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PrivMail: A Privacy-Preserving Framework for Secure Emails

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Computer Security – ESORICS 2023 (ESORICS 2023)

Part of the book series: Lecture Notes in Computer Science ((LNCS,volume 14345))

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Abstract

Emails have improved our workplace efficiency and communication. However, they are often processed unencrypted by mail servers, leaving them open to data breaches on a single service provider. Public-key based solutions for end-to-end secured email, such as Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME), are available but are not widely adopted due to usability obstacles and also hinder processing of encrypted emails.

We propose PrivMail, a novel approach to secure emails using secret sharing methods. Our framework utilizes Secure Multi-Party Computation techniques to relay emails through multiple service providers, thereby preventing any of them from accessing the content in plaintext. Additionally, PrivMail supports private server-side email processing similar to IMAP SEARCH, and eliminates the need for cryptographic certificates, resulting in better usability than public-key based solutions. An important aspect of our framework is its capability to enable third-party searches on user emails while maintaining the privacy of both the email and the query used to conduct the search.

To evaluate our solution, we benchmarked transfer and search operations using the Enron Email Dataset and demonstrate that PrivMail is an effective solution for enhancing email security.

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Notes

  1. 1.

    Mostly the subject and content fields but not other meta data.

  2. 2.

    https://encrypto.de/code/PrivMail.

  3. 3.

    The older Post Office Protocol (POP) downloads the email from the server and optionally deletes it from the server, but in contrast to IMAP provides no server-side search.

  4. 4.

    Later in Sect. 3.2 we describe an optimization to send a seed for a Pseudo Random Function (PRF) instead of the whole share \(\textsf{E}_{1}\).

  5. 5.

    The agent \(\mathcal {A}\) generates the shares of the mask and sends them with the keyword shares.

  6. 6.

    https://encrypto.de/code/PrivMail.

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Acknowledgements

This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 850990 PSOTI). It was co-funded by the Deutsche Forschungsgemeinschaft (DFG) within SFB 1119 CROSSING/236615297 and GRK 2050 Privacy & Trust/251805230.

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Authors and Affiliations

  1. ENCRYPTO, Technical University of Darmstadt, Darmstadt, Germany

    Gowri R. Chandran, Raine Nieminen & Thomas Schneider

  2. Cryptography Research Centre, Technology Innovation Institute, Abu Dhabi, UAE

    Ajith Suresh

Authors
  1. Gowri R. Chandran
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  2. Raine Nieminen
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  3. Thomas Schneider
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  4. Ajith Suresh
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Correspondence to Gowri R. Chandran .

Editor information

Editors and Affiliations

  1. University of California, Irvine, CA, USA

    Gene Tsudik

  2. University of Padua, Padua, Italy

    Mauro Conti

  3. Delft University of Technology, Delft, The Netherlands

    Kaitai Liang

  4. Delft University of Technology, Delft, The Netherlands

    Georgios Smaragdakis

A Comparison of the Different Search Techniques

A Comparison of the Different Search Techniques

Each of the search techniques discussed in Sect. 4, i.e., circuit-based, bucketing-based, and indexing-based, have their own pros and cons depending on the keywords being searched. The user or the email client can therefore choose the most beneficial technique according to their requirements. In Table 2, we give a comparison between the different techniques for various use-cases, highlighting the most efficient techniques for each use-case.

Table 2. Efficiency comparison of the different search techniques for different types of keywords. The most efficient technique is marked in bold.
Full size table

Performance of Circuit-Based Search. Table 3 summarizes our benchmarks for search across four different keyword lengths, \(s\in \{3,8,13,18\}\) (corresponding to the average of our bucket sizes), on email sets of sizes 100 and 200. The total computation and communication overheads grow proportionally to the keyword length and number of emails in the sets as the search circuit size grows.

Table 3. Evaluation of circuit-based search (Sect. 4.2). Runtime (Time) is in seconds and communication (Comm.) between the servers in mebibytes (MiB).
Full size table

We parallelize each equality test circuit (see Eq. (1)) with Single Instruction, Multiple Data (SIMD) operations, which results in an almost linear total runtime with respect to the keyword length. The minor difference in online runtime is caused by runtime fluctuations in our WAN simulation and can be evened out with additional iterations. The remaining cumulative OR in Eq. (2) dominates the online runtime, giving a nearly constant runtime.

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Chandran, G.R., Nieminen, R., Schneider, T., Suresh, A. (2024). PrivMail: A Privacy-Preserving Framework for Secure Emails. In: Tsudik, G., Conti, M., Liang, K., Smaragdakis, G. (eds) Computer Security – ESORICS 2023. ESORICS 2023. Lecture Notes in Computer Science, vol 14345. Springer, Cham. https://doi.org/10.1007/978-3-031-51476-0_8

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