In the 90s and 00s analog pay tv used to be "encrypted" like this. IIRC software decryption relied on deriving the weak shuffling key by manually reordering ima

In the 90s and 00s analog pay tv used to be "encrypted" like this. IIRC software decryption relied on deriving the weak shuffling key by manually reordering image rows and then using that to reverse the key so the next frames could be decrypted more reliably. Doing that kind of video processing in realtime and in software was close to CPU limits. https://en.wikipedia.org/wiki/Nagravision https://en.wikipedia.org/wiki/Nagravision