MILTON FRIEDMAN famously called for the abolition of the Federal Reserve, which he thought ought to be replaced by an automated system which would increase the money supply at a steady, predetermined rate. This, he argued, would put a lid on inflation, setting spending and investment decisions on a surer footing. Now, Friedman's dream has finally been realised—albeit not by a real-world central bank.

Bitcoin, the world's "first decentralised digital currency", was devised in 2009 by programmer Satoshi Nakomoto (thought not to be his—or her—real name). Unlike other virtual monies—like Second Life's Linden dollars, for instance—it does not have a central clearing house run by a single company or organisation. Nor is it pegged to any real-world currency, which it resembles in that it can be used to purchase real-world goods and services, not just virtual ones. However, rather than rely on a central monetary authority to monitor, verify and approve transactions, and manage the money supply, Bitcoin is underwritten by a peer-to-peer network akin to file-sharing services like BitTorrent.

The easiest way to store Bitcoins is to sign up to an online wallet service through which all transactions are carried out. This, of course, means trusting the provider of that service not to cheat, or go out of business, taking clients' savings with it. Warier users can install a personal digital wallet on their own computers. They must then, however, keep it safe from viruses or physical damage. If a laptop went up in smoke, so would the virtual coins stored on its hard drive. (Keeping back-up copies would do the trick.)

All transactions are secured using public-key encryption, a technique which underpins many online dealings. It works by generating two mathematically related keys in such a way that the encrypting key cannot be used to decrypt a message and vice versa. One of these, the private key, is retained by a single individual. The other key is made public. In the case of Bitcoin transactions, the intended recipient's public key is used to encode payments, which can then only be retrieved with the help of the associated private key. The payer, meanwhile, uses his own private key to approve any transfers to a recipient's account.

This provides a degree of security against theft. But it does not prevent an owner of Bitcoins from spending his Bitcoins twice—the virtual analogue of counterfeiting. In a centralised system, this is done by clearing all transactions through a single database. A transaction in which the same user tries to spend the same money a second time (without having first got it back through another transaction) can then be rejected as invalid.

The whole premise of Bitcoin is to do away with a centralised system. But tracking transactions in a sprawling, dispersed network is tricky. Indeed, many software developers long thought it was impossible. It is the problem that plagued earlier attempts to establish virtual currencies; the only way to prevent double spending was to create a central authority. And if that is needed, people might as well stick with the government devil they know.

To get around this problem, Bitcoins do not resemble banknotes with unique serial numbers. There are no virtual banknote files with an immutable digital identity flitting around the system. Instead, there is a list of all transactions approved to date. These transactions come in two varieties. In some, currency is created; in others, nominal amounts of currency are transferred between parties.

In the very first transaction the creator's computer forged 50 units of the currency. The next transaction would have involved subtracting some amount from the creator's account and crediting it to a recipient's. These actions, and any subsequent ones, were automatically broadcast to the entire network. At first, when the network was small and transactions few and far between, verifying them was been straightforward. The first person to confirm the new transactions would offer his updated log as the one against which any future transactions ought to be judged. Once everyone else agreed that this candidate register was indeed accurate, it would be adopted and the new transactions included in it confirmed. If anyone tried to game the system by erasing an old transaction (so he could re-use the same money again) or adding an unwarranted new one (transferring the same money as before, say), he would be promptly found out, his proposed log discarded, and the transactions rejected as invalid.

However, as the network expands from dozens of users to thousands, and transaction volume grows, so does the number of logs vying for the official crown. Getting everybody to scrutinise the first proposal aired across the network for inconsistencies soon becomes impractical; the whole system grinds to a halt. Some way is therefore needed to ensure that the official register can be updated and agreed on in real time (or nearly), while preventing individuals from tampering with it. Mr (or Ms) Nakomoto's ingenious solution involves two related cryptographic techniques: hashing and forced work.

A hashing algorithm converts a message into a number called a hash value, or a digest. If this number is big enough, it provides a unique representation of the original (since the same algorithm could not conceivably yield identical hash values for different messages). Moreover, it is impossible to reconstruct the original on the basis of the digest alone. Nor is it possible to predict what the digest would be for even a slightly tweaked version of the original message; fiddling with a single letter will produce a completely different digest. In that regard, digests appear to be generated at random. As a result, hashing is what computer scientists call an irreversible process.

Consider a hashing algorithm which converts anything fed into it to a whole number between one and 1,000. For random sets of data, the algorithm would spit out a value below 11, say, once in every 100 tries, on average. Now suppose some data are given in advance. How does one find a number that needs to be appended to these given data to produce a hash value below 11? Because hashing is irreversible, and digests are essentially random, the only way to do this is through trial and error: by splicing different numbers onto the old data and hashing the whole lot until the desired result pops out. On average, this will require 100 tries. However, once the answer is found, everyone else can verify whether the problem has indeed been solved by running the hashing algorithm just once, with the proposed solution. This type of puzzle can only be cracked using brute force, which is why it is dubbed forced work.

With Bitcoin, all new transactions are automatically broadcast across the entire network and analysed in portions, called blocks. Besides any new as-yet-unconfirmed transactions, each block contains the digest for the last block to have got the nod from the network. That last block will always come from tip of the longest chain of blocks currently on the network. This chain is, in effect, the official log—confirmation that all the previous blocks tot up.

For a new block to be deemed valid, some computer on the network must create a transaction log for it that dovetails with the previous blocks. To prevent acceptance of bogus logs, giving it a seal of approval has to be prohibitively costly to any individual user, but relatively cheap for the network as a whole. This is done by making it into a forced-work task, which involves using the valid blocks and the new transactions to generate a digest consisting of 256 bits (ie, any number between 0 and 2256). The task is complete when the system's algorithm spits out a hash value below a preset target (like 11 in the example above). The target is set so that the puzzle is solved by someone on the network, and a new block approved, every 10 minutes. To keep this rate constant as the network's ranks swell and its combined computing power grows, the target is lowered in order to make generating a value below it harder. (Conversely, if the network were to shrink, it would get easier again.)

Creating the doctored block and having it validated and attached to the official log would thus require outpacing the network's combined computing power. This can only happen if a fraudster controls more than half of the network's total number-crunching capacity, which is possible, but extremely expensive for any one person.

The system can thus rely on users to police it. As a reward for giving up some computing power to that end, the first user to crack the forced-work task gets 50 coins for the effort. This is done by always making the first new transaction in each block the conjuring up of 50 coins out of nothing. When other participants agree to append the new block to the official chain, they also validate the creation of the new money (they would, of course, reject it if someone tried to game the system by minting more than 50 coins).

This is also how Bitcoin niftily gets around the problem of increasing the money supply without a central mint. Since blocks are created at a constant average rate, and there is a set number of coins minted per block, the total money supply, too, increases at a steady clip. For now, this is 300 coins every hour on average. Every four years, though, the minting rate is set to fall by a half. It will drop to 25 coins per block in 2013, to 12.5 coins in 2017, and so on, until the total supply plateaus at 21m or so around 2030.

The idea is to mimic the extraction of minerals (the transaction-validating software is called the Bitcoin miner). As the most readily accessible resources are exhausted, the supply dwindles. Unlike real resources, however, there is no as-yet-undiscovered, hidden lode a fortunate prospector can strike to disrupt the money supply. Should a powerful new computer be introduced to the network, the difficulty of the forced-work challenge would soar, keeping the rate at which blocks are approved—and new money created—unchanged.

In theory, then, the system ought to keep a lid on inflation—making it attractive to critics of interventionist monetary policy of the sort practised since 2008 by America's Federal Reserve under the label quantitative easing. (The mineral analogy, in particular, appeals to proponents of a return to a gold standard.) It offers other apparent benefits, too. The currency can be used by anyone (unlike credit cards, for instance), anywhere. Transaction costs are also likely to be lower than those for traditional payment systems, though these are not in fact zero. Some are reflected in the hardware and energy used to police the system. Some surely creep in whenever those who have no wish to mine Bitcoins themselves purchase them for dollars, euros and several other currencies at specialised sites like Mt. Gox.

Legally, Bitcoin exchanges are subject to the same regulations as ones trading commodities. For example, an exchange must report any transaction above $15,000, a policy meant to stem money laundering. For the purposes of taxation, meanwhile, reimbursing somebody for a product or service in BitCoins is treated as barter. The tax code makes provisions for such practices, though, admittedly, they can be tough to enforce.

This has not stopped some American politicians from expressing grave concern about the virtual currency. Charles Schumer, a prominent Democratic senator, has inveighed against it, claiming it is just what drug dealers have been waiting for. All the clever cryptography means Bitcoin dealings are difficult to trace. But not impossible. According to Bitcoin's defenders, its users may be more difficult for a government agency to pinpoint than someone paying with a credit card. But they are easier to catch than those using cash. Moreover, any drug trade involves sending physical products to recipients. Authorities already track many packages sent by groups under investigation. When it comes to physical delivery, the method of payment is irrelevant. Another worry, for the authorities at least, is that, in theory, a Bitcoin account cannot be frozen. But, like cash, Bitcoins can be nabbed by seizing the computer on which they are stored.

Ordinary folk, meanwhile, have different concerns. They fear being bilked by a cabal of clever boffins, who can insidiously fiddle with the system's software to take advantage of less geeky types. This queasiness, though understandable, may be misplaced. As an open-source project, the computer code which undergirds Bitcoin can be viewed, and modified, by anyone. As with all such ventures, however, if a change is introduced that most participants do not accept, they will simply refuse to download that version of the software. Since the self-professed geeks who make up the web's open-source communities often delight in (and excel at) scrutinising seemingly impenetrable lines of computer language, it is highly unlikely that someone could get away with surreptitiously inserting a command to create excess Bitcoins and siphon them off to his account, for instance. For the same reason, the open-source nature of the project is also a bulwark against hackers or malware. Indeed, as cybercrime goes, Bitcoin may be safer than traditional financial institutions, which are often on the receiving end of such attacks.

And then there are the currency's economics (discussed in more detail in this week's print edition). These have engendered a surprisingly lively debate. One particular bone of contention is whether it makes sense to decrease the rate of money creation with time. Some people think this will entail disastrous deflation if the demand for Bitcoins grows at a faster rate than new coins are minted. As recent wild swings in their dollar price amply demonstrated, they are not the most predictable of vehicles. The volatility is largely down to the fact that the currency remains illiquid—only 6.5m currency units (divisible to eight decimal places) are currently in circulation among some 10,000 users (including several hundred merchants who accept payment in Bitcoins). This seems unlikely to change in the foreseeable future, as even Bitcoin's most ardent supporters admit. That is not because people are queasy about intangibles. After all, much of modern pecuniary activity already involves bits rather than bob and consumers have embraced credit cards, electronic transfers and the like.

The difference is that established fiat currencies—ones where the bills and coins, or their digital versions, get their value by dint of regulation or law—are underwritten by the state which is, in principle at least, answerable to its citizens. Bitcoin, on the other hand, is a community currency. It requires self-policing on the part of its users. To some, this is a feature, not a bug. But, in the grand scheme of things, the necessary open-source engagement remains a niche pursuit. Most people would rather devolve this sort of responsibility to the authorities. Until this mindset changes, Bitcoin will be no rival to real-world dosh.

Update June 15th: A Bitcoin user claims to have suffered a substantial theft of Bitcoins, worth nearly $500,000.