Use end-to-end encryption for card backup to another seedkeeper

[Toporin]

In the current app implementation, when performing a backup, secrets are exported in plaintext. It is preferable to perform the backup encrypted.

The basic flow is the following:

• scan the backup card to get authentikey (for pairing). (Authentikey can be recovered using cmdSet.cardGetAuthentikey())
  • Check also the status of the card (i.e. is card setup done?)
• Scan the main card and perform the following:
  • import the backup card authentikey => get the backup authentikey SID
  • get the main card authentikey (for later import into the backup card)
  • export all the secret encrypted using the backup authentikey SID
• Scan the backup card again and:
  • get a list of all the secret currently on the card (to check for secret already stored)
  • compare list of existing secret with list of secret to import (based on fingerprint), remove secret already present from list of secret to import
  • import the main card authentikey => get the main authentikey SID from the backup card
  • import each of the encrypted secret using the main authentikey SID

A basic example of export/import secret encrypted is provided in the test app.

Note that the PIN code for the backup card may be different from the PIN of the main card!