Limited key size to 256 bytes, changed default key size to 16

256 bytes is still ridiculous but it's more realistic than 64K.

Formatting changes, time can now dispay minutes and hours, file sizes…

… are now represented in larger units

Optimised encrypt and decrypt functions

Moved the outside loop inside and inside one outside to achieve better data locality. Improvement rate goes up with key size. (~170% faster on standard 256-bit key)

Add files via upload