Compute Expected Loss

By this time, we have gained an understanding of the assets we value, their possible vulnerabilities, and the likelihood that the vulnerabilities will be exploited. Next, we must determine the likely loss if the exploitation does indeed occur. As with likelihood of occurrence, this value is difficult to determine. Some costs, such as the cost to replace a hardware item, are easy to obtain. The cost to replace a piece of software can be approximated reasonably well from the initial cost to buy it (or specify, design, and write it). However, we must take care to include hidden costs in our calculations. For instance, there is a cost to others of not having a piece of hardware or software. Similarly, there are costs in restoring a system to its previous state, reinstalling software, or deriving a piece of information. These costs are substantially harder to measure.

In addition, there may be hidden costs that involve legal fees if certain events take place. For example, some data require protection for legal reasons. Personal data, such as police records, tax information, census data, and medical information, are so sensitive that there are criminal penalties for releasing the data to unauthorized people. Other data are company confidential; their release may give competitors an edge on new products or on likely changes to the stock price. Some financial data, especially when they reflect an adverse event, could seriously affect public confidence in a bank, an insurance company, or a stock brokerage. We are hard pressed to determine the cost of releasing these data.

If a computing system, a piece of software, or a key person is unavailable, causing a particular computing task to be delayed, there may be serious consequences. If a program that prints paychecks is delayed, employees’ confidence in the company may be shaken, or some employees may face penalties from not being able to pay their own bills. If customers cannot make transactions because the computer is down, they may choose to take their business to a competitor. For some time-critical services involving human lives, such as a hospital’s life-support systems or a space station’s guidance systems, the costs of failure are infinitely high.