How does Bitcasa encryption work?

Question

Encryption

Good afternoon community.
Maybe someone knows how the subject works? The site promises that it is encrypted on the spot, then it is uploaded.
But there are two things that are disturbing:
1. We all remember the history of dropboxing. I do not want to run into the same rake again.
2. How, in this case, can the web client work, where does the secret key come from?