S/MIME in VC++

Hi,

i hope somebody can explain what my problem is.
I'm trying to read and write s/mime attachements on mails.
I already manged to crypt, sign, decrypt and verify files using the platform
SDK functions on MSVC++.
But reading in a s/mime attachment using my functions to decrypt files won't
work!
Does anybody know how i have to fight throught my problem?
Thanks.