new, simplified backup proposal

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 26dbbf17..20fce37b 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.
 
 Summary
 =======
diff --git a/design-documents/009-backup.rst b/design-documents/009-backup.rst
new file mode 100644
index 00000000..7ec8be2c
--- /dev/null
+++ b/design-documents/009-backup.rst
@@ -0,0 +1,105 @@
+Design Doc 009: Wallet Backup
+#############################
+
+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 **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 38825b2b..55dd8c1e 100644
--- a/design-documents/index.rst
+++ b/design-documents/index.rst
@@ -18,3 +18,4 @@ and protocol.
   006-anastasis-ux
   007-payment
   008-fees
+  009-backup