This EIP introduces two EVM instructions `AUTH` and `AUTHCALL`. The first sets a context variable `authorized` based on an ECDSA signature. The second sends a call as the `authorized`.
Adding more functionlity to EOAs has been a long-standing feature request. The requests have spanned from implementing batching capabilities, allowing for gas sponsoring, expirations, scripting, and beyond. These changes often mean increased complexity and rigidity of the protocol. In some cases, it also means increased attack surfaces.
This EIP takes a different approach. Instead of enshrining these capabilities in the protocol as transaction validity requirements, it provides developers with a flexible framework for developing novel transaction schemes for EOAs. A good way to think about this is that this EIP allows any EOA to become a smart contract wallet *without* deploying a contract.
Although this EIP provides great benefit to individual users, the leading motivation for this EIP is "sponsored transactions". This is where the fee for a transaction is provided by a different account than the one that originates the call.
With the extraordinary growth of tokens on Ethereum, it has become common for EOAs to hold valuable assets without holding any ether at all. Today, these assets must be converted to ether before they can be used to pay gas fees. However, without ether to pay for the conversion, it's impossible to convert them. Sponsored transactions break the circular dependency.
The context variable `authorized` shall indicate the active account for `AUTHCALL` instructions in the current frame of execution. If set, `authorized` shall only contain an account which has given the contract authorization to act on its behalf. An unset value shall indicate that no such account is set and that there is not yet an active account for `AUTHCALL` instructions in the current frame of execution.
The variable has the same scope as the program counter -- `authorized` persists throughout a single frame of execution of the contract, but is not passed through any calls (including `DELEGATECALL`). If the same contract is being executed in separate execution frames (ex. a `CALL` to self), both frames shall have independent values for `authorized`. Initially in each frame of execution, `authorized` is always unset, even if a previous execution frame for the same contract has a value.
The arguments (`yParity`, `r`, `s`) are interpreted as an ECDSA signature on the secp256k1 curve over the message `keccak256(TYPE || paddedInvokerAddress || commit)`, where:
-`paddedInvokerAddress` is the address of the contract executing `AUTH`, left-padded with zeroes to a total of 32 bytes (ex. `0x000000000000000000000000AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA`).
-`commit`, one of the arguments passed into `AUTH`, is a 32-byte value that can be used to commit to specific additional validity conditions in the invoker's pre-processing logic (e.g. a nonce for replay protection).
If the signature is valid, the `signerAddress` is recovered. Signature validity and signer recovery is handled analogous to transaction signatures, including the stricter `s` range for preventing ECDSA malleability. Note that `yParity` is expected to be `0` or `1`. If `signerAddress != tx.origin`, the context variable `authorized` is set to `signerAddress`.
In any other case, i.e. if the signature is invalid or `signerAddress == tx.origin`, `authorized` is reset to an unset value.
A new opcode `AUTHCALL` shall be created at `0xf7`. It shall take seven stack elements and return one stack element. It matches the behavior of the existing `CALL` (`0xF1`) instruction, except where noted below.
- If `authorized` is unset, execution is considered invalid and must exit the current execution frame immediately (in the same way as a stack underflow or invalid jump).
- Otherwise, the caller address for the call is set to `authorized`.
`AUTHCALL` must increase the call depth by one. `AUTHCALL` must not increase the call depth by two as if it first called into the authorized account and then into the target.
As with `CALL`, the gas cost for the opcode itself (both the static and the dynamic portion) is always charged, independent of whether the call is actually executed. The gas passed into the call is calculated following EIP-150 and is refunded partially if the call returns with unused gas left, or completely if the call is not executed at all because of a failing pre-check.
A well-behaved contract should never reach an `AUTHCALL` without having successfully set `authorized` beforehand. The safest behavior, therefore, is to exit the current frame of execution immediately. This is especially important in the context of transaction sponsoring / relaying, which is expected to be one of the main use cases for this EIP. In a sponsored transaction, the inability to distinguish between a sponsee-attributable fault (like a failing sub-call) and a sponsor-attributable fault (like a failing `AUTH`) is especially dangerous and should be prevented because it charges unfair fees to the sponsee.
While clients should never interpret EIP-3074 signed messages as transactions, reserving an [EIP-2718](./eip-2718.md) transaction type reduces the likelihood of this occurring by accident.
There are two general approaches to separating the "fee payer" from the "action originator".
The first is introducing a new transaction. This requires significant changes to clients to support and is generally less upgradeable than other solutions (e.g. this EIP). This approach is also 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. The main advantage of new transaction types is that the validity requirements are enforced by the protocol, therefore invalid transactions do not pollute block space.
The other main approach is to introduce a new mechanism in the EVM to masquerade as other accounts. This EIP introduces `AUTH` and `AUTHCALL` to make calls as EOAs. There are many different permutations of this mechanism. An alternative mechanism would be add an opcode that can make arbitrary calls based on a similar address creation scheme as `CREATE2`. Although this mechanism would not benefit users today, it would immediately allow for those accounts to send and receive ether -- making it feel like a more first-class primitive.
Besides better compatibility with AA, introducing a new mechanism into the EVM 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.
`AUTHCALL`'s single deviation from `CALL` is to set `CALLER`. It implements the minimal functionality to enable sender abstraction for sponsored transactions. This single mindedness makes `AUTHCALL` significantly more composable with existing Ethereum features.
More logic can be implemented around the `AUTHCALL` instruction, giving more control to invokers and sponsors without sacrificing security or user experience for sponsees.
As originally written, this proposal specified a precompile with storage to track nonces. Since a precompile with storage is unprecedented, a revision moved replay protection into the invoker contract, necessitating a certain level of user trust in the invoker. Expanding on this idea of trusted invokers, the other signed fields were eventually eliminated, one by one, until only `invoker` and `commit` remained.
The `invoker` binds a particular signed message to a single invoker. If invoker was not part of the message, any invoker could reuse the signature to completely compromise the EOA. This allows users to trust that their message will be validated as they expect, particularly the values committed to in `commit`.
### Understanding `commit`
Earlier iterations of this EIP included mechanisms for replay protection, and also signed over value, gas, and other arguments to `AUTHCALL`. After further investigation, we revised this EIP to its current state: explicitly delegate these responsibilities to the invoker contract.
A user will specifically interact with an invoker they trust. Because they trust this contract to execute faithfully, they will "commit" to certain properties of a call they would like to make by computing a hash of the call values. They can be certain that the invoker will only allow they call to proceed if it is able to verify the values committed to (e.g. a nonce to protect against replay attacks). This certainty arises from the `commit` value that is signed over by the user. This is the hash of values which the invoker will validate. A safe invoker should accept the values from the user and compute the commit hash itself. This ensures that invoker operated on the same input that user authorized.
Using `commit` as a hash of values allows for invokers to implement arbitrary constraints. For example, they could allow accounts to have `N` parallel nonces. Or, they could allow a user to commit to multiple calls with a single signature. This would allow mult-tx flows, such as ERC-20 `approve`-`transfer` actions, to be condensed into a single transaction with a single signature verification. A commitment to multiple calls would look something like the diagram below.
The invoker contract is a trustless intermediary between the sponsor and sponsee. A sponsee signs over `invoker` to require they transaction to be processed only by a contract they trust. This allows them to interact with sponsors without needing to trust them.
Choosing an invoker is similar to choosing a smart contract wallet implementation. It's important to choose one that has been thoroughly reviewed, tested, and accepted by the community as secure. We expect a few invoker designs to be utilized by most major transaction relay providers, with a few outliers that offer more novel mechanisms.
An important note is that invoker contracts **MUST NOT** be upgradeable. If an invoker can be redeployed to the same address with different code, it would be possible to redeploy the invoker with code that does not properly verify `commit` and any account that signed a message over that invoker would be compromised. Although this sounds scare, it is no different than using a smart contract wallet via `DELEGATECALL`. If the wallet is redeployed with different logic, all wallet using its code could be compromised.
The reason for banning signatures from `tx.origin` is that subsequent `AUTHCALL`s would result in `msg.sender == tx.origin`. This however is a frequently used pattern to test for top-level execution (i.e. being called directly from an EOA). Banning `tx.origin` as signer keeps this invariant intact.
The EVM limits the maximum number of nested calls, and naively allowing a sponsor to manipulate the call depth before reaching the invoker would introduce a griefing attack against the sponsee. That said, with the 63/64th gas rule, and the cost of `AUTHCALL`, the stack is effectively limited to a much smaller depth than the hard maximum by the `gas` parameter.
It is, therefore, sufficient for the invoker to guarantee a minimum amount of gas, because there is no way to reach the hard maximum call depth with any reasonable (i.e. less than billions) amount of gas.
- Replay protection should be implemented by the invoker, and included in `commit`. Without it, a malicious actor can reuse a signature, repeating its effects.
-`value` should be included in `commit`. Without it, a malicious sponsor could cause unexpected effects in the callee.
-`gas` should be included in `commit`. Without it, a malicious sponsor could cause the callee to run out of gas and fail, griefing the sponsee.
- The current chain id should be included in `commit` and checked on every transaction. Without it, a malicious sponsor could replay a signature on a different chain.
-`addr` and `calldata` should be included in `commit`. Without them, a malicious actor may call arbitrary functions in arbitrary contracts.
