Hypothetical. Lets say you have an application where you generate a master key from a user password using PBKDF2 (static salt) and then use the master key to derive two
passwords keys for an encrypt-then-mac scheme using HKDF (unique salt per encryption).
The password would be shared by two individuals, who would then potentially share many messages. Each message would share the same master key, but would have unique encryption and MAC keys. Does this make sense, or is the unique salts in HKDF unnecessary?