Encrypt as much of the application's data as possible

[powerkangaroo]

Depends on: #28

After the wallet protection mechanism is chosen, encrypt as much of the application's data as possible with the involved secret (including public keys, transaction history, birth block, etc.). Use AES-256 and sufficient rounds of Argon2. Ideally, a user who can't authenticate themselves should learn nothing more than the fact that the application is installed on the device and should get no further than an authentication screen.