Let’s say I want to use some js encryption library (not java) to encrypt a file. Is it possible to do so on client side before sending the file to the server and so upload a file by javascript in some memory on client side ?
Could I use local storage for example at least for latest browsers ?
You could use the File API
Here some examples: https://developer.mozilla.org/en/Using_files_from_web_applications)
Of course, as you imagined, you need latest browsers.