While storing documents on the cloud can be attractive, the question remains whether cloud providers can be trusted with storing private documents. Even if trusted, data breaches are ubiquitous. To prevent information leakage one can store documents encrypted. If encrypted under traditional schemes, one loses the ability to perform simple operations over the documents, such as searching through them. Searchable encryption schemes were proposed allowing some search functionality while documents remain encrypted. Orthogonally, research is done to find attacks that exploit search and access pattern leakage that most efficient schemes have. One type of such an attack is the ability to recover plaintext queries. Passive query-recovery attacks on single-keyword search schemes have been proposed in literature, however, conjunctive keyword search has not been considered, although keyword searches with two or three keywords appear more frequently in online searches. We introduce a generic extension strategy for existing passive query-recovery attacks against single-keyword search schemes and explore its applicability for the attack presented by Damie et al. (USENIX Security '21). While the original attack achieves up to a recovery rate of 85% against single-keyword search schemes for an attacker without exact background knowledge, our experiments show that the generic extension to conjunctive queries comes with a significant performance decrease achieving recovery rates of at most 32%. Assuming a stronger attacker with partial knowledge of the indexed document set boosts the recovery rate to 85% for conjunctive keyword queries with two keywords and achieves similar recovery rates as previous attacks by Cash et al. (CCS '15) and Islam et al. (NDSS '12) in the same setting for single-keyword search schemes.

翻译：虽然将文档存储在云端具有吸引力，但云服务提供商能否信任地存放私密文档仍存疑问。即使信任存在，数据泄露也屡见不鲜。为防止信息泄露，可对文档进行加密存储。若采用传统加密方案，则将丧失对文档执行简单操作（如搜索）的能力。可搜索加密方案应运而生，允许在文档保持加密状态的同时提供部分搜索功能。与之正交地，研究者正致力于发现利用大多数高效方案中存在的搜索及访问模式泄露的攻击手段。其中一类攻击是恢复明文查询的能力。文献中已提出针对单关键词搜索方案的被动查询恢复攻击，但连接关键词搜索尚未被考虑，尽管在线搜索中两关键词或三关键词的查询出现更为频繁。我们提出一种通用扩展策略，用于现有针对单关键词搜索方案的被动查询恢复攻击，并探索其在Damie等人（USENIX Security '21）提出的攻击中的适用性。原始攻击对缺乏精确背景知识的攻击者针对单关键词搜索方案可实现高达85%的恢复率，而我们的实验表明，该通用扩展应用于连接查询时性能显著下降，恢复率最高仅为32%。假设攻击者拥有对被索引文档集的部分知识（即更强的攻击者），则对包含两个关键词的连接关键词查询可提升恢复率至85%，并在相同设定下达到与Cash等人（CCS '15）及Islam等人（NDSS '12）针对单关键词搜索方案的先前攻击相当的恢复率。