Boštjan Šuštar, one of our security gurus, started a formidable journey: he will document all Cisco IOS IPSec design architecture options in a series of IP Corner articles. In the first article, he's describing the venerable technology available "forever" in Cisco IOS – the static and dynamic crypto-maps. I particularly like the introductory paragraph explaining the need for IP encryption:
Once upon a time, in the land of IP, there was a wide area network (WAN) providing connectivity between clients and servers, and all was well. Then, suddenly, bad things started to happen, and paranoia spread throughout the land. Firewalls grew around hamlets to protect them from the unknown beyond the realm of calm, but then packets were forced to travel thorough the dark forests of the WAN. There was a need to provide them with protection.