Bitcoin exchange Bitfinex this week was forced to halt trading after discovering a security breach.

"We are investigating the breach to determine what happened, but we know that some of our users have had their bitcoins stolen," the company wrote. "We are undertaking a review to determine which users have been affected by the breach. While we conduct this initial investigation and secure our environment, bitfinex.com will be taken down and the maintenance page will be left up."

The service also halted all digital token deposits to and withdrawals from Bitfinex.

Bitfinex didn't reveal how much was stolen, but according to Bloomberg, hackers nabbed $65 million. Not surprisingly, the exchange rate took a dive and is down about $100 from last month.

"We will look at various options to address customer losses later in the investigation," the company said. "While we are halting all operations at this time, we can confirm that the breach was limited to bitcoin wallets; the other digital tokens traded on Bitfinex are unaffected."

The cryptocurrency is designed to hide the identity of the owner and is run on a digital ledger based on blockchain. While that is useful for privacy advocates, it can make it exceedingly difficult to find those who might have stolen the cryptocurrency.