Outsourcing data to external providers has gained momentum with the advent of cloud computing. Encryption\nallows data confidentiality to be preserved when outsourcing data to untrusted external providers that may be\ncompromised by attackers. However, encryption has to be applied in a way that still allows the external provider to\nevaluate queries received from the client. Even though confidential database-as-a-service (DaaS) is still an active field\nof research, various techniques already address this problem, which we call confidentiality preserving indexing\napproaches (CPIs). CPIs make individual tradeoffs between the functionality provided, i.e., the types of queries that can\nbe evaluated, the level of protection achieved, and performance.\nIn this paper, we present a taxonomy of requirements that CPIs have to satisfy in deployment scenarios including the\nrequired functionality and the required level of protection against various attackers. We show that the taxonomy�s\nunderlying principles serve as a methodology to assess CPIs, primarily by linking attacker models to CPI security\nproperties. By use of this methodology, we survey and assess ten previously proposed CPIs. The resulting CPI catalog\ncan help the reader who would like to build DaaS solutions to facilitate DaaS design decisions while the proposed\ntaxonomy and methodology can also be applied to assess upcoming CPI approaches.
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