Local Differential Privacy for Evolving Data
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There are now several large scale deployments of differential privacy used to track statistical information about users. However, these systems periodically recollect the data and recompute the statistics using algorithms designed for a single use and as a result do not provide meaningful privacy guarantees over long time scales. Moreover, existing techniques to mitigate this effect do not apply in the "local" model of differential privacy that these systems use. In this paper, we introduce a new local differential privacy technique to maintain persistently up-to-date statistics over time, with privacy guarantees scaling only with the number of changes in the underlying distribution rather than the number of collection periods. The key ideas include batching time into epochs -- varying the epoch size allows us to trade off accuracy against frequency of updates -- and a protocol for users to "vote" to update out-of-date statistics while losing very little privacy. We prove our main results for the setting where users hold a single bit, redrawn at every time period, from a common (but changing) distribution; however, our framework is quite general and we give an application to frequency and heavy-hitter estimation.
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