EIP-5920: PAY opcode

Abstract

This EIP introduces a new opcode, PAY, taking two stack parameters, addr and val, that transfers val wei to the address addr without calling any of its functions.

Motivation

Currently, to send ether to an address requires you to call into that address, which transfers execution context to that address, which creates several issues:

• First of all, it opens a reentrancy attack vector, as the recipient can call back into the sender. More generally, the recipient can unilaterally execute arbitrary state changes, limited only by the gas forwarded, which is not desirable from the point of view of the sender.
  • In practice, when calling into another account and sending ether, 2300 gas (the "gas stipend") is always available "for free" in the newly created call frame. Although it is currently not possible to make storage state changes with the 2300 gas stipend (see the last minute rejection of EIP-1283 from Constantinople to be replaced with the "gas stipend safe" EIP-2200 version in Istanbul), it is possible to do this with transient storage (EIP-1153). This is a security risk where potentially the transient storage gets changed unintentionally. The goal is to send ether, and not have to consider these potential unintuitive security problems.
• Secondly, it opens a DoS vector. Contracts wanting to send ether must be cognizant of the possibility that the recipient will run out of gas or revert.
• Future potential call-like opcodes may not provide a way to restrict the amount of gas being forwarded to the recipient, so the meager mitigation against unintended side effects in use today (gas limit) is not guaranteed to be available.
• The CALL and EXTCALL opcodes will execute code on the recipient, which is unintended when wanting to send ether and which could lead to unintentional operations. The code execution also has to be paid for in gas, even when the intention is to only send ether, which is thus an unnecessary waste of gas.
• Finally, EIP-7702 allows to delegate externally owned accounts (EOAs) to other accounts, which breaks the invariant that EOAs cannot contain code. Therefore, calling such EOA with the intention to send ether will thus also execute code and cost unnecessary gas.

Having a dedicated opcode for ether transfers solves all of these issues, and would be a useful addition to the EVM.

Specification

Constants

Behavior

A new opcode is introduced: PAY (0xfc), which:

• Halt with exceptional failure if the current frame is static, as defined in EIP-214.
• Pops two values from the stack: addr then val.
• Exceptionally halts if addr has any of the high 12 bytes set to a non-zero value (i.e. it does not contain a 20-byte address).
• Charges the gas cost detailed below.
• Marks addr as warm (adding addr to accessed_addresses).
• Transfers val wei from the current address to the address addr, only if the current address has a balance greater than or equal to val.
• Push 1 on the stack if the PAY operation was successful, or 0 if it failed.
  • Currently only insufficient balance would produce a 0 return value.

Gas Cost

The gas cost for PAY is the sum of the following:

• Is addr in accessed_addresses?
  • If yes, WARM_STORAGE_READ_COST;
  • Otherwise, COLD_ACCOUNT_ACCESS_COST.
• Does addr exist or is val zero?
  • If yes to either, zero;
  • Otherwise, GAS_NEW_ACCOUNT.
• Is val zero?
  • If yes, zero;
  • Otherwise, GAS_CALL_VALUE.

PAY cannot be implemented on networks with empty accounts (see EIP-7523).

Rationale

Argument order

The order of arguments mimics that of CALL, which pops addr before val. Beyond consistency, though, this ordering aids validators pattern-matching MEV opportunities, so PAY always appears immediately after COINBASE.

Halting for invalid address

The halting behavior is designed to allow for Address Space Extension. If the high bytes were truncated, as in CALL, contracts could depend on the truncating behavior. If the address space were extended beyond 20 bytes, PAY would either not be able to target those accounts, or code expecting truncation could send ether to the wrong address.

Because this behavior may be changed, contracts should not rely on this halting behavior and use methods designated to intentionally halt (such as the INVALID opcode).

Backwards Compatibility

This change requires a hard fork.

Security Considerations

Existing contracts should not rely on their balance being under their control, since it is already possible to send ether to an address without calling it, by using the SELFDESTRUCT opcode (somewhat restricted in EIP-6780). It is also possible to involuntarily fund an account with priority fees sent to a block.coinbase. However, this opcode does make this process cheaper and easier. It thus does not break an invariant.

Copyright

Copyright and related rights waived via CC0.