The source code for the function is proprietary, and thus, unavailable. The number of rows it would apply to would be relatively small. As the only non-ETL alternative would be to decrypt a significantly larger quantity of rows, there's really no viable alternative ... especially from a performance perspective, where the alternative is known to be a serious performance problem waiting to happen.

I also need detailed step by step so that I can learn the process as well as understand it. I can read the VB code needed as I've been writing VBA and VBScript for more years than most folks have been working with computers, but I don't have any exposure to the details behind how a .NET assembly is compiled / created / deployed, or, once those things are done, referenced by a SQL Server UDF.