I’m pretty bullish on Bitcoin so a few months ago I set out to build a “pure” Bitcoin related application. Specifically, I was looking to build an application that leveraged the Bitcoin network directly, without using any third party APIs or services. The goal behind avoiding third party services was to explore how difficult using the Bitcoin network directly is and also to embrace Bitcoin’s decentralized nature and not rely on another company to move coins.

Conceptually the way the Bitcoin network works is relatively straightforward. You move coins by creating transactions which are just messages written and cryptographically signed in a specific format and then you listen for transactions which include your addresses to keep your balance up to date. Of course, the devil is in the details and there’s a dauntingly large number of them. For example, Ken Shiriff explains how to craft a transaction by hand in Bitcoins the hard way: Using the raw Bitcoin protocol and it’s no easy read. Given that just crafting transactions involved so much code, I started researching existing open source libraries that facilitate working with Bitcoin.

After doing some research, it looked like the most popular approach to interfacing with the network directly was to run the bitcoind deamon and then make RPC calls to the exposed functions. Objectively, using RPC calls to bitcoind qualifies as a “pure” solution but I still didn’t love it. After a bit more searching, I came across bitcoinj which is a pure Java library for working with Bitcoin.

Unlike bitcoind, bitcoinj is a library so its designed to be embedded in other codebases and it supports simplified payment verification (SPV) which allows it to operate without downloading the entire blockchain, ~25GB as of today. On top of this, its written in Java so it’s easy to use from Scala, something I’d been looking to experiment with.

So what are some key takeaways from using bitcoinj?

• Test on the TestNet: Although debugging new code with live money would be thrilling. the TestNet allows you to develop against the Bitcoin network without using “live” coins. With bitcoinj, if you instantiate TestNet3Params you’ll connect into the Testnet.
• TestNet Fountains: Testing is pretty tough without some coins but luckily you can use fountains to grab some. http://faucet.xeno-genesis.com/ worked well for me to grab some coins. Just a heads up, TestNet transactions take awhile to get broadcast and included into the Blockchain so be patient.
• Extend WalletAppKit: WalletAppKit is a utility class which helps bootstrap everything you need for a SPV wallet and will save you some headaches. I started off trying to hand roll everything and quickly switched over to extending WalletAppKit.
• Chain API: Since the Chain API can be used on the TestNet it’s useful for testing your own applications. Specifically, Bitcoinj treats sending and receiving coins to “yourself” weirdly so it’s more effective to use Chain to test transactions.
• Coin.parseCoin: Sorting out how many Satoshis you want to send is surprisingly difficult since sendCoins accepts a Coin object. Luckily, Coin.parseCoin will accept a BTC value and return representative Coin object.
• Blockchain downloads and wallet start up slow down development: Even in SPV mode, it still takes a non-trivial amount of time to “catch up” if your client hasn’t been running for awhile. On top of that, starting up the wallet takes a few minutes since it has to discover and connect to beers. What this translates to is resuming development after being away for a while is slow and the “edit, compile, test” loop isn’t as tight as usual.
• It’s real money: This should go without saying but once you’re running on the live Bitcoin network you’re playing with real money so be careful. If your keys get out anyone will be able to move your coins out of your addresses. So be careful what you check in on git and if you’re building something public definitely take a second to lock down your server.

Anyway, this was my first time building something Bitcoin related and it was a positive experience. The project is still private but I’ll definitely share it once it’s released. As always, questions or comments are welcome!

Posted In: Bitcoin

Tags: bitcoin, bitcoinj, open source, spv wallet