In the aftermath of Judgment Day, we fled to the Exclusion Zone and sought refuge underground. This is what is left of our fallen civilization.

Introduction to Cryptography by Christof Paar

https://www.youtube.com/channel/UC1usFRN4LCMcfIV7UjHNuQg

https://link.springer.com/book/10.1007/978-3-642-04101-3