On the trek, our guide points out plants and trees, as well as spiteful monkeys who hurl twigs from the trees and mount daring ram raids on our backpacks to steal our water bottles.

Here we've collected some of our favorite covers from 1959 to 2000, including a penguin with a high-tech backpack, a self-portrait by Koko the gorilla and a shark attack.

The cryptosystem obtains a high knapsack density, and hence it is secure against low density subset-sum attack.