Execution layer triggerable withdrawals

Abstract

Adds a new mechanism to allow validators to trigger withdrawals and exits from their execution layer (0x01) withdrawal credentials.

These new execution layer exit messages are appended to the execution layer block and then processed by the consensus layer.

Motivation

Validators have two keys -- an active key and a withdrawal credential. The active key takes the form of a BLS key, whereas the withdrawal credential can either be a BLS key (0x00) or an execution layer address (0x01). The active key is "hot", actively signing and performing validator duties, whereas the withdrawal credential can remain "cold", only performing limited operations in relation to withdrawing and ownership of the staked ETH. Due to this security relationship, the withdrawal credential ultimately is the key that owns the staked ETH and any rewards.

As currently specified, only the active key can initiate a validator exit. This means that in any non-standard custody relationships (i.e. active key is separate entity from withdrawal credentials), that the ultimate owner of the funds -- the possessor of the withdrawal credentials -- cannot independently choose to exit and begin the withdrawal process. This leads to either trust issues (e.g. ETH can be "held hostage" by the active key owner) or insufficient work-arounds such as pre-signed exits. Additionally, in the event that active keys are lost, a user should still be able to recover their funds by using their cold withdrawal credentials.

To ensure that the withdrawal credentials (owned by both EOAs and smart contracts) can trustlessly control the destiny of the staked ETH, this specification enables exits triggerable by 0x01 withdrawal credentials.

Note, 0x00 withdrawal credentials can be changed into 0x01 withdrawal credentials with a one-time signed message. Thus any functionality enabled for 0x01 credentials is defacto enabled for 0x00 credentials.

Specification

Constants

Name	Value	Comment
FORK_TIMESTAMP	TBD	Mainnet

Configuration

Name	Value	Comment
WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS	0x0c15F14308530b7CDB8460094BbB9cC28b9AaaAA	Where to call and store relevant details about exit / partial withdrawal mechanism
WITHDRAWAL_REQUEST_TYPE	0x01	The EIP-7685 type prefix for withdrawal request
SYSTEM_ADDRESS	0xfffffffffffffffffffffffffffffffffffffffe	Address used to invoke system operation on contract
EXCESS_WITHDRAWAL_REQUESTS_STORAGE_SLOT	0
WITHDRAWAL_REQUEST_COUNT_STORAGE_SLOT	1
WITHDRAWAL_REQUEST_QUEUE_HEAD_STORAGE_SLOT	2	Pointer to head of the withdrawal request message queue
WITHDRAWAL_REQUEST_QUEUE_TAIL_STORAGE_SLOT	3	Pointer to the tail of the withdrawal request message queue
WITHDRAWAL_REQUEST_QUEUE_STORAGE_OFFSET	4	The start memory slot of the in-state withdrawal request message queue
MAX_WITHDRAWAL_REQUESTS_PER_BLOCK	16	Maximum number of withdrawal requests that can be dequeued into a block
TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK	2
MIN_WITHDRAWAL_REQUEST_FEE	1
WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION	17
EXCESS_INHIBITOR	2**256-1	Excess value used to compute the fee before the first system call

Execution layer

Definitions

• FORK_BLOCK -- the first block in a blockchain with the timestamp greater or equal to FORK_TIMESTAMP.

Withdrawal request operation

The new withdrawal request operation is an EIP-7685 request with type 0x01 and consists of the following fields:

1. source_address: Bytes20
2. validator_pubkey: Bytes48
3. amount: uint64

The EIP-7685 encoding of a withdrawal request is computed as follows. Note that amount is returned by the contract little-endian, and must be encoded as such.

request_type = WITHDRAWAL_REQUEST_TYPE
request_data = read_withdrawal_requests()

Withdrawal Request Contract

The contract has three different code paths, which can be summarized at a high level as follows:

1. Add withdrawal request - requires a 56 byte input, the validator's public key concatenated with a big-endian uint64 amount value.
2. Fee getter - if the input length is zero, return the current fee required to add a withdrawal request.
3. System process - if called by system address, pop off the withdrawal requests for the current block from the queue.

Add Withdrawal Request

If call data input to the contract is exactly 56 bytes, perform the following:

• Ensure enough ETH was sent to cover the current withdrawal request fee (check_fee())
• Increase withdrawal request count by 1 for the current block (increment_count())
• Insert a withdrawal request into the queue for the source address and validator pubkey (insert_withdrawal_request_into_queue())

Specifically, the functionality is defined in pseudocode as the function add_withdrawal_request():

def add_withdrawal_request(Bytes48: validator_pubkey, uint64: amount):
    """
    Add withdrawal request adds new request to the withdrawal request queue, so long as a sufficient fee is provided.
    """

    # Verify sufficient fee was provided.
    fee = get_fee()
    require(msg.value >= fee, 'Insufficient value for fee')

    # Increment withdrawal request count.
    count = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_COUNT_STORAGE_SLOT)
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_COUNT_STORAGE_SLOT, count + 1)

    # Insert into queue.
    queue_tail_index = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_TAIL_STORAGE_SLOT)
    queue_storage_slot = WITHDRAWAL_REQUEST_QUEUE_STORAGE_OFFSET + queue_tail_index * 3
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot, msg.sender)
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot + 1, validator_pubkey[0:32])
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot + 2, validator_pubkey[32:48] ++ uint64_to_little_endian(amount))
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_TAIL_STORAGE_SLOT, queue_tail_index + 1)

Fee calculation

The following pseudocode can compute the cost an individual withdrawal request, given a certain number of excess withdrawal requests.

def get_fee() -> int:
    excess = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, EXCESS_WITHDRAWAL_REQUESTS_STORAGE_SLOT)
    require(excess != EXCESS_INHIBITOR, 'Inhibitor still active')
    return fake_exponential(
        MIN_WITHDRAWAL_REQUEST_FEE,
        excess,
        WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION
    )

def fake_exponential(factor: int, numerator: int, denominator: int) -> int:
    i = 1
    output = 0
    numerator_accum = factor * denominator
    while numerator_accum > 0:
        output += numerator_accum
        numerator_accum = (numerator_accum * numerator) // (denominator * i)
        i += 1
    return output // denominator

Fee Getter

When the input to the contract is length zero, interpret this as a get request for the current fee, i.e. the contract returns the result of get_fee().

System Call

At the end of processing any execution block where block.timestamp >= FORK_TIMESTAMP (i.e. after processing all transactions and after performing the block body withdrawal requests validations), call the contract as SYSTEM_ADDRESS and perform the following:

• The contract's queue is updated based on withdrawal requests dequeued and the withdrawal requests queue head/tail are reset if the queue has been cleared (dequeue_withdrawal_requests())
• The contract's excess withdrawal requests are updated based on usage in the current block (update_excess_withdrawal_requests())
• The contract's withdrawal requests count is reset to 0 (reset_withdrawal_requests_count())

Specifically, the functionality is defined in pseudocode as the function read_withdrawal_requests():

###################
# Public function #
###################

def read_withdrawal_requests():
    reqs = dequeue_withdrawal_requests()
    update_excess_withdrawal_requests()
    reset_withdrawal_requests_count()
    return ssz.serialize(reqs)

###########
# Helpers #
###########

def little_endian_to_uint64(data: bytes) -> uint64:
    return uint64(int.from_bytes(data, 'little'))

def uint64_to_little_endian(num: uint64) -> bytes:
    return num.to_bytes(8, 'little')

class ValidatorWithdrawalRequest(object):
    source_address: Bytes20
    validator_pubkey: Bytes48
    amount: uint64

def dequeue_withdrawal_requests():
    queue_head_index = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_HEAD_STORAGE_SLOT)
    queue_tail_index = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_TAIL_STORAGE_SLOT)
    num_in_queue = queue_tail_index - queue_head_index
    num_dequeued = min(num_in_queue, MAX_WITHDRAWAL_REQUESTS_PER_BLOCK)

    reqs = []
    for i in range(num_dequeue):
        queue_storage_slot = WITHDRAWAL_REQUEST_QUEUE_STORAGE_OFFSET + (queue_head_index + i) * 3
        source_address = address(sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot)[0:20])
        validator_pubkey = (
            sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot + 1)[0:32] + sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot + 2)[0:16]
        )
        amount = little_endian_to_uint64(sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, queue_storage_slot + 2)[16:24])
        req = ValidatorWithdrawalRequest(
            source_address=Bytes20(source_address),
            validator_pubkey=Bytes48(validator_pubkey),
            amount=uint64(amount)
        )
        reqs.append(req)

    new_queue_head_index = queue_head_index + num_dequeued
    if new_queue_head_index == queue_tail_index:
        # Queue is empty, reset queue pointers
        sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_HEAD_STORAGE_SLOT, 0)
        sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_TAIL_STORAGE_SLOT, 0)
    else:
        sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_QUEUE_HEAD_STORAGE_SLOT, new_queue_head_index)

    return reqs

def update_excess_withdrawal_requests():
    previous_excess = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, EXCESS_WITHDRAWAL_REQUESTS_STORAGE_SLOT)
    if previous_excess == EXCESS_INHIBITOR:
        previous_excess = 0

    count = sload(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_COUNT_STORAGE_SLOT)

    new_excess = 0
    if previous_excess + count > TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK:
        new_excess = previous_excess + count - TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK

    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, EXCESS_WITHDRAWAL_REQUESTS_STORAGE_SLOT, new_excess)

def reset_withdrawal_requests_count():
    sstore(WITHDRAWAL_REQUEST_PREDEPLOY_ADDRESS, WITHDRAWAL_REQUEST_COUNT_STORAGE_SLOT, 0)

Each withdrawal request must appear in the EIP-7685 requests list in the order they are returned from dequeue_withdrawal_requests().

Bytecode

caller
push20 0xfffffffffffffffffffffffffffffffffffffffe
eq
push1 0xcb
jumpi

push1 0x11
push0
sload
dup1
push32 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
eq
push2 0x01f4
jumpi

push1 0x01
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jumpdest
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iszero
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dup2
add
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calldatasize
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jumpdest
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push32 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
eq
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pop
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add
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jumpdest
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revert

Deployment

The withdrawal requests contract is deployed like any other smart contract. A special synthetic address is generated by working backwards from the desired deployment transaction:

{
  "type": "0x0",
  "nonce": "0x0",
  "to": null,
  "gas": "0x3d090",
  "gasPrice": "0xe8d4a51000",
  "maxPriorityFeePerGas": null,
  "maxFeePerGas": null,
  "value": "0x0",
  "input": "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",
  "v": "0x1b",
  "r": "0x539",
  "s": "0xeb793ed1dcd82833",
  "hash": "0xe89c48ef3308192a0768aefaa5128559e68ab87b5b9a6d113824551867746a20"
}

Sender: 0xa05D9eED37862daB22b002b8F6668B8Fb0c4D798
Address: 0x0c15F14308530b7CDB8460094BbB9cC28b9AaaAA

Consensus layer

Full specification

Sketch of spec:

• New operation ExecutionLayerWithdrawalRequest
• Will show up in ExecutionPayload as an SSZ List bound by length MAX_WITHDRAWAL_REQUESTS_PER_BLOCK
• New function that has similar functionality to process_voluntary_exit but can fail validations (e.g. validator is already exited) without the block failing (similar to deposit coming from EL)
• This function is called in process_operations for each ExecutionLayerWithdrawalRequest found in the ExecutionPayload

Rationale

validator_pubkey field

Multiple validators can utilize the same execution layer withdrawal credential, thus the validator_pubkey field is utilized to disambiguate which validator is being exited.

Note, validator_index also disambiguates validators.

Message queue

The contract maintains and in-state queue of withdrawal request messages to be dequeued each block into the block and thus into the execution layer.

The number of withdrawal requests that can be passed into the consensus layer are bound by MAX_WITHDRAWAL_REQUESTS_PER_BLOCK to bound the load both on the block size as well as on the consensus layer processing. 16 has been chosen for MAX_WITHDRAWAL_REQUESTS_PER_BLOCK to be in line with the bounds of similar operations on the beacon chain -- e.g. VoluntaryExit and Deposit.

Although there is a maximum number of withdrawal requests that can passed to the consensus layer each block, the execution layer gas limit can provide for far more calls to the withdrawal request predeploy contract at each block. The queue then allows for these calls to successfully be made while still maintaining a system rate limit.

The alternative design considered was to have calls to the contract fail after MAX_WITHDRAWAL_REQUESTS_PER_BLOCK successful calls were made within the context of a single block. This would eliminate the need for the message queue, but would come at the cost of a bad UX of contract call failures in times of high exiting. The complexity to mitigate this bad UX is relatively low and is currently favored.

Rate limiting using a fee

Transactions are naturally rate-limited in the execution layer via the gas limit, but an adversary willing to pay market-rate gas fees (and potentially utilize builder markets to pay for front-of-block transaction inclusion) can fill up the exit operation limits for relatively cheap, thus griefing honest validators that want to make a withdrawal request.

There are two general approaches to combat this griefing -- (a) only allow validators to send such messages and with a limit per time period or (b) utilize an economic method to make such griefing increasingly costly.

Method (a) (not used in this EIP) would require EIP-4788 (the BEACON_ROOT opcode) against which to prove withdrawal credentials in relation to validator pubkeys as well as a data-structure to track requests per-unit-time (e.g. 4 months) to ensure that a validator cannot grief the mechanism by submitting many requests. The downsides of this method are that it requires another cross-layer EIP and that it is of higher cross-layer complexity (e.g. care that might need to be taken in future upgrades if, for example, the shape of the merkle tree of BEACON_ROOT changes, then the contract and proof structure might need to be updated).

Method (b) has been utilized in this EIP to eliminate additional EIP requirements and to reduce cross-layer complexity to allow for correctness of this EIP (now and in the future) to be easier to analyze. The EIP-1559-style mechanism with a dynamically adjusting fee mechanism allows for users to pay MIN_WITHDRAWAL_REQUEST_FEE for withdrawal requests in the normal case (fewer than 2 per block on average), but scales the fee up exponentially in response to high usage (i.e. potential abuse).

TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK configuration value

TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK has been selected as 2 such that even if all ETH is staked (~120M ETH -> 3.75M validators), the 64 validator per epoch target (2 * 32 slots) still exceeds the per-epoch exit churn limit on the consensus layer (defined by get_validator_churn_limit()) at such values -- 57 validators per epoch (3.75M // 65536).

Fee update rule

The fee update rule is intended to approximate the formula fee = MIN_WITHDRAWAL_REQUEST_FEE * e**(excess / WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION), where excess is the total "extra" amount of withdrawal requests that the chain has processed relative to the "targeted" number (TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK per block).

Like EIP-1559, it’s a self-correcting formula: as the excess goes higher, the fee increases exponentially, reducing usage and eventually forcing the excess back down.

The block-by-block behavior is roughly as follows. If block N processes X requests, then at the end of block N excess increases by X - TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK, and so the fee in block N+1 increases by a factor of e**((X - TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK) / WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION). Hence, it has a similar effect to the existing EIP-1559, but is more "stable" in the sense that it responds in the same way to the same total withdrawal requests regardless of how they are distributed over time.

The parameter WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION controls the maximum downwards rate of change of the blob gas price. It is chosen to target a maximum downwards change rate of e(TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK / WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION) ≈ 1.125 per block. The maximum upwards change per block is e((MAX_WITHDRAWAL_REQUESTS_PER_BLOCK - TARGET_WITHDRAWAL_REQUESTS_PER_BLOCK) / WITHDRAWAL_REQUEST_FEE_UPDATE_FRACTION) ≈ 2.279.

Withdrawal requests inside of the block

Withdrawal requests are placed into the actual body of the block (and execution payload in the consensus layer).

There is a strong design requirement that the consensus layer and execution layer can execute independently of each other. This means, in this case, that the consensus layer cannot rely upon a synchronous call to the execution layer to get the required withdrawal requests for the current block. Instead, the requests must be embedded in the shared data-structure of the execution payload such that if the execution layer is offline, the consensus layer still has the requisite data to fully execute the consensus portion of the state transition function.

Backwards Compatibility

This EIP introduces backwards incompatible changes to the block structure and block validation rule set. But neither of these changes break anything related to current user activity and experience.

Security Considerations

Impact on existing custody relationships

There might be existing custody relationships and/or products that rely upon the assumption that the withdrawal credentials cannot trigger a withdrawal request. We are currently confident that the additional withdrawal credentials feature does not impact the security of existing validators because:

1. The withdrawal credentials ultimately own the funds so allowing them to exit staking is natural with respect to ownership.
2. We are currently not aware of any such custody relationships and/or products that do rely on the lack of this feature.

In the event that existing validators/custodians rely on this, then the validators can be exited and restaked utilizing 0x01 withdrawal credentials pointing to a smart contract that simulates this behaviour.

Fee Overpayment

Calls to the system contract require a fee payment defined by the current contract state. Overpaid fees are not returned to the caller. It is not generally possible to compute the exact required fee amount ahead of time. When adding a withdrawal request from a contract, the contract can perform a read operation to check for the current fee and then pay exactly the required amount. Here is an example in Solidity:

function addWithdrawal(bytes memory pubkey, uint64 amount) private {
    assert(pubkey.length == 48);

    // Read current fee from the contract.
    (bool readOK, bytes memory feeData) = WithdrawalsContract.staticcall('');
    if (!readOK) {
        revert('reading fee failed');
    }
    uint256 fee = uint256(bytes32(feeData));

    // Add the request.
    bytes memory callData = abi.encodePacked(pubkey, amount);
    (bool writeOK,) = WithdrawalsContract.call{value: fee}(callData);
    if (!writeOK) {
        revert('adding request failed');
    }
}

Note: the system contract uses the EVM CALLER operation (Solidity: msg.sender) as the target address for withdrawals, i.e. the address that calls the system contract must match the 0x01 withdrawal credential recorded in the beacon state.

Using an EOA to request withdrawals will always result in overpayment of fees. There is no way for an EOA to use a wrapper contract to request a withdrawal. And even if a way existed, the gas cost of returning the overage would likely be higher than the overage itself. If requesting withdrawals to an EOA through the system contract is desired, we recommend that users perform transaction simulations to estimate a reasonable fee amount to send.

Copyright

Copyright and related rights waived via CC0.