SignUtil.java

import org.web3j.crypto.Keys;
import org.web3j.crypto.Sign;
import org.web3j.utils.Numeric;

import java.security.SignatureException;

public class SignUtil {

    private static final String GETH_SIGN_PREFIX = "\u0019Ethereum Signed Message:\n32";

    /**
     * This method is expecting the signed message to be a hash of the original message. The length of the message is
     * then hardcoded to 32. Also, this might only work for messages signed by geth, not sure if other clients
     * add the prefix to the signed message.
     * @param signedHash
     * @param originalMessageHashInHex
     * @return
     * @throws SignatureException
     */
    public static String getAddressUsedToSignHashedMessage(String signedHash, String originalMessageHashInHex) throws SignatureException {
        byte[] messageHashBytes = Numeric.hexStringToByteArray(originalMessageHashInHex);
        String r = signedHash.substring(0, 66);
        String s = "0x"+signedHash.substring(66, 130);
        String v = "0x"+signedHash.substring(130, 132);
        System.out.println();
        byte[] msgBytes = new byte[GETH_SIGN_PREFIX.getBytes().length + messageHashBytes.length];
        byte[] prefixBytes = GETH_SIGN_PREFIX.getBytes();
        System.arraycopy(prefixBytes, 0, msgBytes, 0, prefixBytes.length);
        System.arraycopy(messageHashBytes, 0, msgBytes, prefixBytes.length, messageHashBytes.length);
        String pubkey = Sign.signedMessageToKey(msgBytes,
                new Sign.SignatureData(Numeric.hexStringToByteArray(v)[0],
                        Numeric.hexStringToByteArray(r),
                        Numeric.hexStringToByteArray(s)))
                .toString(16);
        System.out.println("");
        System.out.println("Pubkey: " + pubkey);
        String address = Keys.getAddress(pubkey);
        return address;
    }
}