Creating Ethereum contracts and verifying messages with node.js

by mjbecze 14-Jun-2014

In this article I'll be covering ethereum transaction creation and verifying transactions. First of all; whats the rational for creating transaction with with javascript? Well I, as always just want to explore and play around with ethereum. But I imagine this would be usefull in testing compilers and even more usefull, create contracts programmatically. So lets get started! :D

First of all we will be using ethereumjs-lib in this post. You can install it from npm npm install ethereumjs-lib or directly from git npm install git+

Creating a transaction

After that let create a new file and put the following in it. For the those how are cut and paste handicap here is the full example.

var Ethereum = require("ethereum-lib");
var Transaction = Ethereum.Transaction;

//create a blank transaction
var tx = new Transaction();

// So now we have created a blank transaction but Its not quiet valid yet. We
// need to add some things to it. Lets start with 
tx.nonce = 0;
tx.gasPrice = 100;
tx.gasLimit = 1000; = 0; //Sending to 0, therefore we are creating a contract
tx.value = 0; = "7f4e616d65526567000000000000000000000000000000000000000000000000003057307f4e616d6552656700000000000000000000000000000000000000000000000000573360455760415160566000396000f20036602259604556330e0f600f5933ff33560f601e5960003356576000335700604158600035560f602b590033560f60365960003356573360003557600035335700";

Hopefull this is pretty self explanatory. nonce is the number of transaction that the sending account has sent. gasPrice is the amount you are will to pay for gas. gasLimit if the amount of gas you are will to spend. to is the address we are sending to, in this case the zero address which creates a new contract. If we where to put a valid address here this account would become a message instead of a contract. To check which type of transaction you have, you can do

tx.type === "message"
tx.type === "contract"
//type updates every time you change

value is the is the amount you are sending and data is data that you are sending in this case it is initialization code for a contract. The data is a Name Registrar contract. You can use one of the compilers to generate the code. The two main ones are lllc which is compiled with cpp-ethereum and serpent. Ok now we have a transaction with data. Now we need to sign it. To do this we will need a private key. There are several way to acquire one but for now we are just going to steal one from AlethZero. That way I know that I'm create valid transaction that actually has the ether that this transaction is says that it has. If you have AlethZero running. You can select tools>export selected key, and the copy the private key out. Here is mine.

exporting private key

var privateKey = new Buffer("e331b6d69882b4cb4ea581d88e0b604039a3de5967688d3dcffdd2270c0fd109");

Now we have a signed transaction, but for it to be valid, the account that we signed it with needs to have a certain amount of wei in to. To see how much that this account needs we can use tx.getUpFrontCost

console.log("Total Amount of wei needed:" + tx.getUpFrontCost());

this gives the amount in wei that the account needs to have. If your wondering how this is caculated it is

  • data lenght in bytes * 5
  • + 500 Default transaction fee
  • + gasLimit * gasPrice

Lets serialize the transaction

console.log("---Serialized TX----");
console.log(tx.serialize().toString("hex")); //serialize returns a buffer

Now that we have the serialized transaction we can get AlethZero to except it by selecting debug>inject transaction and pasting in the transaction serialization. It should then show up in the pending transaction pane.

inject transaction

Parsing & Validating transactions

If you have a transaction that you want to verify you can parse it. If you got it directly from the network it will be rlp encoded. You can decode it using the rlp module. After decoding it that you should have something like this.

var rawTx =  [

var tx = new Transaction(rawTx);

Note: rlp.decode will actully produce an array of buffers new Transaction will take either and array of buffers or and array of hex strings. So assuming that you were able to parse the transaction, we will now get the sender's address

console.log("Senders Address" + tx.getSenderAddress());

Cool now we know who sent the tx! Lets verify the signature to make sure it not some poser.

    console.log("Signature Checks out!");

And hopefull its verified. For the transaction to be total valid we would also need to check the account of the sender and see if they have at least tx.getUpFrontCost().

And that is all for now I hope you have enjoyed the read. You can view the documentation for transaction here to see all the exciting things it can do. Let me know if you have any questions or comments.