sign.js

// credits : https://gist.github.com/chafreaky/09296ff58b937056d362b69c2ef59a47

// Do `npm i elliptic sha256` before hand
import sha256 from 'sha256';
import pkg from 'elliptic';
const { ec } = pkg;

function seedHexToPrivateKey(seedHex) {
    const EC = new ec('secp256k1');
    return EC.keyFromPrivate(seedHex);
}

const uvarint64ToBuf = (uint) => {
    const result = [];

    while (uint >= 0x80) {
        result.push((uint & 0xff) | 0x80);
        uint >>>= 7;
    }

    result.push(uint | 0);

    return new Buffer.from(result);
};

function signTransaction(seedHex, transactionHex) {
    const privateKey = seedHexToPrivateKey(seedHex);

    const transactionBytes = new Buffer.from(transactionHex, 'hex');
    const transactionHash = new Buffer.from(sha256.x2(transactionBytes), 'hex');
    const signature = privateKey.sign(transactionHash);
    const signatureBytes = new Buffer.from(signature.toDER());
    const signatureLength = uvarint64ToBuf(signatureBytes.length);

    const signedTransactionBytes = Buffer.concat([
        transactionBytes.slice(0, -1),
        signatureLength,
        signatureBytes,
    ]);

    return signedTransactionBytes.toString('hex');
}

export default signTransaction;