I ended up syncing the data to the server, and emailing the user with a "magic link" with the decryption/auth keys in.
To begin with I didn't store the encryption key, but too many users lost the original email, so now I also store the user's encryption key, which is lame from a privacy point of view, but for most users is what they expect.
To begin with I didn't store the encryption key, but too many users lost the original email, so now I also store the user's encryption key, which is lame from a privacy point of view, but for most users is what they expect.