We're talking about a local installation of SoSci Survey, do we? Which one? What is the program and database version installed on the server?
If you application is on the same server, you could just access the SoSci Survey's database and use PHP's
password_verify() to check the password. If it's alright, you'd create a session like for any other login method.
We also have developed a login PlugIn to access the SoSci Survey user database from a different server. It works with strong encryption to transmit the requests. As you can image, this PlugIn is currently optimized for Q2A, but could work with nearly any software that allows for auth plugins.