I'm not entirely sure what they mean by a "magic cookie" here.
The document does say it uses public/private keys for data transfer. You would not usually use public/private for data storage because of the huge keysizes required.
I think the cookie bit is just an authorization credential.
If they use asymmetric encryption for data transfer, then how does that work as part of a convergent encryption scheme? Wouldn't all of the file hashes be different at that point?
I would assume the asymmetric crypto is just for the transfer. In other words you encrypt the data and then send it wrapped in another layer of public/private crypto. Not entirely sure though.
The document does say it uses public/private keys for data transfer. You would not usually use public/private for data storage because of the huge keysizes required.