The developer (my customer) of the Atari ST graphics program was wicked :)
When he detected (in runtime at random places) that the program was cracked, he added a invisible watermark to the output of the graphics program.
When someone produced public, commercial work with a cracked version of his software, he could prove it was done with a non-paid cracked version of his software. He could "convince" several user to pay him retrospective.
My copy protection was made for a niche high quality Atari ST graphics program that only got sold in very small numbers. The developer (my customer) want to get sure he really get money for every copy.
And I put a lot of effort into randomization of my copy protection detection software. I want get sure nobody developed an automatic cracking program for my copy protection. But I left a easy to find exit point where cracker could easy remove my copy protection detection software. Manually. That worked. Everything can be cracked. But my copy protection got never in deep analysed because I made it really really hard to do it.
This reminds me about the copy protection I once developed for the Atari ST.
The Atari ST had a proper floppy disk controller from Western Digital.
They never changed a bit about the WD controller. You could rely on its characteristics.
My Schrodinger's cat of copy protection for the Atari ST needed a new, never formatted disk, that gave different track reading data every time you read it to start with.
I produced some hundred pseudo random bytes carefully avoiding the magic sync bit sequence to init the read circuit of the controller.
At the end of the pseudo random bytes where exact one controller sync sequence following a 32 bit serial number. Than I wrote them as track data.
But I exactly timed the write track command and terminated it in the middle of execution to keep the random state of the rest of the floppy track.
When you than read the track you got every time different data, except that 32 bit serial number.
It worked very reliable.
Even if a floppy disk had read errors and you could not read the directory or the program data any more, the copy protection worked still reliable.
It was the only floppy disk copy protection for the Atari ST that could never be copied. :)
* Intel i7-4600U CPU DMips/Mhz (I found about 10 DMips/Mhz)
* RISC-V VRoom = 10.3 DMips/MHz (still in development)
* RISC-V XiangShan = ?
* RISC-V others ?