For this specific use case, you don’t need anything fancy like a constant time implementation, and I found it easy enough to implement from the paper—except that, mindbogglingly, they didn’t address endianness at all, even though you have to take it into account; so you need to read https://www.spinics.net/lists/arm-kernel/msg633602.html as well.

Look at https://git.chrismorgan.info/tesid/blob/HEAD:/rust/src/fpeck..., it’s very simple.