Solidity: extract data from signed message?

Question

In an effort to reduce gas costs for users, I'm looking for a way to delegate certain transactions to a contract owner so that the owner makes a transactions that would have been otherwise sent by the user. So instead of sending a transaction directly to the blockchain, the user could sign a message in MetaMask and send the message as an authorization to the server, which would then send a transaction to the blockchain, assuming other conditions are met.

However, to provide maximum assurance to users that only authorized transactions are processed/sent by the contract owner, I'd like to be able to unpack the signed message within the contract so that the following are verified:

1) The user's address

2) The transactions data

At a high level, the solidity function would look something like this.

function processDelegatedTransaction(
    address userAddress,
    bytes32 msgHash,
    uint8 v,
    bytes32 r,
    bytes32 s,
    uint16 param1,
    uint16 param2
) public requireOwner {

    require(userAddress == ecrecover(msgHash, v, r, s));

    // Do something else to verify that param1 and param2 are packed in msgHash
}

The require statement should take care of #1 above, but #2 is the issue.

The message signing process could encode multiple parameters into a single uint, which would be included in the hash and then, if I'm able to unpack the hash, decode them similarly to the method described here:

https://medium.com/@novablitz/storing-structs-is-costing-you-gas-774da988895e

So the only sticking point is being able to extract the params from the hash or recreate the hash from the params. Is this possible?

Answer 1

The typical solution here is to not accept a hash at all but just recreate it. Something like this:

function processDelegatedTransaction(
    address userAddress,
    uint8 v,
    bytes32 r,
    bytes32 s,
    uint16 param1,
    uint16 param2
) public requireOwner {
    bytes32 msgHash = keccak256(abi.encodePacked(param1, param2));
    require(userAddress == ecrecover(msgHash, v, r, s));
}

Depending on how you're producing the signature in the first place, you may need to do either EIP712 or prefixed (geth/Parity-style) messages. For the latter, see this example:

https://programtheblockchain.com/posts/2018/02/17/signing-and-verifying-messages-in-ethereum/