Bitcoin—a pseudonymous cryptographic currency designed by an enigmatic, freedom-loving hacker, and currently used by the geek underground to buy and sell everything from servers to cellphone jammers. No, this isn't a cyberpunk artifact from Snow Crash or Neuromancer; it's a real currency currently valued several times higher than the US dollar, the British pound, and the Euro.

Bitcoin is a virtual currency, designed to allow people to buy and sell without centralized control by banks or governments, and it allows for pseudonymous transactions which aren't tied to a real identity. In keeping with the hacker ethos, Bitcoin has no need to trust any central authority; every aspect of the currency is confirmed and secured through the use of strong cryptography.

Over the last few months, Bitcoin's value has risen by an order of magnitude as the sagas of Wikileaks and Anonymous (among others) have highlighted the limits of a financial system which relies on centralized intermediaries. With a current estimated market capitalization of about $100 million, Bitcoin has recently graduated from a theoretical techno-anarchic project patronized by libertarians and hackers to a full-fledged currency prompting comment from technologists and economists. At the time of this writing, one Bitcoin (BTC) is worth about US$15.

So how does Bitcoin work? Is it really secure? And is it here to stay—or just another digital currency fad? Glad you asked.

Complexities of cryptographic currencies

The problem with purely digital currencies is that of double-spending. Economists in the audience will note that digital products like a movie or a text file are non-rivalrous. If you have a copy of my pseudo-trip-rock band's new MP3 album, there's still just as much MP3 to go around for everyone else who wants one. That's not a problem for files, but it is a problem with currency, since the whole point is that there's a limited supply. If you use a dollar at the grocery store today, you can't go out and spend that same dollar at a bar tomorrow.

The usual solution to the double-spending problem is a trusted intermediary. PayPal makes sure that you can't spend the same dollars twice by deducting them from your account before they get added to someone else's account. Visa, MasterCard, and every other bank and payment processor do the same. However, this centralized approach is the one that enigmatic creator Satoshi Nakamoto specifically tried to avoid in the original Bitcoin design. The idea was to use cryptography to create verifiable transaction records without the need to trust anyone but your own calculations.

The Bitcoin solution uses cryptography and an open transaction register. Whenever you spend a Bitcoin, you cryptographically sign a statement saying that you have transferred the coin to a new owner and you identify the new owner by their public crypto key. Whenever they need to spend the coin, the new owner uses his private key to sign it over to some further owner. As soon as a transaction takes place, the recipient (who has a very strong incentive to ensure that you don't spend the coin twice) publishes the transaction to the global Bitcoin network. Now every Bitcoin user has incontrovertible evidence that the coin has been spent, and users won't accept that coin from anyone but the new owner.

Mining and make-work

As a digital currency, Bitcoin suffers from a tangibility problem. Unlike other currencies traded online, you can't go to a bank and withdraw physical coins, so what are they? More importantly, where do they come from? Coins are essentially agreements between all the Bitcoin nodes to accept a particular coin as currency. They are created gradually according to a precise protocol in order to reward those who contribute and maintain the network, control the rate of creation of the currency, and maintain the integrity of the transaction list.

In a process known as mining, individual Bitcoin users attempt to generate new coins by checking the integrity of the transactions list. They confirm the previous transactions and attempt to solve a difficult proof-of-work problem which involves exhaustively trying different solutions. There are a very large number of such potential solutions, so the likelihood of finding the solution depends how many other people are looking for it and how much computing power you devote to the problem. The first client to find the solution announces its good fortune to the whole network and earns a little reward for itself in the form of some shiny new Bitcoins.

By finding the newest solution to the proof-of-work problem, a Bitcoin client confirms the history of previous transactions and moved the transaction register forward, allowing new debits and credits to form part of the next block that can be mined to earn more coins. Future coins can't be mined in advance, because the computation to find the new block (and hence create new Bitcoins) relies on the the chain of previous blocks and the history of transactions since the most recent block.

The number of new coins generated per block gradually decreases over time. It started out at 50 BTC, but will dwindle to zero sometime in future when all 21 million coins have been generated. Fortunately, coins can be divided down to the eighth decimal place, which may prove increasingly useful if their value grows.

What's a few coins between friends?

One of the difficulties with a novel currency like Bitcoin is adoption and valuation. The same was true when the greenback paper dollar was first introduced, and it's a real problem with any means of exchange. After all, a currency is little more than something useless but rare which everyone agrees to trade for useful things, whether apples or assault rifles. National currencies have the advantage that governments demand them in taxes and require them to be accepted, which provides both a particular market and a high rate of adoption.

So, why would anyone exchange their hard-won dollars for Bitcoins, or accept Bitcoins in exchange for real products like a carton of milk or a subway ride? As a currency, Bitcoin has a number of desirable features which are not found together in any other currency. Cash has features like anonymity and eminent portability, but also comes with the downside that you have to physically move it from place to place to use it. Credit cards and other trust-based electronic currencies can be used instantly over any distance, but you have to attach your real identity to the purchase.

Bitcoins combine the advantages of the two methods. Using Bitcoins, I can buy a racy t-shirt from Tibet and computer time from China without either merchant knowing who I am, or my bank knowing what I bought. This is useful not just for those purchasing questionable items (the downside of anonymous currency flows), but also for those who don't want merchants, banks, or card companies to be able to build up detailed profiles of their life, likes, and habits.

Since they're useful, some people want to use Bitcoins. Since some people want to use them, merchants have an incentive to accept them in order to attract the business of those customers.

This simplified economic model is not uncontested. Ars tech policy contributor Tim Lee has publicly criticized Bitcoin's economic model, both from the point of view of external market forces and over the internal incentive structures inherent to the protocol. Tech and economic policy commentator Jerry Brito provides a counterpoint, emphasizing Bitcoin's decentralizaion, which makes it very hard to control, but concedes that it is very hard to distinguish between a currency bubble and currency value.

Bitcoin's anonymity has already attracted Congressional attention. Sen. Chuck Schumer (D-NY) this weekend blasted Silk Road, an online drugs outlet that allegedly relies on TOR to obfuscate Internet traffic and Bitcoins for payment. "It's an online form of money laundering used to disguise the source of money, and to disguise who's both selling and buying the drug," Schumer said.