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## 19.5 OBFUSCATION TECHNIQUES FOR USER-PRIVACY

To guarantee user location privacy, we introduce three basic obfuscation techniques that modify a user location to reduce the associated relevance (henceforth the accuracy) until a given level.

### 19.5.1 Obfuscation by Enlarging the Radius

Obfuscating a location measurement area by increasing its radius (see Figure 19.3(a)) is the technique that most solutions exploit, either explicitly or implicitly by scaling a location to a coarser granularity (e.g., from few meters to hundred of meters, from a city block to the whole town, and so on). The obfuscation is a probabilistic effect due to a decrease in the corresponding joint probability density function (joint pdf), which can be expressed as ∀r, r′, r < r′ : fr(x, y) > fr(x, y). The following proposition allows us to calculate the obfuscated area.

Table 19.2 Examples of access control policies for a healthcare scenario

Proposition 19.1. Given a location area of radius r with relevance Init and an obfuscated area of radius rderived by enlarging the original radius, the relevance Final of the obfuscated area is calculated by reducing Init of the ratio fr(x, y)/fr(x, y) of corresponding joint ...

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