mission: emulate "trusted login"

I would like FreeTDS to be able to run on Win64 in just the way
NTWDBLIB.DLL runs in Win32.  

The biggest missing component is "trusted login", whereby the user does
not provide his username and password.  Instead, cached credentials are
retrieved from the operating system.   Adding that feature to FreeTDS will
give us a bone-for-bone Microsoft-compatible Win64 DB-Library.  

Question: how to retrieve said credentials via the Win32 API?  

Windows surely doesn't store the user's password verbatim, but rather
keeps the hashed/salted/whatever version of it.  Part of FreeTDS's current
functionality is to turn a raw password in to a [something] and then pass
that to the server.  I want to pass the same [something], but starting
from whatever I can get from Win32, not the password itself.  

If someone could point me to the right API function, I would consider it a
service, with gratitude.  Google terms also gladly accepted.  If you could
tell me what I mean by [something], that would also be helpful.  

I found getSchannelClientHandle().  Is that in the right neighborhood?  

Many thanks.  

--jkl