I didn't say never copy and paste. I'm saying when you push a commit you should understand what all the LOC in that commit do (not counting vendored dependencies). If you don't understand how something works, like crypto (not sure what Hamilton or Euler refers to in this context), ideally you would use a library. If you can't, you should still understand the code sufficiently well to be able to explain how it implements the underlying algorithm. For example if you're writing a CRC function you should be able to explain how your function implements the CRC operations, even if you don't have a clue why those operations work.

There's a huge difference between copy-pasting code you don't understand and using a library with the assumption that the library does what it says on the tin. At the very least there's a clear boundary between your code and not-your-code.