Backing up your password manager is a good idea in case your house burns down, but where do you store the password to the off-site backup?
Remembering the master password is easy, but re-using the master password directly for my off-site backup is a bad idea. But what if I used a key derivation function to generate a password for the backup site from my master password using a predictable salt?
Let's say that I documented publicly, for example in this very post, that I would use PBKDF2-HMAC-SHA256 with one million iterations, using the backup service domain name and an iteration number as the salt, to generate the exact number of bits for a 64-character Base64 encoding, which would then be the password (the iteration number is necessary in case the backup service doesn't accept the first password generated, e.g. missing numbers or special characters).
- Would this scheme be secure assuming the derived password is compromised, but the master password remains secret?
- The master password needs to be strong, so how many bits of entropy does it need to protect against a brute-force attack by all but a nation-state attacker?