– As customers ate lunch at a popular restaurant in Washington's Adams Morgan neighborhood, few had any inkling they were on the front lines of an international monetary war that's featured FBI raids, the Deep Web, allegations of money laundering and millions vanishing in a nanosecond.

Not to mention predictions of the demise of cash.

The Diner, as the Adams ­Morgan eatery is aptly named, recently installed Washington's first ATM for bitcoin, a controversial digital currency with fierce advocates and ferocious critics.

Bitcoin fans say it's a revolutionary way to move money quickly and anonymously from one point to another — whether around the corner or across international borders — with no middlemen, no fees, no central banks, little regulation, no collection of personal data and almost impenetrable computer security.

Former Republican Rep. Ron Paul said the bitcoin was a welcome newcomer in "the terrible monetary system" that holds sway in the United States.

"Anything that wants to substitute for the American dollar should be permitted," Paul told the Quora online-interview website last spring.

Because bitcoin is an encrypted "cryptocurrency," with each transaction tied to a unique 32-digit code of randomly generated numbers and letters, it can't be stolen from a gas pump the way thieves can use a simple device to lift credit card account numbers.

Six "core developers" — some of the world's best software engineers — oversee the bitcoin's international network, tied to 100,000 computers.

Helped by hundreds of anonymous "miners," they maintain a public "blockchain," a digital ledger that records every bitcoin transaction with a 32-digit code. Each public code, in turn, is linked to a 32-digit private code known only to the bitcoin owner. The miners use mathematical equations and algorithms that bounce around the globe instantaneously to verify the authenticity of each transaction.

"Before I got into bitcoin I thought it was impossible," said Jeff Garzik, an Atlanta-based software engineer who's one of the core developers. "They proved me wrong."

True to its enigmatic status, the bitcoin's origins are shrouded in mystery.

In 2008, someone using the alias Satoshi Nakamoto posted a white paper outlining the bitcoin in concept to an e-group of computer geeks. The following year, the anonymous genius uploaded open-source software to the Internet to implement it.

Garzik and the other core developers e-mailed with Nakamoto for two years as they expanded the digital currency that Nakamoto had invented and learned how to run it. Despite their skill at tracking all things digital, however, they never learned Nakamoto's real name, gender, nationality or location. Then, circa 2011, Nakamoto went silent.

"Basically, he signed off a few years ago," Garzik said.

Using the kind of understatement brilliant software engineers are prone to employ, Garzik added: "He was above average at protecting his ­anonymity. He didn't leave any data points."

By now, bitcoin has attracted an intense, if limited, following.

"I'll tell you why the bitcoin is one of the best things ever invented," said Andy Mahoney, an ATM scout who finds merchants willing to accept the machines. "For the first time, a person can send another person money anywhere on the planet instantaneously with virtually no costs. It's completely safe and secure and encrypted, which means you're sending your money anonymously."

Detractors counter that these purported strengths of the bitcoin, which is trading for about $298 apiece — it varies slightly on different exchanges — are really weaknesses.

"The underlying technology has great potential, but I'm skeptical of the raw, unregulated, anonymous form of bitcoin," said Mark Williams, a former Federal Reserve examiner who teaches finance at Boston University. "It avoids the controls that have been created by our banking system, and it's void of consumer protections. If your 'bitcoin wallet' gets picked digitally, there's no way to trace it back to the culprit. You've lost everything."

Hard-to-trace transactions

Most people who use a BTM — insider lingo for a bitcoin ATM — have an app-like "bitcoin wallet" downloaded to their smartphones, which has an embedded bar code that they hold up to the BTM screen during an exchange.

Bitcoins flow into or out of the virtual wallet depending on whether they're bought or sold. Ninety percent of transactions at the roughly 310 BTMs worldwide, with about 60 in the United States, involve exchanges of dollars for bitcoins.

There's thought to be about $4 billion worth of bitcoin in circulation, but only a portion of that is being used — partly because speculators are hoarding it, partly because few merchants accept it.

Most bitcoin transactions are direct person-to-person transfers. Want to send your college daughter some money? Open your phone's bitcoin wallet, enter the amount and punch in her wallet's unique code. She'll have the money moments after you hit send.

Williams and other critics, however, think the bitcoin is tailor-made for Internet pornography, gambling, money laundering and other illicit activities because it's hard to trace and operates outside long-standing fiscal institutions.

In one bust, on Oct. 21, 2013, FBI agents raided the San Francisco offices of Silk Road, a bitcoin-based online black market. They shut down its operations and arrested its proprietor, Ross William Ulbricht.

Under the pseudonym Dread Pirate Roberts, Ulbricht is accused of selling illegal drugs via a hidden Internet service called Tor on what's called the Deep Web, or Deepnet, because it's configured to elude standard search engines. His trial begins Jan. 13.

For Williams, the bitcoin's wildly fluctuating value — which spiked to nearly $1,200 a year ago but has dropped to about $298 — makes it much riskier than standard currency and even gold or stocks.

Williams acknowledges Andreessen, venture capitalist Tim Draper and other ­Silicon Valley heavyweights are investing heavily in bitcoin-related firms.

"You have a lot of smart money behind bitcoins, but smart money can make bad decisions," Williams said.