The code is published, so anyone may inspect it. That openness is a security argument as much as a price one — flaws can be found by people who do not work there.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a recipe printed on the packet.
For example
Anybody may read how it works. Somebody who does not work there spots a flaw and reports it, which is exactly the point.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.