Protecting privacy by adding noise

Fascinating piece by FTC technologist Ed Felten that I just got around to reading:

When I wrote previously about differential privacy, a mathematical framework that allows rigorous reasoning about privacy preservation, I promised to work through an example to show how the theory works. Here goes.

Suppose that Alice has access to a detailed database about everyone in the United States. Bob wants to do some statistical analysis, to get aggregate statistics about the population. But Alice wants to make sure Bob can’t infer anything about an individual. Rather than giving Bob the raw data—which would surely undermine privacy—Alice will let Bob send her queries about the data, and Alice will answer Bob’s queries. The trick is to make sure that Alice’s answers don’t leak private information.