Query Obfuscation by Semantic Decomposition

Proceedings of the 13th Conference on Language Resources and Evaluation (LREC 2022), pages 6200–6211 Marseille, 20-25 June 2022 © European Language Resources Association (ELRA), licensed under CC-BY-NC-4.0 6200 Query Obfuscation by Semantic Decomposition Danushka Bollegala1,2, Tomoya Machide3, Ken-ichi Kawarabayashi3 University of Liverpool1, Amazon2, National Institute of Informatics3 danushka@liverpool.ac.uk, machide@nii.ac.jp, k keniti@nii.ac.jp Abstract We propose a method to protect the privacy of search engine users by decomposing the queries using semantically related and unrelated distractor terms. Instead of a single query, the search engine receives multiple decomposed query terms. Next, we reconstruct the search results relevant to the original query term by aggregating the search results retrieved for the decomposed query terms. We show that the word embeddings learnt using a distributed representation learning method can be used to find semantically related and distractor query terms. We derive the relationship between the obfuscity achieved through the proposed query anonymisation method and the reconstructability of the original search results using the decomposed queries. We analytically study the risk of discovering the search engine users’ information intents under the proposed query obfuscation method, and empirically evaluate its robustness against clustering-based attacks. Our experimental results show that the proposed method can accurately reconstruct the search results for user queries, without compromising the privacy of the search engine users. Keywords: Query Obfuscation, Information Retrieval, Word Embeddings, Reconstrutability 1. Introduction As web search engine users, we are left with two options regarding our privacy. First, we can trust the search en- gine not to disclose the keywords that we use in a search session to third parties, or even to use for any other purpose other than providing search results to the users who issued the queries. However, the user agreements in most web search engines do not allow such user rights. Although search engines pledge to protect the privacy of their users by encrypting queries and search results,1 the encryption is between the user and the search engine – the original non-encrypted queries are still available to the search engine. The keywords issued by the users are a vital source of information for improving the relevancy of the search engine and displaying relevant adverts to the users. For example, in learning to rank (He et al., 2008), keywords issued by a user and the documents clicked by that user are recorded by the search engine to learn the optimal dynamic ranking of the search results, user interests and extract attributes related to frequently searched entities (Pasca, 2014; Sadikov et al., 2010; Santos et al., 2010; Richardson, 2008; Pasca, 2007). Considering the fact that placing advertisements for the highly bid keywords is one of the main revenue sources for search engines, there are obvious commer- cial motivations for the search engines to exploit the user queries beyond simply providing relevant search results to their users. For example, it has been reported that advertisements contribute to 96% of Google’s revenue.2 Therefore, it would be unrealistic to assume that the user queries will not be exploited in a …

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