From what I can tell the splitters essentially convince the playback device to play by relaying whatever HDCP messages the TV sends:
Playback Device (Roku etc)
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Splitter ------ Unauthorized Player (Recorder)
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Authorized Player (TV)
Without this the Playback Device will show a black screen, an HDCP error message or something similar, but even with the splitter & an authorized player it seems the content can be encrypted.
Since the TV obviously has the capability of decrypting the content it seems like there should be a way to split the signal somewhere between the main TV board and the display panel(s), which sounds like a big deal. Or maybe someone could make an incomplete splitter / recorder that just needs a legit Samsung board attached to it?
I did a Google because I was curious, and some of them do indeed strip the HDCP.
I think what you're saying is basically what's happening. The HDMI splitter pretends to be a display/TV, so the playback device sends encrypted data to the splitter, which decrypts it and forwards the decrypted part over the HDMI connections.
From what I could find, it's usually the cheap HDMI splitters that emit unencrypted data over the HDMI ports. That kind of makes sense; they implement HDCP inbound because they have to, but they leave HDCP off the output ports because it reduces their manufacturing cost.
I think you can't just "forward" the encrypted packets, because then both outputs on the splitter would try to communicate back to the player, causing issues. The splitter has to MITM the connection.
Since the TV obviously has the capability of decrypting the content it seems like there should be a way to split the signal somewhere between the main TV board and the display panel(s), which sounds like a big deal. Or maybe someone could make an incomplete splitter / recorder that just needs a legit Samsung board attached to it?