import ABC,abstractmethod from eip_4881 import DEPOSIT_CONTRACT_DEPTH,Hash32,sha256,to_le_bytes,zerohashes @dataclass class DepositTreeSnapshot: finalized: List[Hash32, DEPOSIT_CONTRACT_DEPTH] deposit_root: Hash32 deposit_count: uint64 execution_block_hash: Hash32 execution_block_height: uint64 def calculate_root(self) -> Hash32: size = self.deposit_count index = len(self.finalized) root = zerohashes for level in range(0, DEPOSIT_CONTRACT_DEPTH): if (size & 1) == 1: index -= 1 root = sha256(self.finalized[index] + root) else: root = sha256(root + zerohashes[level]) size >>= 1 return sha256(root + to_le_bytes(self.deposit_count)) def from_tree_parts(finalized: List[Hash32], deposit_count: uint64, execution_block: Tuple[Hash32, uint64]) -> DepositTreeSnapshot: snapshot = DepositTreeSnapshot( finalized, zerohashes, deposit_count, execution_block, execution_block) snapshot.deposit_root = snapshot.calculate_root() return snapshot @dataclass class DepositTree: tree: MerkleTree mix_in_length: uint finalized_execution_block: Optional[Tuple[Hash32, uint64]] def new() -> DepositTree: merkle = MerkleTree.create(, DEPOSIT_CONTRACT_DEPTH) return DepositTree(merkle, 0, None) def get_snapshot(self) -> DepositTreeSnapshot: assert(self.finalized_execution_block is not None) finalized =  deposit_count = self.tree.get_finalized(finalized) return DepositTreeSnapshot.from_tree_parts( finalized, deposit_count, self.finalized_execution_block) def from_snapshot(snapshot: DepositTreeSnapshot) -> DepositTree: # decent validation check on the snapshot assert(snapshot.deposit_root == snapshot.calculate_root()) finalized_execution_block = (snapshot.execution_block_hash, snapshot.execution_block_height) tree = MerkleTree.from_snapshot_parts( snapshot.finalized, snapshot.deposit_count, DEPOSIT_CONTRACT_DEPTH) return DepositTree(tree, snapshot.deposit_count, finalized_execution_block) def finalize(self, eth1_data: Eth1Data, execution_block_height: uint64): self.finalized_execution_block = (eth1_data.block_hash, execution_block_height) self.tree.finalize(eth1_data.deposit_count, DEPOSIT_CONTRACT_DEPTH) def get_proof(self, index: uint) -> Tuple[Hash32, List[Hash32]]: assert(self.mix_in_length > 0) # ensure index > finalized deposit index assert(index > self.tree.get_finalized() - 1) leaf, proof = self.tree.generate_proof(index, DEPOSIT_CONTRACT_DEPTH) proof.append(to_le_bytes(self.mix_in_length)) return leaf, proof def get_root(self) -> Hash32: return sha256(self.tree.get_root() + to_le_bytes(self.mix_in_length)) def push_leaf(self, leaf: Hash32): self.mix_in_length += 1 self.tree = self.tree.push_leaf(leaf, DEPOSIT_CONTRACT_DEPTH) class MerkleTree(): @abstractmethod def get_root(self) -> Hash32: pass @abstractmethod def is_full(self) -> bool: pass @abstractmethod def push_leaf(self, leaf: Hash32, level: uint) -> MerkleTree: pass @abstractmethod def finalize(self, deposits_to_finalize: uint, level: uint) -> MerkleTree: pass @abstractmethod def get_finalized(self, result: List[Hash32]) -> uint: # returns the number of finalized deposits in the tree # while populating result with the finalized hashes pass def create(leaves: List[Hash32], depth: uint) -> MerkleTree: if not(leaves): return Zero(depth) if not(depth): return Leaf(leaves) split = min(2**(depth - 1), len(leaves)) left = MerkleTree.create(leaves[0:split], depth - 1) right = MerkleTree.create(leaves[split:], depth - 1) return Node(left, right) def from_snapshot_parts(finalized: List[Hash32], deposits: uint, level: uint) -> MerkleTree: if not(finalized) or not(deposits): # empty tree return Zero(level) if deposits == 2**level: return Finalized(deposits, finalized) left_subtree = 2**(level - 1) if deposits <= left_subtree: left = MerkleTree.from_snapshot_parts(finalized, deposits, level - 1) right = Zero(level - 1) return Node(left, right) else: left = Finalized(left_subtree, finalized) right = MerkleTree.from_snapshot_parts(finalized[1:], deposits - left_subtree, level - 1) return Node(left, right) def generate_proof(self, index: uint, depth: uint) -> Tuple[Hash32, List[Hash32]]: proof =  node = self while depth > 0: ith_bit = (index >> (depth - 1)) & 0x1 if ith_bit == 1: proof.append(node.left.get_root()) node = node.right else: proof.append(node.right.get_root()) node = node.left depth -= 1 proof.reverse() return node.get_root(), proof @dataclass class Finalized(MerkleTree): deposit_count: uint hash: Hash32 def get_root(self) -> Hash32: return self.hash def is_full(self) -> bool: return True def finalize(self, deposits_to_finalize: uint, level: uint) -> MerkleTree: return self def get_finalized(self, result: List[Hash32]) -> uint: result.append(self.hash) return self.deposit_count @dataclass class Leaf(MerkleTree): hash: Hash32 def get_root(self) -> Hash32: return self.hash def is_full(self) -> bool: return True def finalize(self, deposits_to_finalize: uint, level: uint) -> MerkleTree: return Finalized(1, self.hash) def get_finalized(self, result: List[Hash32]) -> uint: return 0 @dataclass class Node(MerkleTree): left: MerkleTree right: MerkleTree def get_root(self) -> Hash32: return sha256(self.left.get_root() + self.right.get_root()) def is_full(self) -> bool: return self.right.is_full() def push_leaf(self, leaf: Hash32, level: uint) -> MerkleTree: if not(self.left.is_full()): self.left = self.left.push_leaf(leaf, level - 1) else: self.right = self.right.push_leaf(leaf, level - 1) return self def finalize(self, deposits_to_finalize: uint, level: uint) -> MerkleTree: deposits = 2**level if deposits <= deposits_to_finalize: return Finalized(deposits, self.get_root()) self.left = self.left.finalize(deposits_to_finalize, level - 1) if deposits_to_finalize > deposits / 2: remaining = deposits_to_finalize - deposits / 2 self.right = self.right.finalize(remaining, level - 1) return self def get_finalized(self, result: List[Hash32]) -> uint: return self.left.get_finalized(result) + self.right.get_finalized(result) @dataclass class Zero(MerkleTree): n: uint64 def get_root(self) -> Hash32: if self.n == DEPOSIT_CONTRACT_DEPTH: # Handle the entirely empty tree case. This is included for # consistency/clarity as the zerohashes array is typically # only defined from 0 to DEPOSIT_CONTRACT_DEPTH - 1. return sha256(zerohashes[self.n - 1] + zerohashes[self.n - 1]) return zerohashes[self.n] def is_full(self) -> bool: return False def push_leaf(self, leaf: Hash32, level: uint) -> MerkleTree: return MerkleTree.create([leaf], level) def get_finalized(self, result: List[Hash32]) -> uint: return 0
The upcoming switch to PoS will require newly synced nodes to rely on valid weak subjectivity checkpoints because of long-range attacks. This proposal relies on the weak subjectivity assumption that clients will not bootstrap with an invalid WS checkpoint.
Care must be taken not to send a snapshot which includes deposits that haven’t been fully included in the finalized checkpoint. Let
state be the
BeaconState at a given block in the chain. Under normal operation, the
Eth1Data stored in
state.eth1_data is replaced every
EPOCHS_PER_ETH1_VOTING_PERIOD epochs. Thus, finalization of the deposit tree proceeds with increments of
eth1data be some
Eth1Data. Both of the following conditions MUST be met to consider
- A finalized checkpoint exists where the corresponding
state.eth1_data == eth1data
- A finalized checkpoint exists where the corresponding
state.eth1_deposit_index >= eth1data.deposit_count
When these conditions are met, the tree can be pruned in the reference implementation by calling
The proposed design could fail if the deposit queue becomes so large that deposits cannot be processed within the EIP-4444 Pruning Period (currently set to 1 year). The beacon chain can process
MAX_DEPOSITS/SECONDS_PER_SLOT deposits/second without skipped slots. Even under extreme conditions where 25% of slots are skipped, the deposit queue would need to be >31.5 million to hit this limit. This is more than 8x the total supply of ether assuming each deposit is a full validator. The minimum deposit is 1 ETH so an attacker would need to burn >30 Million ETH to create these conditions.
Copyright and related rights waived via CC0.
Please cite this document as:
Mark Mackey, "EIP-4881: Deposit Contract Snapshot Interface [DRAFT]," Ethereum Improvement Proposals, no. 4881, January 2021. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-4881.