Use WebCrypto API instead of SJCL
Investigating around #27 made me discover JS crypto libraries and I found that the one used by Lufi is probably one of the slowlest. To greatly improve performances of Lufi, I would suggest to use Web Crypto API with GCM mode.
It can be 5 to 10 times faster than the actual algorithm and could save a lot of time (and would be less resource consuming) for the end user.
To be still able to detect links generated with the old algorithm, we would have to modify the lufi-down.js to be able to detect if the file has been uploaded with the old algorithm or with the new one (just using a base64 key of different size would be enough).
Also, that would be the perfect occasion to implement #178.