path: root/design-documents
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authorFlorian Dold <>2020-11-06 17:42:35 +0100
committerFlorian Dold <>2020-11-06 17:43:04 +0100
commitacb799f34f2d576b39203012d0c603ffaf31a232 (patch)
tree934a33ad4aa48b2e270c5e4559992f4a02a214b8 /design-documents
parentdae4c834b69efbabb796506a5dbdead9be3464a1 (diff)
new, simplified backup proposal
Diffstat (limited to 'design-documents')
3 files changed, 111 insertions, 1 deletions
diff --git a/design-documents/005-wallet-backup-sync.rst b/design-documents/005-wallet-backup-sync.rst
index 26dbbf1..20fce37 100644
--- a/design-documents/005-wallet-backup-sync.rst
+++ b/design-documents/005-wallet-backup-sync.rst
@@ -3,7 +3,11 @@ Design Doc 005: Wallet Backup and Sync
.. warning::
- This is an unfinished draft.
+ This document is deprecated. We have decided to first
+ implement backup, and tackle sync later.
+ The multi-device sync described in this document would lead
+ to a bad/unexpected user experience that does not justify the
+ conceptual / implementation complexity.
diff --git a/design-documents/009-backup.rst b/design-documents/009-backup.rst
new file mode 100644
index 0000000..7ec8be2
--- /dev/null
+++ b/design-documents/009-backup.rst
@@ -0,0 +1,105 @@
+Design Doc 009: Wallet Backup
+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.
+* 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 **if** requested by the user
+* 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.
+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
+* 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.
+* Real-time synchronization, either over some signaling server
+ or P2P connectivity (WebRTC, etc.)
diff --git a/design-documents/index.rst b/design-documents/index.rst
index 38825b2..55dd8c1 100644
--- a/design-documents/index.rst
+++ b/design-documents/index.rst
@@ -18,3 +18,4 @@ and protocol.
+ 009-backup