I remember writing a small program to decrypt image stills in the late 90s and it was surprisingly easy and straightforward, though obviously doing it in real time may have been another story...
You Never needed to actually use the cards for Nagra. But The competitor DTV system built by NDS had an ASIC on the chip, so all solutions required a card at some level. But some setups could use one slave card for multiple receivers. Even over IP.
Fun times!
Instead of using OEM receivers and compromised cards, they’ve gone to running emulated receivers and regular full-sub cards.
I used to be really into FTA satellite, but have not played with it in awhile.