Five Algorithms Walk into a CTF (Only One Walks Out)

Not only the title, but the entire article, and every reference that pops up in a search, fails to explain the initialism "CTF".

This is pure gatekeeping by immature little boys in a treehouse labelled "NGA".

If we had awards for bad initialisms, this one would be in the running for the top spot. I'm still not entirely sure what it means.