Yes, it is tricky to add IDECHashAuthentication because the class methods, and to solve this either add the same naming methods in TDECHash, so you can redirect all these methods to the class ones in TDECHashAuthentication, or introduce new global function ValidPasswordHash just like ValidHash that return TDECHashAuthentication.
Both approaches are not ideals, the best or the better is MAY BE dropping the class methods and making them simple local methods, but i have no idea what this might break in current build, as these class methods popular or even needed as class methods ?
I can't tell, and believe you have better inside knowledge in
DEC design.