Square Inc. /zigman2/quotes/205989440/composite SQ -2.13% is contemplating building a system for bitcoin /zigman2/quotes/31322028/realtime BTCUSD +0.29% mining, according to Chief Executive Jack Dorsey. The system would be open source and "based on custom silicon," Dorsey announced in a late Friday tweet. He argued that bitcoin mining needs to become more distributed, efficient, and accessible. "Bitcoin mining should be as easy as plugging a rig into a power source," he wrote. "There isn't enough incentive today for individuals to overcome the complexity of running a miner for themselves." Dorsey has long been interested in bitcoin and looked for tie-ins with Square's business. He introduced the ability for users of Square's Cash App mobile wallet to buy and sell bitcoin a few years back and announced in July that the company was working on an open-developer platform mainly focused on bitcoin. Square shares are off 0.6% in premarket trading Monday. They've gained 14% so far this year as the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.90% has risen 19%.