Looking for an encrypted mail provider I checked ProtonMail and asked them why do they use “semi-random password salts (one random part, one non-random part)” compared to SRP 6a fully random salt. Got this answer:
The advantage is that in the event another service’s password database is compromised, and the user used the same password on ProtonMail and this compromised third-party service (don’t do this!), and that service uses the same hash algorithm we do, the attacker would not be able to impersonate the ProtonMail server in the SRP auth sequence, because ProtonMail clients know that the salt begins with a non-random sequence and check this, whereas the salt stolen from 3rd party service would not.
Now given that the client is Open Source, what is the point of this? They didn't answer this one. I haven't read the source, so I don't know how it's implemented, but does anybody know how it could enhance auth security?