There are multiple elements of the Graft user wallet:
Private spend key – secret – required to spend money; stored in the wallet app
Private view key – “semi-secret” – required to see the balance and previous transactions
Payment address – public – required to receive money
In Graft, the Private spend key is always stored at the client (wallet) and never shared with the supernodes. Therefore, it is not the same as credit card when you share you card account number with the merchant, payment processor, and the bank every time you make a payment. If one of them is breached, your credit card can be stolen and used to make fraudulent payments. If any or even all supernodes are “breached”, they don’t have your private spend key so no one will be able to “use” your Graft account.
The Graft wallet balance is a “hidden” set of previous transactions. It is calculated by scanning all the previous transactions which are stored on the public blockchain but invisible without view key. Since mobile wallet app does not have direct access to the blockchain, the private view key is temporary shared with a single “proxy” (relay) supernode in order to retrieve the wallet balance; the supernode will not store this view key in any database so even if it’s “breached” the view key will not be disclosed in most cases. However, even if the view key is disclosed, it only allows to see transactions, not to spend any money.
Users (either buyers or merchants) with higher requirements for privacy can host their own private “proxy” supernode with full copy of blockchain. This way they will never share their view keys with the random “foreign” supernodes. Most probably, due to limited processing power and other resource limitations, such a private proxy supernode will not be able to participate in transaction processing and earn any block rewards or transaction fees, but it will still be able to validate transactions and view balances privately by scanning the local copy of the blockchain. For users who cannot or don’t want to host their own supernode but still don’t trust the entire network, Graft creates a special (free) cloud service with trusted proxy supernodes that are protected by multiple levels of security.