Keep client group membership state management

[pdyraga]

Keep client does not store information about group membership and generated keys in any persistent way. Everything is kept in memory and single client restart is enough to lose all this information.

This card covers work on introducing basic state management to Keep client:

• group memberships should be persisted once obtained and read when the client starts
• group key shares should be persisted once obtained and read when the client starts
• registry of active groups should be synced from the chain when the client starts

Important technical aspects:

• Keep client data should be stored in a secure manner (password-encrypted file?)
• persistent storage should be quasi-transactional - update failure should not break all the other data, multiple writes should be synchronized to avoid writing garbage

This card does not cover storing the current protocol execution state.

Flowdock thread: https://www.flowdock.com/app/cardforcoin/keep/threads/hib9k937zQqi6txJyDnXYqw5fm-

[pdyraga]

First implementation guidelines:

• sensitive information such as key shares should be encrypted
• we encrypt data using KEEP_ETHEREUM_PASSWORD
• the Keep client configuration file should contain an entry pointing to the state data directory
• store group data in a separate file for each group to mitigate data corruption risk (?)
• information about groups and membership should be loaded from the chain when client starts and we shouldn't cache this information locally

[pdyraga]

• PoC for on-disk key shares storage (bolt, sqlite3, vault, our own solution?); estimate: 4.2
• interface for the state store; estimate: 2.7
• encrypting/decrypting data in between the storage; estimate: 3.2
• on-chain view for getting groups & membership information; estimate: 1.9
• off-chain code calling on-chain group API when client starts; estimate: 1.9
• off-chain code for loading keys from storage when client starts 1.9
• failure & recovery tests 3.8

19.6 in total ~ 20 days

For future:

• delete key shares once group expires