I have a server which encrypts files with the same AES key. Users can upload a file and download its encrypted version. A user can upload as many files as he wants. Users can decrypt the documents via the server after a D date. A user may see an other user's encrypted document, but it would be a disaster if he could decrypt it before the D date!
I've read in other stack overflow post that using the same initialization vector can make chosen-plaintext attack easy. So I want to generate a different initialization vector for every uploaded file. Is it a standard secure way to add this IV to the encrypted document?
My first idea is that I encrypt it with a different AES key and simply concat it to the encrypted byte array. But I'd rather use a more standard solution for this problem.