12.10. Design Doc 009: Wallet Backup

12.10.1. 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.

12.10.2. 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.

    Note

    CG: This boils down to the existing ‘reset’ button (developer mode). Very dangerous. Could be OK if we had some way to notice the number of wallets using the same backup and then allow this ‘reset’ as longa as # wallets > 1. Still, doing so will require a handshake with the other wallets to ensure that the user doesn’t accidentally reset on both wallets at the same time, each believing the other wallet is still sync’ed. So we would need like a 2-phase commit “planning to remove”, “acknowledged” (by other wallet), “remove”. Very bad UX without real-time sync.

  • 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.

12.10.3. Solution Overview

Each wallet has a 64 (CG: 32 should be enough, AND better for URLs/QR codes/printing/writing down) byte wallet root secret, which is used to derive all other secrets used during backup, which are currently:

  1. The private account key for a sync provider, derived using the sync provider’s base URL as salt. The base URL must be normalized to end with a “/”. The schema (“http://” or “https://”) is part of the base URL, thus different account keys would be used for “http://” vs. “https://” (reduces linkability).
  2. The symmetric key used to encrypt/decrypt the backup blob. FIXME: document exact KDF salt here once implemented.

If the user chooses to use Anastasis, the following information is backed up in Anastasis (as the core secret in Anastasis terminology):

  • Taler Wallet core secret tag (new GANA registry) and format version
  • List of used backup providers (sync)
  • Wallet root secret
  • Tor constraint (boolean) advising wallets that the backup should only be accessed via Tor and that users must be warned before attempting to restore the backup without Tor.

Supported Operations

  • restore-from-anastasis: Start Anastasis recovery process. This requires the wallet backup state to be uninitialized. FIXME: The last sentence makes no sense, as the user may have to pay for recovery!
  • restore-from-recovery-secret: This requires the wallet backup state to be uninitialized. FIXME: Again, I do not think we can require this. User could make backup, then loose device. Create new wallet. Use new wallet (including making yet another backup). THEN user remembers that he had a backup (or find root key) and now want to restore backup. This should MERGE the two states (you can consider it a ‘poor’ version of ‘sync’). Note that the lost device cannot ‘abandon’ the backed up state here!
  • 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 sync provider URL list in the Anastasis core secret (forcing a new policy to be uploaded).
  • 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. Uploads the latest wallet state to all sync providers. If sync provider state has changed unexpectedly, downloads backup, merges, and then uploads the reconciled state.
  • 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 sync providers.
  • backup-to-anastasis is missing.

Backup Format

TBD. Considerations from Design Doc 005: Wallet Backup and 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.

12.10.4. Open Questions

  • Should the exchange tell the wallet about available sync/Anastasis providers? Otherwise, what do we do if the wallet does not know any providers for the currency of the user?

  • 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?

    Note

    CG would definitively solve this using a more complex format for the master secret, basically serializing multiple root secret values with meta data (which wallet/device/name).

12.10.5. 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.)