how to confirm ethers signatures onchain?

[grapevinegizmos]

It's fairly easy to get use ethers to get a metamask signature of a given piece of text, but how to confirm the signer onchain seems to be a bigger challenge than I thought). On a client side app with metamask installed I can call:

let provider =  new ethers.providers.Web3Provider(window.ethereum);
let signer = provider.getSigner();
let signature = await signer.signMessage("Hello world")

Metamask will pop the signing verification dialog, and return a signature.
You can pretty easily validate the address that produced the signature:

let signerAddress = ethers.utils.verifyMessage(ethers.utils.verifyMessage('Hello world', sig);

But if you want to send the signature to a smart contract, and confirm that a string message was signed by a particlar account, its not so easy.

The ECDSA package from OpenZeppelin has a recover() function but it only takes a hash as the "message', not a string, like "hello world". You can hash the string, but that doesn't return the right answer either:

bytes32 messageHash = keccak256(abi.encodePacked("Hello world");
address signer = ECDSA.recover(messageHash, signature);

Does not return the address of the signer.

Nor does

address signer = ECDSA.recover(ECDSA.toEthSignedMessageHash("Hello world"), signature);

So how can validate you validate on-chain that the signature of a message produced off-chain by ethers is in fact the expected signer address??

[grapevinegizmos]

Was able to determine that Metamask prepends the header ""\x19Ethereum Signed Message:\n(message length) to the message before it computes the hash. If you prepend the same header to the message before calling the ECDSA.recover it will return the right address.