Design Doc 009: Wallet Backup
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Summary
=======

This document describes the backup system used by Taler wallets.
This is the second, simplified iteration of the proposal, which leaves
out multi-device synchronization.


Requirements
============

* Backup must work both with and without Anastasis

  * When not using Anastasis, the user is responsible for keeping
    their wallet's root secret safe.

* Arbitrary number of backup providers must be supported
* Minimize information leaks / timing side channels (user might be able to change
  some setting to allow more frequent backup with less potential data loss but more
  leakage)
* Minimize potential to lose money or important information
* Since real-time sync is not supported yet, wallets should have a feature
  where their whole content is "emptied" to another wallet, and the wallet is
  reset.
* Even without real-time sync, the backup data must support merging with old, existing wallet
  state, as the device that the wallet runs on may  be restored from backup or be offline
  for a long time.


Solution Overview
=================

Each wallet has a 64 byte wallet root secret, which is used to derive all other secrets
used during backup, which are currently:

1. The sync account key for a sync provider, derived via the sync provider's base URL.
2. The symmetric key used to encrypt the backup blob

If the user chooses to use Anastasis, the following information is backed up in Anastasis:

* list of used backup providers
* wallet root secret


Supported Operations
--------------------

* **restore-from-anastasis**:  Start Anastasis recovery process.  This requires
  the wallet backup state to be "uninitialized".
* **restore-from-recovery-secret**:  This requires the wallet backup state to be uninitialized.
* **add-provider** / **remove-provider**:  Add/remove a sync provider from the
  list of providers.  Adding a provider will cause payment(s) to the provider
  to be scheduled according to the provider's terms.  If the wallet backup
  state is "uninitialized", adding a provider will set the backup state to
  "initialized" with a fresh wallet root key.  Changing the provider list will
  also update the backup provider URL list in the  anastasis core secret.
* **abandon** / **takeover**: When the user wants to stop using a wallet on a particular
  device, another wallet can "take over" by reading the recovery secret of the abandoned wallet.
  The abandoned wallet marks explicitly in its backup blob that it is abandoned.
  Abandoning a wallet will set the backup state to "uninitialized".
* **backup**: Do a backup cycle.
* **rekey**:  Change to a new wallet root secret, in case the old one has been
  compromised.  Only protectes future funds of the wallet from being
  compromised.  Requires a new payment to all configured backup providers.


Backup Format
-------------

TBD.  Considerations from :doc:`005-wallet-backup-sync` still apply,
especially regarding the CRDT.


Initial User Experience
-----------------------

The user will be asked to set up backup&sync (by selecting a provider)
after the first withdrawal operation has been confirmed.  After selecting
the backup&sync providers, the user will be presented with a "checklist" that
contains an option to (1) show/print the recovery secret and (2) set up Anastasis.

The wallet will initially only withdraw enough money to pay the
backup&sync/anastasis providers.  Only after successful backup of the wallet's
signed planchets, the full withdrawal will be completed.


Open Questions
==============

* Should the wallet root secret and wallet database be locally encrypted
  and protected via a passphrase?
* What happens if the same Anastasis user has multiple wallets?  Can Anastasis somehow
  support multiple "instances" per application?

Future Work / Ideas
===================

* Incremental backups?

  * Instead of one big blob that always needs to be read/written, we could have (1) a
    limited length append-only journal and (2) a merkle tree so that the backup blob can
    be updated incrementally once the journal is full.
  * Leaks more information and is more complex.

* Mult-device synchronization, with synchronous communication either over some signaling server
  or P2P connectivity (WebRTC, etc.)

  * Destroys the "wallet" metaphor, now the wallet is more like an account.
  * We should first agree on the requirements from the perspective of end users
  * P2P payments in Taler might also make sync less important
  * Maybe only parts of the state (purchases / contracts, but not coins) should be synchronized?
  * WhatsApp web model:  The wallet runs only on one devices, but other devices
    can connect to it as clients.  (Allows my browser wallet to temporarily access
    money from my phone wallet and vice versa.)