Is Kraken legal?

Could you kindly clarify for me the legality status of Kraken? I've been hearing quite a buzz about it in the crypto community, but I'm still a bit hazy on whether it's operating within the confines of the law. I'm particularly interested in understanding its regulatory standing in major financial hubs like the United States and Europe. Could you also elaborate on any potential legal challenges or risks associated with using Kraken as a platform for cryptocurrency transactions? Thank you in advance for shedding some light on this matter.