The currency known as bitcoin — a much-hyped and much-doubted type of digital cash that can be bought with traditional money — has mostly attracted attention for its popularity in the black market, and for its wildly gyrating valuation.

But some entrepreneurs, investors and even merchants are eyeing a far more mainstream use for it. They are convinced that bitcoin, though not widely understood, offers a path to lower payment processing and more secure transactions. Instead of using bitcoin to buy illegal guns in the recesses of the web, they say, ordinary consumers will use it to buy legal goods from legal retailers — and as easily as they now swipe their credit cards or exchange paper bills.

“I’m confident you will see major worldwide retailers adopting systems built on bitcoin,” said Jim Breyer, the Silicon Valley venture capitalist and early Facebook investor who also served on the board of Walmart Stores for more than a decade.

Mr. Breyer is an investor in Circle Internet Financial, one of the host of start-ups trying to find a way to make bitcoin a widely adopted currency for retail payments. The company was started by Jeremy Allaire, a serial entrepreneur, and it aims to be a payment processing system for online and physical merchants, similar to the service PayPal offers online. Along with his venture firm, Accel Partners, and another called General Catalyst Partners, Mr. Breyer has invested $9 million in the company.