On my site, users can download files which are stored on public hosting, so I need to encrypt them. A downloaded file is divided into blocks of 1 kB and I want encrypt them individually, using the same key. I have read that the best option for performing the encryption is AES, but I don't know which mode should I use.
Also, I want also ask, is using similar keys insecure? If that practice is safe, I needn't generate long keys and store them, because all can have same beginning. The main problem is user could find the encrypted files, then they will have the file both before we perform encryption, and afterways; does this raise the possibility of finding the key?