Protecting file after decryption

[spman]

Hi friends

I'm producing a secure program. The program opens files with secret contents. suppose the file is video or any other specific format that I don't know about its structure and I use some libraries in my source project to open the file. So I can't make change in the section of code which opens the file. In order to protect the file, encryption is used. But on the other hand, the file must be decrypted to be opened by the program, meaning that the neat file must be passed to the section of program which uses file contents. In the program I do it before passing the file to that section. But in this time the decrypted file is subjected to illegal read. How can I Prevent the file to be read after decryption?

I think if I can make the decrypted file in RAM, perhaps accessing to it become hard. Right?

What's your suggestion?

Other cracking methods like obtaining encryption key are ignored nonce.

Thanks in advance.