## 1 Introduction  
Dynamic searchable symmetric encryption (DSSE) is a kind of searchable symmetric encryption (SSE) specifically designed to support data dynamics such as addition and deletion operations in an encrypted database (EDB) [1; 2]. Although DSSE schemes benefit from the flexible operations without decryption, they are likely to leak sensitive information. For example, an adversary can observe added or deleted documents that are accessed by users by exploiting the access pattern leakage [3], or identify the underlying keyword of queries by exploiting the search pattern leakage [3]. To formally address this information leakage problem, [PERSON] et al. [4; 5] introduced the notions of forward and three different types (i.e., Type-I, II, III) of backward privacy in DSSE, mainly focusing on addressing security concerns regarding the linkability between queries (e.g., update and search) and updated data. Although leaking less information implies higher security, it inevitably incurs higher computational overhead. For example, Type-I backward privacy has been only achieved by adopting cryptographically heavy operations such as oblivious RAM (ORAM) [6]. Recently, a trusted execution environment (TEE) such as Intel SGX [7] has been considered for DSSE to mitigate efficiency problems caused in favor of the forward/backward privacy preservation [8; 9; 10; 11]. [PERSON] et al. [8] proposed several forward and various types of backward private DSSE schemes using Intel SGX. Their first scheme, called Fort, achieves the highest security guarantee (i.e., Type-I), but suffers from high communication overhead due to the usage of ORAM [6]. To balance security and efficiency, they proposed another scheme, called Bunker-B (Type-II), aiming to reduce communication costs. However, it still suffers from scalability degradation. To resolve this problem, [PERSON] et al. proposed SGX-SE1, providing Type-II backward privacy [11], and Maiden providing Type-I backward privacy [12], both leveraging the server-side SGX enclave as a proxy to reduce the communication cost and enhance the scalability of the scheme. Unfortunately, despite its efficiency improvement, we observed that SGX-SE1 does not fully guarantee Type-II backward privacy as expected. SGX-SE1 has additional information leakage related to deletion history, which is information leakage only allowed in Type-III backward privacy, by exploiting secondary leakage, which is naturally allowed in the protocol construction. Thus, in this paper, we seek to answer the following questions: _What is the root cause or conditions for the leakage, and how can it be exploited to break the backward privacy of DSSE schemes?_  
In search of the answer, we first conduct an in-depth analysis of the information leakages in existing Type-II backward private DSSE schemes [8; 11; 13; 14] in terms of both theoretical definitions and scheme constructions. Consequently, we found that there exist information leakages in several schemes [11; 14], which can be utilized to extract a deletion history of the encrypted data, leading to violation of Type-II backward privacy. Then, we examine the condition allowing the vulnerability, and demonstrate how it can affect the Type-II backward privacy in practice by exploiting the secondary information leakage on [PERSON] et al. 's [11] and [PERSON] et al. 's [14] DSSE schemes. Next, we propose a novel forward and Type-II backward private DSSE scheme based on SGX. To this end, we design an obfuscation technique to hide the access and the search patterns in order to prevent the information leakages related to update operations. To minimize extra communication and computation costs caused by the obfuscation technique, we selectively cache the top \(k\)-frequently accessed documents inside the SGX enclave for fast data retrieval and obfuscation.