Every bitcoin has an address with a matching encrypted key—a secret number that unlocks the bitcoin so the owner can access it for making payments. The key is designed to determine the bitcoin's address, but the bitcoin address cannot be used to do identify the private key. So if someone gets your private key, they can spend your bitcoin.
Currently, there are a few ways to store bitcoins, Botezatu said.
People can store bitcoins on the Internet with an online service, like an online stock exchange or bitcoin wallet. They also can be stored on a computer in an encrypted wallet on an offline wallet, which may consist of copying the wallet to a computer that is not connected to the Internet or to an external storage medium. People can also print out their encryption keys on paper and store them in a safe place.
One way cybercriminals are stealing bitcoins is by using malware to target bitcoin wallets stored on their machine that is connected to the Internet, Schouwenberg said.
They are also hacking into severs of bitcoin exchanges and other third-party platforms, he said. Many of these storage services are run with little funds and bare-bones infrastructure, so they can be easy targets, Schouwenberg said.
And since these online bitcoin wallets and other storage businesses are not regulated, it's not difficult to set up a rogue service claiming to secure bitcoins.
"Anyone can start their own bitcoin service, their own mining tool, their own thing and just walk away," Schouwenberg said. "In recent weeks we have seen a number of cases where we are not entirely sure if the online services are being hacked or the people running the site are just walking away from the services with people's bitcoins."