## 4 Our Construction
### Scheme Construction
## 8 Conclusions

In this paper, we first conduct a comprehensive analysis of Type-II backward privacy in the aspects of both theoretic definition and practical constructions. We then demonstrate how information leakage that is not covered by the current notions can be utilized to extract the deletion history from Type-II backward private schemes under specific conditions and violate their Type-II privacy in practice. Finally, we propose a novel forward and Type-II backward private DSSE scheme based on Intel SGX, and formally prove the security. The

Figure 5: Retrieval rate of the \(i\)th most frequent keywords. (**a**) Synthetic dataset. (**b**) Enron email dataset. proposed scheme outperforms the previous works in either search latency or security level with negligible utility loss. To the best of our knowledge, this is the first work that investigates the importance of secondary information leakages that are not captured in traditional backward privacy notions and discusses their practical implications on the backward privacy schemes. Conceptualization, H.Y., C.H., D.K. and J.H. ; methodology, H.Y. and M.Y. ; validation, H.Y., C.H., D.K. and J.H. ; formal analysis, H.Y., C.H., D.K. and J.H. ; investigation, H.Y. and M.Y. ; writing--original draft preparation, H.Y. and C.H. ; writing--review and editing, H.Y., C.H., D.K. and J.H. ; supervision, C.H., D.K. and J.H. All authors have read and agreed to the published version of the manuscript. This work was supported as part of the Military Crypto Research Center (UD210027 XD) funded by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD). Not applicable. The data presented in this study are available in [MonashCybersecurityLab /SGXSSE] at [[[https://github.com/MonashCybersecurityLab/SGXSSE](https://github.com/MonashCybersecurityLab/SGXSSE)]([https://github.com/MonashCybersecurityLab/SGXSSE](https://github.com/MonashCybersecurityLab/SGXSSE))], and in [Enron Email Dataset at [[[https://www.cs.cmu.edu/~enron/](https://www.cs.cmu.edu/~enron/)]([https://www.cs.cmu.edu/~enron/](https://www.cs.cmu.edu/~enron/))], reference number [15]. The authors declare no conflicts of interest. ## References

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