I was wondering about the choice of numbers on the steps up to the slide; then I realised they were prime numbers. I decided to look up the uses of prime numbers. This site explains well the use of prime numbers in encryption, saying that the use of very large prime numbers just means that at the moment the decryption problem is just that it takes so long for even a super computer to work out all the possibilities. This site explains the cicada life cycle issue very well.
