EIPs/EIPS/eip-3074.md

eip	title	author	discussions-to	status	type	category	created
3074	Native Sponsored Transactions	Sam Wilson (@SamWilsn)	https://ethereum-magicians.org/t/eip-3074-sponsored-transaction-precompile/4880	Draft	Standards Track	Core	2020-10-15

Simple Summary

Creates a new EVM instruction, analogous to CALL (0xF1), that sets CALLER (0x33) based on an ECDSA signature.

Abstract

This EIP creates an EVM instruction (CALLFROM) which forwards a CALL, setting CALLER according to an ECDSA signature.

Motivation

Sponsored transactions—the separation of fee payment from transaction content—have been a long standing feature request. Unlike similar proposals, this EIP specifies a method of implementing sponsored transactions that allows both externally owned accounts (EOAs) and EIP-2938 contracts to act as sponsors.

With the explosion of tokens built on Ethereum, especially stable coins, it has become common for EOAs to hold valuable assets without holding any Ether at all. These assets must be converted to Ether before they can be used to pay gas fees, but without Ether to pay for the conversion, it's impossible to convert them. Sponsored transactions break the circular dependency.

While it is possible to emulate sponsored transactions (ex. Gas Station Network), these solutions require specific support in callee contracts.

Specification

An opcode, at 0xf9, shall function like a CALL (0xF1) instruction:

• To the to address,
• Transferring value wei from the invoker to the callee,
• Passing gas gas for execution,
• With calldata in the memory region specified by argsOffset and argsLength,
• With a return data region specified by retOffset and retLength.

Additionally, the opcode shall:

• Set the caller address based on an ECDSA signature.

CALLFROM shall increase the call depth by one, in the same way as CALL. CALLFROM shall not increase the call depth by two (as it would if it first called into the sponsee account and then into the callee.)

In a static context (such as the one created by STATICCALL), CALLFROM with a non-zero value shall exit the current execution frame immediately (in the same way CALL behaves with a non-zero value in a static context.)

Definitions

• CALLFROM - the specific instruction encoded as 0xf9, introduced by this EIP, which implements the CALL analogue.
• Transaction-like Package (TLP) - the signed arguments passed to CALLFROM.
• Sponsor - the account which is responsible for paying gas fees and sending the transaction.
• Sponsee - the account which signed the transaction-like package.
• Invoker - the account or contract which contains CALLFROM.
• Callee - the target of the call from CALLFROM.

Conventions

• top - N - the Nth most recently pushed value on the EVM stack, where top - 0 is the most recent.
• || - byte concatenation operator.

Constants

Constant	Value	Description
SPONSORED_TYPE	0x03	EIP-2718 transaction type reserved for transaction-like packages.

API

Inputs

CALLFROM shall require the following stack arguments:

Stack	Value
top - 0	yParity
top - 1	r
top - 2	s
top - 3	sponsee
top - 4	type
top - 5	extra
top - 6	gas
top - 7	callee
top - 8	value
top - 9	argsOffset
top - 10	argsLength
top - 11	retOffset
top - 12	retLength

The signature (yParity, r, s) arguments shall be computed from secp256k1(keccak256(type || abi.encode(invoker, chainid, extra))).

The arguments are defined to be:

• type: uint8 - EIP-2718 transaction type (currently always SPONSORED_TYPE);
• invoker: address - the address of the invoker contract;
• chainid: uint256 - the chain id, as returned by the CHAINID (0x46) opcode;
• extra: uint256 - extra data, which can be used in the invoker to implement replay protection;
• callee: address - address of the callee;
• gas: uint256 - exact gas limit which must be provided with the call into CALLFROM;
• value: uint256 - exact amount of Ether in wei to be received by the callee;
• sponsee: address - address of the sponsee;
• argsOffset: uint256, argsLength: uint256 - region of memory used as the calldata for the call into the callee;
• retOffset: uint256, retLength: uint256 - region of memory filled with the return data from the call into the callee; and
• yParity: uint8, r: bytes32, s: bytes32 - signature for the package.

Outputs

CALLFROM pushes the following two values onto the stack:

Stack	Value
top - 0	valid
top - 1	success

valid

valid shall be zero in the following cases:

• type != SPONSORED_TYPE
• Invalid signature
• The address recovered from yParity, r, and s does not match sponsee

valid shall be a one in all other cases.

success

success shall be zero in the following cases:

• success == 0
• the code execution failed due to an exceptional halting or revert
• call depth limit has been reached

success shall be a one in all other cases.

Return Data

The memory region defined by retOffset and retLength shall be filled in the same way as the CALL instruction with similar arguments.

The return data area accessed with RETURNDATASIZE (0x3d) and RETURNDATACOPY (0x3e) shall be set in the same way as the CALL instruction.

Gas Fees

The gas fees for CALLFROM are calculated according to the following pseudo-code:

fee = 3200

if preconditions_good(...):
    return fee + cost_of_call(...)
else:
    return fee

Where cost_of_call(...) is the cost of a CALL (0xF1) instruction with the same gas, value, argsOffset, argsLength, retOffset, and retLength arguments.

Rationale

Omitting Arguments

The signature arguments chainid and invoker are not included in arguments to the instruction because they can be calculated by the instruction itself.

Two Return Values

It is important to differentiate between a failure in CALLFROM's preconditions versus a failure in the callee. Correctly implementing replay protection requires the invoker to change its state (i.e. burn the nonce) even if the callee fails; but doing so if, for example, the signature failed would be nonsensical. Several options exist for encoding these two failure cases: returning two stack elements, reserving a specific revert reason, or choosing different values in a single stack element.

First, it's important to note that all three options are a deviation from the semantics of other CALL opcodes, but this deviation is unavoidable.

Reserving a specific revert reason, for example CALLFROM failed, is a large departure from other instructions. An invoker would need to inspect the revert reason to determine whether the callee reverted, or the CALLFROM pre-conditions were invalidated, which implies reading and comparing memory values. Further, to remain sound if a callee were to revert with CALLFROM failed, CALLFROM would need to replace the return data with some other value.

Returning a single stack element with different values depending on the situation (ex. 0 on success, 1 when the pre-conditions are violated, and 2 when the callee reverts) introduces the opportunity for a subtle bug: it's trivially easy to misinterpret the return value (CALL returns non-zero on success), but it's much harder to ignore a whole new stack value.

Sponsee in Arguments

Including sponsee in the arguments to CALLFROM is a gas optimization for invoker contracts implementing some replay protection based on the sponsee address. Without the sponsee argument, invokers would have to do their own ecrecover before calling into CALLFROM to verify/adjust any state for replay protection.

Reserving an EIP-2718 Transaction Type

While clients should never directly interpret transaction-like packages as true transactions, reserving an EIP-2718 transaction type for transaction-like packages reduces the likelihood of a transaction-like package being misinterpreted as a true transaction.

Another Sponsored Transaction EIP

Other approaches to sponsored transactions, which rely on introducing a new transaction type, are not immediately compatible with account abstraction (AA). These proposals require a signed transaction from the sponsor's account, which is not possible from an AA contract, because it has no private key to sign with.

Besides better compatibility with AA, an instruction is a much less intrusive change than a new transaction type. This approach requires no changes in existing wallets, and little change in other tooling.

CALLFROM's single purpose is to set CALLER. It implements the minimal functionality to enable sender abstraction for sponsored transactions. This single mindedness makes CALLFROM significantly more composable with existing Ethereum features.

More logic can be implemented around the CALLFROM instruction, giving more control to invokers and sponsors without sacrificing security or user experience for sponsees.

What to Sign?

Earlier approaches to this problem included mechanisms for replay protection, and also signed over value, gas, and other arguments to CALLFROM. Instead, this proposal explicitly delegates these responsibilities to the invoker contract.

As originally written, this proposal specified a precompile with storage to track nonces. Since a precompile with storage is unprecedented, a later revision moved replay protection into the invoker contract, necessitating a certain level of user trust in the invoker, while also opening the door to more creative replay protection schemes in the future. Building on this idea of trusted invokers, the other signed fields in the transaction-like package were eliminated until only invoker, chainid, and extra remained.

The motivation for including invoker is to bind a particular transaction-like package to a single invoker. If invoker was not part of the TLP, a malicious invoker could reuse the TLP to impersonate the EOA.

While chainid is not strictly necessary, and could be enforced in the invoker contract, it's reasonable to assume every well-behaved invoker will check the chain id. Without chainid, a TLP would be valid on multiple chains.

Finally, extra should be used by invoker contracts to implement replay protection and security around calldata, value, and other parameters. For example, an invoker may assume extra to be keccak256(abi.encode(gas, value, nonce)), guaranteeing that the sponsee intended to set those parameters to those specific values. Without extra, invokers would not be able to determine if other values (eg. gas, value, calldata, etc.) had been tampered with.

Backwards Compatibility

No known issues.

Test Cases

TODO

Implementation

TODO

Security Considerations

Reentrancy

• Checking msg.sender == tx.origin no longer prevents reentrancy. Adding the pre-condition that sponsor != sponsee would restore this property.

Signature Verification, Reply Protection & extra

• Potential impersonation attacks if there is a bug in the signature verification.
• Replay protection can be poorly implemented (or even maliciously broken) in the invoker.
• A naive invoker can allow a malicious sponsor to take near complete control over a sponsee's EOA, including but not limited to:
  • Causing a transaction-like package to fail if CALLFROM is provided insufficient gas; and
  • Replacing the sponsee's callee and/or calldata with other values.

Frontrunning

• Transaction-like packages can be extracted from the original sponsor's transaction and resent by another sponsor.

Copyright

Copyright and related rights waived via CC0.