--- eip: 7739 title: Readable Typed Signatures for Smart Accounts description: A defensive rehashing scheme which prevents signature replays across smart accounts and preserves the readability of the signed contents author: vectorized (@vectorized), Sihoon Lee (@push0ebp), Francisco Giordano (@frangio), Hadrien Croubois (@Amxx), Ernesto García (@ernestognw), Im Juno (@junomonster), howydev (@howydev), Atarpara (@Atarpara), 0xcuriousapple (@0xcuriousapple) discussions-to: https://ethereum-magicians.org/t/erc-7739-readable-typed-signatures-for-smart-accounts/20513 status: Draft type: Standards Track category: ERC created: 2024-05-28 requires: 191, 712, 1271, 5267 --- ## Abstract This proposal defines a standard to prevent signature replays across multiple smart accounts when they are owned by a single Externally Owned Account (EOA). This is achieved through a defensive rehashing scheme for [ERC-1271](./eip-1271.md) verification using specific nested [EIP-712](./eip-712.md) typed structures, which preserves the readability of the signed contents during wallet client signature requests. ## Motivation Smart accounts can verify signatures with via [ERC-1271](./eip-1271.md) using the `isValidSignature` function. A straightforward implementation as shown below, is vulnerable to signature replay attacks. ```solidity /// @dev This implementation is NOT safe. function isValidSignature( bytes32 hash, bytes calldata signature ) external override view returns (bytes4) { uint8 v = uint8(signature[64]); (bytes32 r, bytes32 s) = abi.decode(signature, (bytes32, bytes32)); // Reject malleable signatures. require(uint256(s) <= 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0); address signer = ecrecover(hash, v, r, s); // Reject failed recovery. require(signer != address(0)); // `owner` is a storage variable containing the smart account's owner. if (signer == owner) { return 0x1626ba7e; } else { return 0xffffffff; } } ``` When multiple smart accounts are owned by a single EOA, the same signature can be replayed across the smart accounts if the `hash` does not include the smart account address. Unfortunately, this is the case for many popular applications (e.g. Permit2). As such, many smart account implementations perform some form of defensive rehashing. First, the smart account computes a final hash from minimally: (1) the hash, (2) its own address, (3) the chain ID. Then, the smart account verifies the final hash against the signature. Defensive rehashing can be implemented with [EIP-712](./eip-712.md), but a straightforward implementation will make the signed contents opaque. This standard provides a defensive rehashing scheme that makes the signed contents visible across all wallet clients that support [EIP-712](./eip-712.md). It is designed for minimal adoption friction. Even if wallet clients or application frontends are not updated, users can still inject client side JavaScript to enable the defensive rehashing. ## Specification The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174. ### Overview The following dependencies are REQUIRED: - [EIP-712](./eip-712.md) Typed structured data hashing and signing. Provides the relevant typed data hashing logic internally, which is required to construct the final hashes. - [ERC-1271](./eip-1271.md) Standard Signature Validation Method for Contracts. Provides the `isValidSignature(bytes32 hash, bytes calldata signature)` function. - [ERC-5267](./eip-5267.md) Retrieval of EIP-712 domain. Provides the `eip712Domain()` function which is required to compute the final hashes. This standard defines the behavior of the `isValidSignature` function for [ERC-1271](./eip-1271.md), which comprises of two workflows: (1) the `TypedDataSign` workflow, (2) the `PersonalSign` workflow. ### `TypedDataSign` workflow The `TypedDataSign` workflow handles the case where the `hash` is originally computed with [EIP-712](./eip-712.md). #### `TypedDataSign` final hash The final hash for the `TypedDataSign` workflow is defined as: ``` keccak256(\x19\x01 ‖ APP_DOMAIN_SEPARATOR ‖ hashStruct(TypedDataSign({ contents: hashStruct(originalStruct), name: eip712Domain().name, version: eip712Domain().version, chainId: eip712Domain().chainId, verifyingContract: eip712Domain().verifyingContract, salt: eip712Domain().salt })) ) ``` where `‖` denotes the concatenation operator for bytes. In Solidity, this can be written as: ```solidity finalTypedDataSignHash = keccak256( abi.encodePacked( hex"1901", // Application specific domain separator. Passed via `signature`. bytes32(APP_DOMAIN_SEPARATOR), keccak256( abi.encode( // Computed on-the-fly with `contentsType`, which is passed via `signature`. typedDataSignTypehash, // This is the `contents` struct hash, which is passed via `signature`. bytes32(hashStruct(originalStruct)), // `eip712Domain()` is from ERC-5267. keccak256(bytes(eip712Domain().name)), keccak256(bytes(eip712Domain().version)), uint256(eip712Domain().chainId), uint256(uint160(eip712Domain().verifyingContract)), bytes32(eip712Domain().salt) ) ) ) ); ``` where `typedDataSignTypehash` is: ```solidity typedDataSignTypehash = keccak256( abi.encodePacked( "TypedDataSign(", contentsName, " contents,", "string name,", "string version,", "uint256 chainId,", "address verifyingContract,", "bytes32 salt" ")", contentsType ) ); ``` If `contentsType` is `"Mail(address from,address to,string message)"`, then `contentsName` will be `"Mail"`. The `contentsName` is the substring of `contentsType` up to (excluding) the first instance of `"("`: In Solidity, this can be written as: ```solidity // `slice(string memory subject, uint256 start, uint256 end)` // returns a copy of `subject` sliced from `start` to `end` (exclusive). // `start` and `end` are byte offsets. // // `indexOf(string memory subject, string memory search)` // Returns the byte index of the first location of `search` in `subject`, // searching from left to right. Returns `2**256 - 1` if `search` is not found. contentsName = LibString.slice( contentsType, 0, // Start byte index. LibString.indexOf(contentsType, "(") // End byte index (exclusive). ); ``` [A copy of the `LibString` Solidity library is provided for completeness](../assets/eip-7739/contracts/utils/LibString.sol). For safety, it is RECOMMENDED to treat the signature as invalid if any of the following is true: - `contentsName` is the empty string (i.e. `bytes(contentsName).length == 0`). - `contentsName` starts with any of the following bytes `abcdefghijklmnopqrstuvwxyz(`. - `contentsName` contains any of the following bytes `, )\x00`. #### `TypedDataSign` signature The `signature` passed into `isValidSignature` will be changed to: ``` originalSignature ‖ APP_DOMAIN_SEPARATOR ‖ contents ‖ contentsDescription ‖ uint16(contentsDescription.length) ``` where: - `contents` is the bytes32 struct hash of the original struct. - `contentsDescription` is either: - `contentsType` (implicit mode), where `contentsType` starts with `contentsName`. - `contentsType ‖ contentsName` (explicit mode), where `contentsType` may not necessarily start with `contentsName`. In Solidity, this can be written as: ```solidity signature = abi.encodePacked( bytes(originalSignature), bytes32(APP_DOMAIN_SEPARATOR), bytes32(contents), bytes(contentsDescription), uint16(contentsDescription.length) ); ``` The appended `APP_DOMAIN_SEPARATOR` and `contents` struct hash will be used to verify if the `hash` passed into `isValidSignature` is indeed correct via: ```solidity hash == keccak256( abi.encodePacked( hex"1901", bytes32(APP_DOMAIN_SEPARATOR), bytes32(contents) ) ) ``` If the `hash` does not match the reconstructed hash, then the `hash` and `signature` are invalid under the `TypedDataSign` workflow. ### `PersonalSign` workflow This `PersonalSign` workflow handles the case where the `hash` is originally computed with [EIP-191](./eip-191.md). #### `PersonalSign` final hash The final hash for the `PersonalSign` workflow is defined as: ``` keccak256(\x19\x01 ‖ ACCOUNT_DOMAIN_SEPARATOR ‖ hashStruct(PersonalSign({ prefixed: keccak256(bytes(\x19Ethereum Signed Message:\n ‖ base10(bytes(someString).length) ‖ someString)) })) ) ``` where `‖` denotes the concatenation operator for bytes. In Solidity, this can be written as: ```solidity finalPersonalSignHash = keccak256( abi.encodePacked( hex"1901", // Smart account domain separator. // Can be computed via `eip712Domain()` from ERC-5267. bytes32(ACCOUNT_DOMAIN_SEPARATOR), keccak256( abi.encode( // `PERSONAL_SIGN_TYPEHASH`. keccak256("PersonalSign(bytes prefixed)"), // `hash` is from `isValidSignature(hash, signature)` hash ) ) ) ); ``` Here, `hash` is computed in the application contract and passed into `isValidSignature`. The smart account does not need to know how `hash` is computed. For completeness, this is how it can be computed: ```solidity hash = abi.encodePacked( "\x19Ethereum Signed Message:\n", // `toString` returns the base10 representation of a uint256. LibString.toString(someString.length), // This is the original message to be signed. someString ); ``` #### `PersonalSign` signature The `PersonalSign` workflow does not require additional data to be appended to the `signature` passed into `isValidSignature`. ### Support detection Smart accounts SHOULD return `bytes4(0x77390001)` for `isValidSignature(0x7739773977397739773977397739773977397739773977397739773977397739, "")` to indicate support for this standard. The magic number `bytes4(0x77390001)` MAY be incremented if this standard gets updated. ### Signature verification workflow deduction As the `isValidSignature` signature function signature is unchanged, the implementation MUST be able to deduce the type of workflow required to verify the signature. If the signature contains the correct data to reconstruct the `hash`, the `isValidSignature` function MUST perform the `TypedDataSign` workflow. Otherwise, the `isValidSignature` function MUST perform the `PersonalSign` workflow. In Solidity, the check can be written as: ```solidity // If this is true, it means that the `signature` contains // the correct `APP_DOMAIN_SEPARATOR` and `contents`, // and the `TypedDataSign` workflow MUST be performed. // Otherwise, the `PersonalSign` workflow MUST be performed. hash == keccak256( abi.encodePacked( hex"1901", bytes32(APP_DOMAIN_SEPARATOR), bytes32(contents) ) ) ``` ### Conditional skipping of defensive rehashing Smart accounts MAY skip the defensive rehashing workflows if any of the following is true: - `isValidSignature` is called off-chain. - The `hash` passed into `isValidSignature` has already included the address of the smart account. As many developers may not update their applications to support the nested EIP-712 workflow, smart account implementations SHOULD try to accommodate by skipping the defensive rehashing where it is safe to do so. ## Rationale ### `TypedDataSign` structure The `typedDataSignTypehash` must be constructed on-the-fly on-chain. This is to enforce that the signed contents will be visible in the signature request, by requiring that `contents` be a user defined type. The fields of `eip712Domain` are flattened into the `TypedDataSign` structure instead of being included as a field of type `EIP712Domain` in order to avoid a conflict with the domain type of the verifying contract in case it's different. The `bytes1 fields` bitmap and `uint256[] extensions` array in [ERC-5267](./eip-5267.md) have been omitted. Differentiating between an absent field versus a zero field (e.g. `bytes32(0)`) offers no additional security benefits for on-chain defensive rehashing. The `extensions` parameter is a list of EIP numbers used for off-chain signaling. ### `contentsDescription` with implicit and explicit modes When the `contents` structure contains nested types, EIP-712 lexicographical sorting can result in the `contentsName` not being positioned exactly at the start of the `contentsType`. As such, we need the explicit mode. ### Support detection with `isValidSignature` For easier implementation in modular smart accounts, we have decided to utilize the `isValidSignature` method to return a magic number instead of defining new functions. ### Rejecting `contentsName` beginning with any lowercase 7-bit ASCII character This recommendation is to keep the standard language agnostic and future-proof. Atomic types such as `uint256` may be named differently in other languages (e.g. `u256`). ## Backwards Compatibility ### Detection of previous draft In an earlier draft, we have designated a `supportsNestedTypedDataSign()` function for support detection, which returns `bytes4(0xd620c85a)`. ## Reference Implementation [A production ready and optimized implementation is provided for reference](../assets/eip-7739/contracts/accounts/ERC1271.sol). It includes relevant complementary features required for safety, flexibility, developer experience, and user experience. The reference implementation is intentionally not minimalistic. This is to avoid repeating the mistake of [ERC-1271](./eip-1271.md), where a minimalist reference implementation is wrongly assumed to be safe for production use. ## Security Considerations ### Rejecting invalid `contentsName` Current major implementations of `eth_signTypedData` do not sanitize the names of custom types. A phishing website can craft a `contentsName` with control characters to break out of the `PersonalSign` type encoding, resulting in the wallet client asking the user to sign an opaque hash. Requiring on-chain sanitization of `contentsName` will block this phishing attack vector. ### Impossible to chain multiple signers of this kind An account that uses this method as replay protection for ERC-1271 signatures cannot have a signer that uses the same method. This is because a signature defines a `TypedDataSign` struct type with a member that has the type of the message being signed, and if the message being signed is another `TypedDataSign` struct, the resulting EIP-712 message will contain in its body two separate `TypedDataSign` types with incompatible contents, something that can't be represented in an EIP-712 request. ## Copyright Copyright and related rights waived via [CC0](../LICENSE.md).