Encryption was export-restricted until 1996. Actually that isn't really relevant - most of the systems the authors are talking about (and that they worked on)

Encryption was export-restricted until 1996. Actually that isn't really relevant - most of the systems the authors are talking about (and that they worked on) would have been running on enterprise or campus LANs, and it wasn't totally clear (at least to the sysadmins at the place where I worked) whether it was even legal to connect corporate networks to the internet at that point in time if you weren't a govt contractor. Based on what I recall, most of the security threats we were worried about had to do with corporate espionage and information theft, and would have required physical access to the network. In that environment, encryption isn't important in the same way as it is on today's Internet.