Hoofdstuk 14. Beveiliging

Het belang van een informatie systeem is afhankelijk van de omgeving waarin het gebruikt wordt. In sommige gevallen is het van vitaal belang voor het voortbestaan van een bedrijf. Dan moet het beschermd worden tegen een veelvoud van gevaren. Het proces van evalueren van de gevaren, en het definiëren en implementeren van de beveiliging tegen deze gevaren wordt samengevat als het "beveiligingsproces".

14.1. Vastleggen van het beveiligingsbeleid

Het woord "beveiliging" omvat een enorme reikwijdte aan concepten, gereedschappen en procedures, die geen van allen universeel toepasbaar zijn. Om hieruit te kunnen kiezen dien je een precies omschreven doel voor ogen te hebben. Het beveiligen van een systeem begint met het beantwoorden van enkele vragen. Als een kip zonder kop een willekeurige set beveiligingsinstrumenten implementeren levert het risico dat je je focus op de verkeerde aspecten van de beveiliging richt.

Het eerste wat men dus moet vastleggen is het doel. Een goede manier om daarmee te beginnen is door het beantwoorden van de volgende vragen:

Wat willen we beschermen? Het beveiligingsbeleid zal anders zijn afhankelijk van of we computers of data willen beveiligen. In het laatste geval moeten we ook weten welke data.
Waar willen we tegen beschermen? Tegen het lekken van vertrouwelijke gegevens? Abusievelijk gegevens verlies? Verlies van omzet omdat services niet beschikbaar zijn?
Daarnaast, tegen wie willen we beschermen? Beveiligingsmaatregelen zijn sterk afwijkend voor het detecteren van een type fout van een gewone gebruiker van het systeem, of voor het beschermen tegen een geconcentreerde groep aanvallers.

The term “risk” is customarily used to refer collectively to these three factors: what to protect, what needs to be prevented from happening, and who will try to make it happen. Modeling the risk requires answers to these three questions. From this risk model, a security policy can be constructed, and the policy can be implemented with concrete actions.

NOTE Permanent questioning

Bruce Schneier, a world expert in security matters (not only computer security) tries to counter one of security's most important myths with a motto: “Security is a process, not a product”. Assets to be protected change in time, and so do threats and the means available to potential attackers. Even if a security policy has initially been perfectly designed and implemented, one should never rest on one's laurels. The risk components evolve, and the response to that risk must evolve accordingly.

Extra constraints are also worth taking into account, as they can restrict the range of available policies. How far are we willing to go to secure a system? This question has a major impact on the policy to implement. The answer is too often only defined in terms of monetary costs, but the other elements should also be considered, such as the amount of inconvenience imposed on system users or performance degradation.

Once the risk has been modeled, one can start thinking about designing an actual security policy.

NOTE Extreme policies

There are cases where the choice of actions required to secure a system is extremely simple.

For instance, if the system to be protected only comprises a second-hand computer, the sole use of which is to add a few numbers at the end of the day, deciding not to do anything special to protect it would be quite reasonable. The intrinsic value of the system is low. The value of the data is zero since they are not stored on the computer. A potential attacker infiltrating this “system” would only gain an unwieldy calculator. The cost of securing such a system would probably be greater than the cost of a breach.

At the other end of the spectrum, we might want to protect the confidentiality of secret data in the most comprehensive way possible, trumping any other consideration. In this case, an appropriate response would be the total destruction of these data (securely erasing the files, shredding of the hard disks to bits, then dissolving these bits in acid, and so on). If there is an additional requirement that data must be kept in store for future use (although not necessarily readily available), and if cost still isn't a factor, then a starting point would be storing the data on iridium–platinum alloy plates stored in bomb-proof bunkers under various mountains in the world, each of which being (of course) both entirely secret and guarded by entire armies…

Extreme though these examples may seem, they would, nevertheless, be an adequate response to defined risks, insofar as they are the outcome of a thought process that takes into account the goals to reach and the constraints to fulfill. When coming from a reasoned decision, no security policy is less respectable than any other.

In most cases, the information system can be segmented in consistent and mostly independent subsets. Each subsystem will have its own requirements and constraints, and so the risk assessment and the design of the security policy should be undertaken separately for each. A good principle to keep in mind is that a short and well-defined perimeter is easier to defend than a long and winding frontier. The network organization should also be designed accordingly: the sensitive services should be concentrated on a small number of machines, and these machines should only be accessible via a minimal number of check-points; securing these check-points will be easier than securing all the sensitive machines against the entirety of the outside world. It is at this point that the usefulness of network filtering (including by firewalls) becomes apparent. This filtering can be implemented with dedicated hardware, but a possibly simpler and more flexible solution is to use a software firewall such as the one integrated in the Linux kernel.