How to sign a transaction automatically?

Question

An api will call the smart contract function. It should sign the transaction on its own instead of human intervention.

I think raw transaction can work here. Are there any other options?

Any help is much appreciated. Thanks!

--Edit-- The question is updated!

It should sign the transaction on its own - the API or the contract? (I don't see how the contract could possibly do that, but you just wrote it in a comment above). — goodvibration, Sep 14 '18 at 11:13
I know contract can't sign the transaction on its own. I found my answer it should be done by raw transaction. — Shubham Chadokar, Sep 14 '18 at 15:24

goodvibration · Answer 1 · 2018-09-14T11:03:49.100

Try this (using web3.js v1):

let Web3 = require("web3");
let web3 = new Web3(NODE_ADDRESS);

async function send(transaction, reestimate = function(gas) {return gas;}) {
    let gas = await transaction.estimateGas({from: PUBLIC_KEY});
    let options = {
        to  : transaction._parent._address,
        data: transaction.encodeABI(),
        gas : reestimate(gas)
    };
    let signedTransaction = await web3.eth.accounts.signTransaction(options, PRIVATE_KEY);
    return await web3.eth.sendSignedTransaction(signedTransaction.rawTransaction);
}

Usage example #1:

let receipt = await send(myContract.methods.myFunc(arg1, arg2, arg3));
...

Usage example #2:

let reestimate = function(gas) {return gas * 2;};
let receipt = await send(myContract.methods.myFunc(arg1, arg2, arg3), reestimate);
...

score 0 · Answer 2 · answered Sep 14 '18 at 10:48

0

A contract can not sign transaction by its own. The transaction must be signed by externally owned accounts. If a transaction is not signed by an externally owned account then it won't be included in the blockchain.

answered Sep 14 '18 at 10:48

Soham Lawar

2,567
14
38

This is why I'm pretty sure that this user doesn't really mean to ask how the contract can sign its own transaction, but how the (external) API should sign the transaction (even though he/she stated the opposite in response to your comment). An evidence to that is the fact that the question is tagged under web3.js. In addition, there's no sense in asking how to sign a transaction from the on-chain, because signing a transaction is performed from the off-chain by definition. – goodvibration Sep 14 '18 at 11:40
@goodvibration I agree with you. Even I was confused after reading the question and asked for more clarification. – Soham Lawar Sep 14 '18 at 12:51

How to sign a transaction automatically?

2 Answers2