Add GCM cypher #32
Description
Is your feature request related to a problem? Please describe.
The signing / integrity-check is currently kind of hand-made.
Describe the solution you'd like
Use existing integrity check of GCM library. Think about making it default ON. For backwards kompatibility a "turn GCM off" flag might be necessary.
Describe alternatives you've considered
tbd
Additional context
the encrption method: https://en.wikipedia.org/wiki/Galois/Counter_Mode
there seem to be some disadvantages with memory usage: https://stackoverflow.com/a/26370130