Download encrypted HLS content with ffmpeg

I maintain an archive of videos, especially documentaries from public media libraries from tv channels etc. on my NAS. Whilst I can use youtube-dl for the most part, it’s rather difficult for sites that use HTTP live streaming (HLS) to stream their content. In the most cases this is even AES-128 encrypted, which makes it difficult to download it. But since there’s players that support playback, it’s obviously possible to decode the stream on the client side.