Common code for classic asp and asp.net

Hi Expert,
we have an application that has both classic asp and dot net pages hosted on IIS.We have COM DLL's that get called from classic asp pages and do the talking with db.and we have a solution that contains All dot net modules.Now we have a requirement of applying an algorithm say masking of certain specific data which gets displayed both on asp and asp.net pages .How do we write a common code, that caters to both the pages.Writing logic on db side is not an option i am looking for.I have thought of wcf service that could be called from both asp and asp.net.but is there any other option may be like creating a dll and putting into gac or any thing else.Please suggest some articles or case studies that can help me achieve this.