# Tag Info

The problem is almost exactly the same as in password based key derivation, so you could use a similar solution. Derive a master secret from your password and a unique salt using e.g. PBKDF2 or scrypt: $S_m = PBKDF(p, s)$. Derive a site-specific secret from the master using e.g. HKDF and the site URL: $S_u = HKDF(S_m, u)$. Turn the site secret into a ...
Your idea is no stronger than simply having a common shared password $P_1$ from which the symmetric encryption key is derived. If Alice encrypts a message with Bob's hashed password, even if someone knows the shared password, only Bob can decrypt the message You assumed the hash of Bob's password – $H(B_1)$ – is public, so if Eve knows both it and the ...