Hi,
My name is Oleg Andreev, I work on iOS/OSX wallet and CoreBitcoin - a clean and well-documented Bitcoin toolkit in Objective-C.
As you all know, wallets are typically encrypted with a password (using some key stretching algorithm like PBKDF2 or Scrypt). Since the password is weaker than a purely random 128+ bit key, it's better if the user keeps their wallet in some private location that is relatively hard to access. Such backup is better not to be thrown around on popular hosting services like Gmail or Dropbox. HD wallets (BIP32) improve user experience by requiring to secure only the master key and only once. The rest of the keys can be derived later to retrieve the funds.
The problem is, wallets may have extra metadata which cannot be derived from the master key. E.g. user notes, invoice info, or even more importantly, multisig pubkeys and P2SH scripts. To redeem a P2SH payment one needs to know original script which must be stored somewhere and securely backed up before any transaction is made involving that script. Asking the user to backup his password-protected wallet before each such transaction would be cumbersome.
I suggest additional backup scheme where the user's wallet is encrypted using a truly unpredictable AES key derived from the wallet's master key. If the master key itself is not derived from a weak passphrase, but has 128+ bits on entropy, the AES key would be equally strong. Therefore the wallet can be automatically encrypted and uploaded to one or more backup services without any user action. When the user needs to restore the backup, he will have to restore the original master key first and then make his wallet connect to backup servers and retrieve the most recent backup of full wallet contents. Backup servers cannot possibly decrypt wallets with bruteforce, they only need to allow reliable retrieval. User's wallet may download the backup at regular intervals to detect if one of the servers lost his data or went offline. In such case, another server may be used or the user may be warned to make a manual backup as soon as possible.
Proposal:
https://github.com/oleganza/bips/blob/master/bip-0081.mediawikiUPD: https://github.com/oleganza/bips/blob/master/bip-oleganza-backups.mediawikiUPD2:
https://github.com/oleganza/bitcoin-papers/blob/master/AutomaticEncryptedWalletBackups.mdPS. I didn't want to create a pull request as the text might change and I don't want to have troubles with rebase (and accidentally lose connection to a pull request). Github Issues seem to be disabled in the bitcoin/bips repo. So lets discuss it here for now.