Mining bitcoins chews up a lot of computing power for no other purpose than to sustain the network. Wouldn’t it be useful to mine cryptocurrency in some other, more productive way?

Now a team of Serbian developers has tried to crack the problem with a proof-of-concept altcoin that lets you mine coins simply by exercising.

Mangocoinz was launched as a computer science project by three students at Belgrade's School of Computing, with a simple concept: instead of using computational work to generate coins, miners have to do some actual, physical work.

For example, repeatedly moving a smartphone (typically by walking or jogging) will result in 'proof of work', which the client software will turn into a portion of a coin.

While the coin is still in its very early days and problems remain to be ironed out, the concept may hold promise as a tool for institutions promoting fitness or as a way to lower health insurance premiums.

Mangocoinz is the latest idea in a long-running quest to make proof-of-work models that do more than simply crank out coins. Some have been frankly bizarre: bumbacoin, for example, bafflingly enables people to ‘mine’ coins by trolling newsgroups with Jamaican expletives.

Some attempts have stuck with computation, but have tried to make the computations scientifically useful. Primecoin solves prime numbers while mining coins, for example.

Peercoin, by the same developer, uses proof of stake, at least in part, which mines coins based on how many a person already owns, in an attempt to cut computing cycles and experiment with economics.

Another alternative, gridcoin, rewards people for useful research. Its users still crank out compute cycles, but they use them for crowdsourced scientific computing projects such as SETI@home. Gridcoins are tokens proving that their computers did that work.

All of these projects, even ridiculous ones like bumbacoin, have one thing in common: they involve some kind of effort on the miner’s part.

“There are many kinds of proofs-of-work. All have their trade-offs. We discover more new ideas/avenues every day,” said core bitcoin protocol developer Jeff Garzik. “The general idea is that copying digital data is trivial and frictionless. You must discover a method to make that process slower, more difficult.”

If a proof of work is simply about effort, then a physical proof should theoretically be possible. Running entails a measurable effort, which has a positive fitness effect.

There are problems with the idea, though. It would seem trivially easy to game this system so that you didn’t have to work at it – and the cost of attack is very low. Putting the phone on a washing machine during the spin cycle might do it. Theoretically, you could make yourself a mangocoinz millionaire and get a nice, fresh-smelling pair of socks at the same time, for a double win.

The trio’s answer for that involves a daily mining limit of 10 mangocoinz (MCZ) per day, verified using a centralized, cloud-based service.

“Coins only become valid when you sync them,” said the company’s founders, who declined to give their full names. “If you mine three coins a day and click sync, you make three coins valid and [can send them] to anyone you like, and you have seven coins left to mine in the day. Then it resets.”

With no rollovers, that places a limit on the profit that anyone could make per day. However, were mangocoinz ever to gain sufficient real-world value to make them interesting, people could still game the system.

Attackers could generate multiple accounts, using phones on washing-machines or hacked clients, and continue to mine coins to the limit. That would raise the mining difficulty, and penalize people who were trying to mine the coins legitimately by working out.

A technical solution to that problem would be difficult. Theoretically, the software could use the GPS to check for a change in geographical position while the coins are being mined, to check that the miner is really walking or running. But that would penalize those using a treadmill or exercise bike at home.

A better alternative might be to marry the accelerometer to the kind of heart monitor expected to be incorporated into wearable devices like Motorola’s Moto360 smart watch. Google’s Fit software development kit is already out in preview form, for developers to play with.

However, this brings us back to the same problem: all of these signals could be falsified using software hacks. And this in turn boils down to a fundamental design characteristic: mangocoinz mining and transactions are validated by a centrally-managed server. The system does not use a decentralized network to cryptographically verify the proof of work as bitcoin does.

“We made the system for honest people,” say the founders. Well, quite, but the Internet is full of scam artists and tricksters who will quickly subvert such a system at the cost of those honest users.

To take a nascent project like mangocoinz to the next level, it may be necessary to revamp its trust model.

Honesty and trust are difficult to codify, but it is possible, believes Jesse Heaslip, a board member at digital currency innovation and branding lab Humint. A big believer in the use of cryptocurrency for social good, Heaslip muses about reputation systems.

If the network could be seeded with trusted nodes – the “honest people” that mangocoinz’ founders describe – then perhaps they could be used to raise the reputation of other nodes that run with them, Heaslip suggested, adding:

“You’d have to build a group dynamic into it. So before you were trusted individually, you’d run in a group.”

He also suggested getting institutional support to launch a coin with potential health benefits to greater heights. “Who’s going to get the value from performing the exercises? You have to throw it to a government or health insurance provider,” he said.

Such an organization might be able to prove that exercise reduces overall healthcare costs by a certain amount.

“You can prove that people exercise that certain amount, which reduces your insurance costs. Thereby you should contribute this much to the project to ensure that it goes,” he concludes.

These concepts are far beyond where mangocoinz is today, but its founders say they have received inquiries from more than one technology accelerator, indicating that there is some interest. And as any good investor knows, product ideas are malleable: it’s the team that counts.

Even if mangocoinz needs further development, it’s an interesting taste of how proof of work may expand in innovative directions in the future. Perhaps we could call it ‘proof of sweat’.

Runner image via Shutterstock.