Refactoring: removal of lastDecrypt

[YoshiMan]

The lastDecrypt property is not needed anymore. It should be removed. Could be a bit tricky ;)

[annejan]

But will make the whole thing more robust by removing this weird "global" that might be filled with secret data 😄

[tezeb]

@annejan you can assign it to me
I think I have it mostly done, but there is few cases that I need to analyze carefully.