Add legacy EIP-1271 support (i.e. 'isValidSignature(bytes,bytes)')

src/AccountV3.sol

@@ -216,6 +216,30 @@ contract AccountV3 is
         return _isValidSigner(signer, "");
     }
 
+    /**
+     * Determines if a given data and signature are valid for this account
+     * @param data Data payload of signed data
+     * @param signature ECDSA signature or encoded contract signature (v=0)
+     */
+    function _isValidSignature(bytes calldata data, bytes calldata signature)
+        internal
+        view
+        virtual
+        override(Signatory)
+        returns (bool)
+    {
+        bytes32 hash = keccak256(data);
+        bytes32 digest;
+        // Adds EIP-191 signature prefix to hashed data
+        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/ea736bd45bd844d7968a64c5707d97710fe1c077/contracts/utils/cryptography/MessageHashUtils.sol#L30
+        assembly ("memory-safe") {
+            mstore(0x00, "\x19Ethereum Signed Message:\n32") // 32 is the bytes-length of hash
+            mstore(0x1c, hash) // 0x1c (28) is the length of the prefix
+            digest := keccak256(0x00, 0x3c) // 0x3c is the length of the prefix (0x1c) + messageHash (0x20)
+        }
+        return _isValidSignature(digest, signature);
+    }
+
     /**
      * @notice Returns whether a given account is authorized to execute transactions on behalf of
      * this account

src/abstract/Signatory.sol

@@ -28,4 +28,25 @@ abstract contract Signatory is IERC1271 {
         view
         virtual
         returns (bool);
+
+    /**
+     * @dev See {IERC1721-isValidSignature}
+     */
+    function isValidSignature(bytes calldata data, bytes calldata signature)
+        external
+        view
+        returns (bytes4 magicValue)
+    {
+        if (_isValidSignature(data, signature)) {
+            return bytes4(keccak256(bytes("isValidSignature(bytes,bytes)")));
+        }
+
+        return bytes4(0);
+    }
+
+    function _isValidSignature(bytes calldata data, bytes calldata signature)
+        internal
+        view
+        virtual
+        returns (bool);
 }