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Differential privacy is a mathematical definition for the privacy loss that results to individuals when their private information is used in the creation of a data product. The term was coined by Cynthia Dwork in 2006, but the correct reference is actually an earlier publication by Dwork, Frank McSherry, Kobbi Nissim and Adam D. Smith. The work is based, in part, on work by Nissim and Irit Dinur which showed that it is impossible to publish information from a private statistical database without revealing some amount of private information, and that the entire database can be revealed by publishing the results of a surprisingly small number of queries.
A result of Dinur and Nissim's "database reconstruction" work was the realization that the approach of providing privacy in statistical databases using semantic definitions of privacy (mostly dating to work of Tore Dalenius in the 1970s) was impossible, and that new techniques for limiting the increased privacy risk resulting from inclusion of private data in a statistical database needed to be developed. A result of the work and subsequent research is the development of techniques that make it possible, in many cases, to provide very accurate statistics from the database while still ensuring high levels of privacy.
Principle and illustrationEdit
Differential confidentiality is a process that introduces randomness into the data.
A simple example, especially developed in the social sciences, is to ask a person to answer the question "Do you own the attribute A?", according to the following procedure:
- Throw a coin.
- If head, then answer honestly.
- If tail, then throw the coin again and answer "Yes" if head, "No" if tail.
The confidentiality arises from the refutability of the individual responses.
But, overall, these data with many responses are significant, since positive responses are given to a quarter by people who do not have the attribute A and three-quarters by people who actually possess it. Thus, if p is the true proportion of people with A, then we expect to obtain (1/4)(1-p) + (3/4)p = (1/4) + p/2 positive responses. Hence it is possible to estimate p.
In particular, if the attribute A is synonymous with illegal behavior, then answering "Yes" is not incriminating, insofar as the person has a probability of a "Yes" response, whatever it may be.
Although this example, inspired by randomized response, might be applicable to microdata (i.e., releasing datasets with each individual response), by definition differential privacy excludes microdata release and is only applicable to queries (i.e., aggregating individual responses into one result) as this would violate the requirements, more specifically the plausible deniability that a subject participated or not.
Formal definition and example applicationEdit
Let be a positive real number and be a randomized algorithm that takes a dataset as input (representing the actions of the trusted party holding the data). Let denote the image of . The algorithm is -differentially private if for all datasets and that differ on a single element (i.e., the data of one person), and all subsets of ,
According to this definition, differential privacy is a condition on the release mechanism (i.e., the trusted party releasing information about the dataset) and not on the dataset itself. Intuitively, this means that for any two datasets that are similar, a given differentially private algorithm will behave approximately the same on both datasets. The definition gives a strong guarantee that presence or absence of an individual will not affect the final output of the algorithm significantly.
For example, assume we have a database of medical records where each record is a pair (Name, X), where is a Boolean denoting whether a person has diabetes or not. For example:
|Name||Has Diabetes (X)|
Now suppose a malicious user (often termed an adversary) wants to find whether Chandler has diabetes or not. Suppose he also knows in which row of the database Chandler resides. Now suppose the adversary is only allowed to use a particular form of query that returns the partial sum of the first rows of column in the database. In order to find Chandler's diabetes status the adversary executes and , then computes their difference. In this example, and , so their difference is 1. This indicates that the "Has Diabetes" field in Chandler's row must be 1. This example highlights how individual information can be compromised even without explicitly querying for the information of a specific individual.
Continuing this example, if we construct by replacing (Chandler, 1) with (Chandler, 0) then this malicious adversary will be able to distinguish from by computing for each dataset. If the adversary were required to receive the values via an -differentially private algorithm, for a sufficiently small , then he or she would be unable to distinguish between the two datasets.
Let be a positive integer, be a collection of datasets, and be a function. The sensitivity  of a function, denoted , is defined by
where the maximum is over all pairs of datasets and in differing in at most one element and denotes the norm.
In the example of the medical database above, if we consider to be the function , then the sensitivity of the function is one, since changing any one of the entries in the database causes the output of the function to change by either zero or one.
There are techniques (which are described below) using which we can create a differentially private algorithm for functions with low sensitivity.
Trade-off between utility and privacyEdit
There is a trade-off between the accuracy of the statistics estimated in a privacy-preserving manner, and the privacy parameter ε. This tradeoff must also account the number of queries (done and expected in the future), by multiplying the epsilon parameter to the number of queries.
Other notions of differential privacyEdit
Since differential privacy is considered to be too strong for some applications, many weakened versions of privacy have been proposed. These include (ε, δ)-differential privacy, randomised differential privacy, and privacy under a metric.
Differentially private mechanismsEdit
Since differential privacy is a probabilistic concept, any differentially private mechanism is necessarily randomized. Some of these, like the Laplace mechanism, described below, rely on adding controlled noise to the function that we want to compute. Others, like the exponential mechanism and posterior sampling sample from a problem-dependent family of distributions instead.
The Laplace mechanismEdit
Many differentially private methods add controlled noise to functions with low sensitivity. The Laplace mechanism adds Laplace noise (i.e. noise from the Laplace distribution, which can be expressed by probability density function , which has mean zero and standard deviation ). Now in our case we define the output function of as a real valued function (called as the transcript output by ) as where and is the original real valued query/function we planned to execute on the database. Now clearly can be considered to be a continuous random variable, where
which is at most . We can consider to be the privacy factor . Thus follows a differentially private mechanism (as can be seen from the definition above). If we try to use this concept in our diabetes example then it follows from the above derived fact that in order to have as the -differential private algorithm we need to have . Though we have used Laplacian noise here, other forms of noise, such as the Gaussian Noise, can be employed, but they may require a slight relaxation of the definition of differential privacy.
If we query an ε-differential privacy mechanism times, and the randomization of the mechanism is independent for each query, then the result would be -differentially private. In the more general case, if there are independent mechanisms: , whose privacy guarantees are differential privacy, respectively, then any function of them: is -differentially private.
Furthermore, if the previous mechanisms are computed on disjoint subsets of the private database then the function would be -differentially private instead.
In general, ε-differential privacy is designed to protect the privacy between neighboring databases which differ only in one row. This means that no adversary with arbitrary auxiliary information can know if one particular participant submitted his information. However this is also extendable if we want to protect databases differing in rows, which amounts to adversary with arbitrary auxiliary information can know if particular participants submitted their information. This can be achieved because if items change, the probability dilation is bounded by instead of , i.e., for D1 and D2 differing on items:
Thus setting ε instead to achieves the desired result (protection of items). In other words, instead of having each item ε-differentially private protected, now every group of items is ε-differentially private protected (and each item is -differentially private protected).
A transformation is -stable if the hamming distance between and is at most -times the hamming distance between and for any two databases . Theorem 2 in  asserts that if there is a mechanism that is -differentially private, then the composite mechanism is -differentially private.
This could be generalized to group privacy, as the group size could be thought of as the hamming distance between and (where contains the group and doesn't). In this case is -differentially private.
Adoption of differential privacy in real-world applicationsEdit
Several uses of differential privacy in practice are known to date:
- U.S. Census Bureau, for showing commuting patterns.
- Google's RAPPOR, for telemetry such as learning statistics about unwanted software hijacking users' settings  (RAPPOR's open-source implementation).
- Google, for sharing historical traffic statistics.
- On June 13, 2016 Apple announced its intention to use differential privacy in iOS 10 to improve its intelligent assistance and suggestions technology.
- Some initial research has been done into practical implementations of differential privacy in data mining models.
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- Irit Dinur and Kobbi Nissim. 2003. Revealing information while preserving privacy. In Proceedings of the twenty-second ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems (PODS '03). ACM, New York, NY, USA, 202–210. DOI=https://dx.doi.org/10.1145/773153.773173
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