Small refactoring

[buldo]

Made code a bit easy to understand:

• Big processing method splitted
• Removed decryptor global state(because this state make sense only in tests)

[Consti10]

In general, I like all those changes, the only issue i see are the changes to the encryption - while it makes sense on a first glance to seperate authenticate and decrypt and re-use authenticate in decrypt, i am not sure if

1. that 'works' since for the decrypt path, libsodium provides a method that authenticates and decrypts in one go
2. it migh reduce performance, since we (unneccessarily) call authenticate first, then authenticate an decrypt again

[Consti10]

actually, not sure if what i wrote above is true ;) got a bit confused looking at the changes in git ;)