C++ simple lib for encryption, with c# wrapper

I need a c++ lib (dll) that encrypt and decrypt files using AES256, and implement simple encryption/decryption schema. It should use standard encryption libs. The task is to build a lib that use the following: Step 1,2: init Step 1:read key K1 from a place in memory that is hard coded and can not be shown in simple "strings" look. for example - something like adding the chars in run time, or similar: char K1a = 80; char K2b = K1a - 15; char K3b = K2a + 23; char K4b = K3b; // together they make a "pass" key NOTE- the actual key length should be large (~20chars). Step 2: read a file in some place that is encrypted with K1, and contain another key (say K2). Step 3 - run time: supply an encrypt and decrypt function that uses K2 to encrypt and decrypt byte arrays. I need: -- all the code above in c++, as DLL -- wrapper functions in c# for all the (public) functions in the DLL the algorithem used for the encryption/decryption should be AES128 or AES256. please use standard lib and dont write encryption/decrytion code.