Unable to recover signer address using api/v1/safes/{address}/multisig-transactions

Question

I am trying to sign a valid transaction hash (obtained from the onchain Gnosis Safe wallet itself) but when I sign the hash and then provide it to the multisig-transactions endpoint to propose the tx, the signing address that is recovered from the signature I provide does not match the address that I actually used to sign.

*note I unfortunately cannot use the SDK in this scenario

The signature is wrong because of some reason like ethersJS is prepending 'x19 Ethereum Signed Message' to the hash or because I am signing an ethers.utils.arrayify (ie bytes) version of the hexstring hash. I have tried all permutations I can think of to fix the invalid signature but I keep receiving response 422: "Signer=0xC2278C77001F5c5164ed97Ff945f6B0E6e006363 is not an owner or delegate. Current owners=[${owners}] The code in question:

const getAndSignTxHash = () => {
  // get txhash from contract
  const safe = new ethers.Contract(state.safeAddress, abiJson, signer);

  // obtain txHash
  const txHash = safe
    .getTransactionHash(
      state.recipient,
      state.value,
      "0x", //state.data,
      state.operation,
      state.safeTxGas,
      state.baseGas,
      state.gasPrice,
      state.gasToken,
      state.refundReceiver,
      state.nonce
    )
    .then((res) => {
      State.update({ txHash: res });
      // sign contractTransactionHash using private key of Gnosis Safe owner (or deployer)
      const signature = signer
        .signMessage(ethers.utils.arrayify(res))
        .then((sig) => {
          State.update({ signature: sig });
          console.log(sig);
        });
    });
};

const postToSafeApi = () => {
  // craft transaction from state vars
  const transaction = {
    safe: state.safeAddress,
    to: state.recipient,
    value: state.value,
    data: "0x", //state.data,
    operation: state.operation,
    gasToken: state.gasToken,
    safeTxGas: state.safeTxGas,
    baseGas: state.baseGas,
    gasPrice: state.gasPrice,
    refundReceiver: state.refundReceiver,
    nonce: state.nonce, 
    contractTransactionHash: state.txHash,
    sender: state.sender,
    signature: state.signature, 
    origin: state.origin,
  };

  const transactionsUrl = state.baseUrl + `multisig-transactions/`;
  const params = JSON.stringify(transaction);
  const proposalOptions = {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    mode: "no-cors",
    body: params,
  };

  // post to gnosis API backend
  const proposed = asyncFetch(transactionsUrl, proposalOptions).then((res) =>
    console.log(res)
  );
};

If there is a better place to ask this question, please let me know since I have been stuck for a long time on this

Answer 1

Constant part per signature: {(max) 64-bytes signature data}{1-byte signature type}

eth_sign signature signature type > 30 To be able to use eth_sign we need to take the parameters r, s and v from calling eth_sign and set v = v + 4