A "Game-Theoretic" Analysis of De-perimeterization

De-perimeterization is a word which (despite being impossible to pronounce or spell correctly) is used more and more in discussions about security of organizations. Studying the effects of the disappearing perimeter in practice is difficult because organizations are complex and it is difficult to measure the quality of newly deployed security measures. Instead, let’s describe some of the issues of de-perimeterization here using an analogy with the well known board game Risk.

In Risk players occupy countries by placing armies on them. Given a configuration of the board where every player has a number of countries with armies, players can attack countries owned by other players from a neighboring country. If all armies of the defending player are completely defeated then that country is conquered and the attacker can place a number of armies on it.

Although luck is certainly a factor (the game uses no less than five dice) the general rule is that the more armies you bring to a fight, the bigger the odds that the country will (still) be yours at the end of the attack. When attacking, a great number of armies can be moved on to the newly conquered country. Armies can also be moved from one country to a neighboring country if owned by the same player when not attacking, but the number of movements is limited per turn. Playing Risk demonstrates that logistics is one of the most difficult parts of administering security.

Countries are organized in six continents. Continents are a lot like organizations: they contain assets (countries, armies) and they have a perimeter. A player receives bonus armies at the start of every turn in which a continent was completely owned by that player and was successfully defended.

Countries on the border of a continent form the perimeter of that continent. Perimeter countries need special attention because enemies need to first travel through perimeter countries before they can attack an inner country. Recall that if an attacker occupies any country of a continent held by a player, then the defender will not get his bonus at the beginning of their next turn. For the defender, moving most armies to the border countries seems therefore a good strategy. We will call this strategy Perimeter Defense.

At first, Perimeter Defense seems like a good idea. All players are each other’s enemies, after all. In practice, however, what happens is that players form temporary alliances so as to effectively attack a common enemy. The common enemy is typically the player with the most armies. This means, for example, that the members of an alliance agree to follow a certain attack strategy and agree not to attack each other for a number of turns so that they can keep borders between alliance-owned continents minimally manned. The armies no longer needed to defend alliance-owned borders can be better used to attack the common enemy with greater force.

But there are far more complex forms of cooperation possible within an alliance. A pattern that is often seen is that one player in the alliance allows another player to move troops over territory owned by the first player. The first player creates a corridor of countries occupied with only 1 army on them. The countries in the corridor are easily conquered by the second player when he attacks them with a great number of armies. Since moving armies during an attack is free, this allows a player to move a great number of troops towards the common enemy’s border, circumventing the per-turn troop movement limits. The second player also leaves only 1 army on the countries in the corridor, allowing the first player to easily recover the original countries of his continent later on.

So what are the alternatives to perimeter defense? It is tempting to think of Defense in Depth as the complete opposite of Perimeter Defense. In the Risk analogy naive Defense in Depth means equally distributing one’s armies over every country of a continent, both inner and border countries. Obviously this means that it becomes easier for a single enemy to occupy a border country (which means the defender won’t get his bonus armies). Yet at least the continent is more difficult to completely conquer by attackers. It very much depends on the situation (the agenda of other players, alliance agreements) whether Defense in Depth is a good strategy.

Defense in Depth also makes it more difficult to move armies to specific places, for example to allow a fellow alliance member to move troops across your continent. Yet, if one doesn’t completely trust the other players in the alliance a certain degree of Defense in Depth is actually a good thing. After all, when alliance member are moving troops through our corridors they should not be tempted too much to occupy our complete continent while they’re at it.

The real world consisting of real organizations is in many aspects much more complex than than simple board game world, if only because the goals of organizations are much more complex than simply ‘winning the game’. Still, real organizations also deal with security strategies. Two organizations will work together if it is of benefit to both of them (although usually not to mount an attack on the security perimeter of some competitor). At the same time organizations need to restrict access to their assets from outsiders as much as possible.

The problem is not that the perimeter is disappearing. The problem is that it is continually changing. The quality of a security strategy depends greatly on external forces such as the goals of other organizations. That these external forces change dynamically makes things even more complex.

Perimeter Defense and Defense in Depth are still good concepts to use when defining a mixed security strategy but much more important seems to be the ability to quickly change strategy. If security controls are resilient rather than brittle (see Schneier’s book Beyond Fear for an explanation of these concepts) then they can easily be used as part of a dynamically configurable perimeter.

(Thanks to Tim, Marcella, Victor, Suzana, Dragan, and Georgi for playing numerous games of Risk. Disclaimer: The author lost most of these games.)