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authorFlorian Dold <florian.dold@gmail.com>2020-04-27 23:58:38 +0530
committerFlorian Dold <florian.dold@gmail.com>2020-04-27 23:58:38 +0530
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wallet backup&sync
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+Design Doc 005: Wallet Backup and Sync
+######################################
+
+.. warning::
+
+ This is an unfinished draft.
+
+Summary
+=======
+
+This document discusses considerations for backup and synchronization of wallets.
+
+
+Requirements
+============
+
+* Backup and sync must not require any synchronous communication between the
+ wallets
+* Wallets operating (payments/withdrawals/...) for longer periods of time without
+ synchronizing should be handled well
+* Conflicts should be resolved automatically in pretty much all cases
+* One wallet can be enrolled with multiple sync servers, and a wallet can
+ join
+* Other wallets connected to the sync server are trusted.
+
+Proposed Solution
+=================
+
+The blob stored on the backup/sync server is a compressed and encrypted JSON file.
+
+The various entity types managed by the wallet are modeled LWW-Sets (Last Write
+Wins Set CRDT). Timestamps for inserts/deletes are are Lamport timestamps. Concurrent, conflicting insert/delete
+operations are resolved in favor of "delete".
+
+The managed entities are:
+
+ * set of exchanges with the data from /keys, /wire
+ * set of directly trusted exchange public keys
+ * set of trusted auditors for currencies
+ * set of reserves together with reserve history
+ * set of accepted bank withdrawal operations
+ * set of coins together with coin history and blinding secret (both for normal withdrawal and refresh)
+ and coin source info (refresh operation, tip, reserve)
+ * set of purchases (contract terms, applied refunds, ...)
+
+(Some of these might be further split up to allow more efficient updates.)
+
+Entities that are **not** synchronized are:
+
+* purchases before the corresponding order has been claimed
+* withdrawal operations before they have been accepted by the user
+
+Entities that **could** be synchronized (to be decided):
+
+* private keys of other sync accounts
+* coin planchets
+* tips before the corresponding coins have been withdrawn
+* refresh sessions (not only the "meta data" about the operation,
+ but everything)
+
+
+Garbage collection
+------------------
+
+There are two types of garbage collection involved:
+
+1. CRDT tombstones / other administrative data in the sync blob. These can be deleted
+ after we're sure all wallets enrolled in the sync server have a Lamport timestamp
+ larger than the timestamp of the tombstone. Wallets include their own Lamport timestamp
+ in the sync blob:
+
+ .. code:: javascript
+
+ {
+ clocks: {
+ my_desktop_wallet: 5,
+ my_phone_wallet: 3
+ },
+ ...
+ }
+
+ All tombstones / overwritten set elements with a timestamp smaller than the
+ smallest clock value can be deleted.
+
+2. Normal wallet GC. The deletion operations resulting from the wallet garbage
+ collection (i.g. deleting legally expired denomination keys, coins, exchange
+ signing keys, ...) are propagated to the respective CRDT set in the sync
+ blob.
+
+
+Ghost Entities
+--------------
+
+Sometimes a wallet can learn about an operation that happened in another synced
+wallet **before** a sync over the sync server happens. An example of this is a
+deposit operation. When two synced wallets spend the same coin on something,
+one of them will receive an error from the exchange that proves the coin has
+been spent on something else. The wallet will add a "ghost entry" for such an
+event, in order to be able to show a consistent history (i.e. all numbers
+adding up) to the user.
+
+When the two wallets sync later, the ghost entry is replaced by the actual
+purchase entity from the wallet that initiated the spending.
+
+Ghost entities are not added to the sync state.
+
+
+References
+==========
+
+* Shapiro, M., PreguiƧa, N., Baquero, C., & Zawirski, M. (2011). A
+ comprehensive study of convergent and commutative replicated data types. [`PDF <https://hal.inria.fr/inria-00555588/document>`__]
+
+Discussion / Q&A
+================
+
+* Why is backup/sync not split into two services / use-cases?
+
+ * For privacy reasons, we can't use some interactive sync service. Thus we
+ use the backup blob as a CRDT that also synchronization for us.
+
+* Do we synchronize the list of other backup enrollments? How
+ do we handle distributing the different private keys for them?
+
+ * If we automatically sync the sync enrollments and the old sync account
+ is compromised, the new sync account would automatically be compromised as well!
+
+ * If every wallet had its own sync key pair, we could select which existing wallets
+ to roll over as well.
+
+* How do we handle a synced wallet that becomes malicious deleting all coins or purchased products?
+
+ * This needs to balance the genuine need to permanently delete data.
+ * Should the sync server allow to fetch previous versions of the sync blob?
+ * Should the individual wallets keep tombstones (i.e. entities just marked as deleted)
+ around for some time, or should they delete and "sanitize" (delete data not needed for the CRDT)
+ tombstones as soon as possible?
+
+* How are wallets identified for backup/sync?
+
+ * UUID / EdDSA pub and nick name? When nickname clashes,
+ some number is added based on lexical sort of the random id ("phone#1", "phone#2").
+
+* Do we have a passphrase for our backup account key(s)?
+
+ * ???
diff --git a/design-documents/index.rst b/design-documents/index.rst
index aa0ca65..590cd45 100644
--- a/design-documents/index.rst
+++ b/design-documents/index.rst
@@ -14,3 +14,4 @@ and protocol.
002-wallet-exchange-management
003-tos-rendering
004-wallet-withdrawal-flow
+ 005-wallet-backup-sync