Back in the iMIS 4/iMIS 10 world, ASIUtil.dll required ASIDBUtil and ASIDBUpgrade. I can tell that the .NET parts of iMIS use Secure.dll instead of ASIUtil, and I know some things changed in 15.1.2 for the Advanced PCI support.
Does ASIUtil 15.1.2 still require either ASIDBUtil or ASIDBUpgrade?
Is ASIUtil 15.1.2 different enough from previous versions that I would need a new Interop?
The three .dlls are still a set...
You need all three .dlls in 15.1.2; the references haven't changed. There's a new method, Init2, added to pass through the iMIS login ID so it can be added to the PciAuditLog table. Init is still available and valid, but you'll end up with a blank UserId column if you're using PCI logging. So depending on what you want to do, you may need a new interop.
For more on the .NET encryption / hashing code, see: http://www.imiscommunity.com/web_cc_encryption_and_password_hashing