In a case testing the government’s ability to unmask individuals who do business in cryptocurrency, San Francisco digital currency company Coinbase Inc. is challenging an IRS request for information about customers.

Coinbase, which claims to have more than 5 million users worldwide, provides “wallets” that allow consumers and merchants to conduct transactions in currencies such as bitcoin and ethereum. Tax enforcers want to sift through reams of Coinbase data to look for unreported income belonging to the company’s U.S. account holders.