Hi Edgar,
I don't have an experience with integration with SF but I can tell you the standard method offered for file encryption in ABAP AS. ABAP AS offers support for PKCS7. It's a standard that describes how to encrypt a message. You can more find info how to use it on this forum. Search for SSF_KERN_ENVELOPE. I am not sure if SF supports this but from your description it seems like yes. Btw DSA can be used only for signing, not for encryption.
Cheers