NV-Verlag is using encryption. For this purpose they bought a external encryption program that is Windhoze only. For the decryption you need also windows. (and the NV-verlag plugin)
For linux and Northsea there is S63 if you need recent charts, or you can search the internet for a CM93 copy.
Navigation is know where you are and what to do to get where you want.
But also: Know where you don't want to be and what to do to don't get there.