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Dynamic Multi-server Updatable Encryption

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Information Security (ISC 2023)

Part of the book series: Lecture Notes in Computer Science ((LNCS,volume 14411))

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Abstract

In this paper, we propose the Dynamic Multi-Server Updatable Encryption (DMUE) primitive as an extension of standard public-key updatable encryption. Traditional UE aims to have efficient ciphertext updates performed by an untrusted server such that the compromise of several cryptographic keys and update tokens does not reduce the standard security of encryption. The update token supports outsourced ciphertext updates without requiring the server to decrypt and re-encrypt the ciphertext and it is typically derived from old and new keys. To mitigate the risk of a single point of failure in single-server UE and thus improve the resilience of the scheme, we formalise a multi-server variant of UE to treat the issue of token leakage. We can achieve a distributed update process by providing each server with an update token and requiring a threshold of servers to engage honestly. However, servers may act dishonestly or need to be replaced over time, so our primitive must cater to dynamic committee changes in the servers participating across epochs. Inspired by the work of Benhamouda et al. (TCC’20) on dynamic proactive secret sharing, we propose a generic DMUE scheme built from public-key UE and dynamic proactive secret sharing primitives and prove the ciphertext unlinkability of freshly encrypted versus updated ciphertexts.

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Notes

  1. 1.

    The authors of [11, 12] established definitions for updatable public-key encryption (UPKE) using an alternative update procedure. One can view UPKE as a distinct primitive to PKUE [20] since the token mechanism used in a UPKE scheme only updates the public key. By contrast, PKUE updates public and secret key pairs as well as the ciphertext.

  2. 2.

    We note that in the multi-server setting, the update process is interactive and is therefore a protocol. However, we chose to use the term algorithm to stay in keeping with the single-server PKUE terminology as \(\varPi _{\textsf {DMUE}}\) can reduce to the single-server setting when \(n=t=1\).

  3. 3.

    Note in the definition of DMUE that the data owner chooses the committee of servers \(\{S_{e_{i+1}}\}_{\forall {i}\in \mathbb {N}}\).

  4. 4.

    A predicate is a statement or mathematical assertion that contains variables. The outcome of the predicate may be true or false depending on the input values.

  5. 5.

    Algorithm \(\textsf {UpdateCh}\) is used as compact notation, following the notation of [8], to denote the process of repeated application of the update algorithm from epoch \(\{e+1,\ldots ,\tilde{e}\}\).

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Authors and Affiliations

  1. University of Surrey, Guildford, England

    Jodie Knapp

  2. Information Security Group, Royal Holloway, University of London, London, England

    Jodie Knapp & Elizabeth A. Quaglia

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  1. Jodie Knapp
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  2. Elizabeth A. Quaglia
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Correspondence to Jodie Knapp .

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Editors and Affiliations

  1. University of Cyprus, Nicosia, Cyprus

    Elias Athanasopoulos

  2. Radboud University, Nijmegen, The Netherlands

    Bart Mennink

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Knapp, J., Quaglia, E.A. (2023). Dynamic Multi-server Updatable Encryption. In: Athanasopoulos, E., Mennink, B. (eds) Information Security. ISC 2023. Lecture Notes in Computer Science, vol 14411. Springer, Cham. https://doi.org/10.1007/978-3-031-49187-0_24

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  • DOI: https://doi.org/10.1007/978-3-031-49187-0_24

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