[Ethereum] Deriving the v value from an ECDSA signature without web3j

1) This line is probably not doing what you expect:

Log.d(TAG, String.valueOf(rc.toString()));

rc has type RemoteCall<TransactionReceipt>, which means it needs to be executed. Instead, your code converts the invocation to a String (twice): once from the toString() and once from String.valueOf.

2) You create a future, then wait for it to complete, and I know you know this is pointless. I dispensed with the future, no idea why Android insists on a future. My simpler code gives the same result as your code, so at least we know that the future is not introducing the problem you report.

3) The error arises after this JSON-RPC payload is sent:

{"jsonrpc":"2.0","method":"eth_gasPrice","params":[],"id":1}

The ClientTransactionManager.sendTransaction() method dies when it attempts to execute:

return web3j.ethSendTransaction(transaction)
            .send();

Although web3j.ethSendTransaction(transaction) seems to work fine, the chained send() method returns an unsuccessful response. I suspect authentication is required, but of course no credentials were provided so the transaction was rejected.

I don't work in Android, so here is the code I worked with:

import org.web3j.protocol.Web3j;
import org.web3j.protocol.core.RemoteCall;
import org.web3j.protocol.core.methods.response.TransactionReceipt;
import org.web3j.protocol.http.HttpService;
import org.web3j.tx.ClientTransactionManager;
import org.web3j.utils.Convert;

import java.math.BigDecimal;

public class SO {
    private static final String FROM_ADDRESS = "0x6861B070f43842FC16eAD07854eE5D91B9D27C13";
    private static final String TO_ADDRESS = "0x31B98D14007bDEe637298086988A0bBd31184523";

    public static void main(String[] args) throws Exception {
        TransactionReceipt value = new SO().test();
        System.out.println(value);
    }

    private TransactionReceipt test() throws Exception {
        //Credentials credentials = obtainCredentials(WALLET_DIRECTORY);

        Web3j web3 = Web3j.build(
            new HttpService("https://rinkeby.infura.io/SxLC8uFzMPfzwnlXHqx9")
        );
        System.out.println("ethGasPrice=" + web3.ethGasPrice().getJsonrpc());

        ClientTransactionManager clientTransactionManager =
            new ClientTransactionManager(web3, FROM_ADDRESS);
        System.out.println("From address=" + clientTransactionManager.getFromAddress());

        org.web3j.tx.Transfer transfer = new org.web3j.tx.Transfer(web3, clientTransactionManager);
        System.out.println("transfer=" + String.valueOf(transfer.getSyncThreshold()));

        RemoteCall<TransactionReceipt> rc = transfer.sendFunds(
            TO_ADDRESS,
            BigDecimal.valueOf(1.0),
            Convert.Unit.ETHER
        );
        TransactionReceipt receipt = rc.send();
        System.out.println("receipt=" + receipt.toString());
        return receipt;
    }
}

So there were two problems with my code. Firstly, I read online about the need to append "\x19Ethereum Signed Message:\n32" to the hash in Solidity, however, this is only required when using Web3js not Web3j. Secondly, I was hashing in my smart contract when the data was not hashed originally.