It seems they are using the service of bitgo to store bitcoin though the following post is old. A reddit user seems to say that it talk to them via the support

It seems they are using the service of bitgo to store bitcoin though the following post is old. A reddit user seems to say that it talk to them via the support 2 months ago and they were saying they are using bitgo. https://webcache.googleusercontent.com/search?q=cache:LV7jr0XYDgoJ:https://www.nicehash.com/news/31+&cd=4&hl=en&ct=clnk&gl=fr https://webcache.googleusercontent.com/search?q=cache:LV7jr0...