Bitcoin Signature Verification

by shubham saxena   Last Updated September 09, 2019 13:27 PM

I am trying to implement the OP_CHECKSIG opcode of Bitcoin and verify whether a signature is valid or not. I have the signature that I recieve from the transaction data and the public key(the hex field of a P2PKH transaction should contain both). I am trying to figure out what is the message digest itself that is signed. From what I know these are the required items -

1.The version number(4 bytes)
2.The number of Inputs(1 byte)
3.Transaction hash of the input being redeemed
4.Index of the output being redeemed (4 bytes)
5.ScriptPubKey of the output being redeemed prepended with the length of the script
6.Sequence Number(4 bytes)
7.No of outputs we are signing over(1 byte)
8.Total value of the output(8 bytes)
9.ScriptSig for the output prepended with the length
10.locktime field(4 bytes)
11.hashcode type(4 bytes)

So If I hash this data in byte format twice using the SHA256 Algorithm would my signature verification pass? I am trying to implement this using Java and this is my code -

public static boolean verifyUsingSecp256k1(byte[] pub, byte[] dataForSigning,
    BigInteger[] rs) throws Exception {
ECDSASigner signer = new ECDSASigner();
X9ECParameters params = SECNamedCurves.getByName("secp256k1");
ECDomainParameters ecParams = new ECDomainParameters(params.getCurve(),
        params.getG(), params.getN(), params.getH());
ECPublicKeyParameters pubKeyParams = new ECPublicKeyParameters(ecParams
        .getCurve().decodePoint(pub), ecParams);
signer.init(false, pubKeyParams);

return signer.verifySignature(dataForSigning, rs[0].abs(), rs[1].abs());}

However when I try to verify my Signature it returns as false. Am I missing something over here?

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