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One way to do it would be: 1) Your plaintext password is used to generate two derivative tokens. One for user authentication with Google, and one for encrypting data stored with Google. The authentication password token is PBKDF2(N, password) and the encryption token is PBKDF2(N-1, password). You give Google the authentication token on signup. They are not capable of deriving the encryption token from it, but the user is capable of generating both from the original password. 2) For backup initialization, you prompt the user for their password, calculate the encryption token, generate an ECC keypair, encrypt the private ECC key with the encryption token, and send the encrypted private key to the backup server. This is the last time you'll need to prompt the user for their password until a restore. 3) Each time you wish to send updates to the backup server, you generate a symmetric encryption key, encrypt that using the user's public key, encrypt the data to backup with the symmetric key, and send the encrypted symmetric key and data together to the backup server. This does not require prompting the user for their password, or storing it locally. 4) On restore, you prompt the user for their password, use it to decrypt the private key that you pull down from the backup server, which you can then use to decrypt any encrypted blocks you retrieve as well. |