Cryptology ePrint Archive: Report 2021/268

Puncture 'Em All: Stronger Updatable Encryption with No-Directional Key Updates

Daniel Slamanig and Christoph Striecks

Abstract: Updatable encryption (UE; CRYPTO 2013) is a symmetric encryption primitive that allows to periodically rotate encryption keys without the need to decrypt and re-encrypt already encrypted data. This is achieved by means of an update token that allows to perform the ciphertext update. In existing UE constructions, the update token thereby allows bi-directional updates of keys and ciphertexts, which leads to undesired information leakage and rather involved security models. A recent work by Jiang (ASIACRYPT 2020) shows that in the currently strongest UE model due to Boyd et al. (CRYPTO 2020), UE with bi-directional key and ciphertext updates implies schemes with uni-directional ones. While this might suggests that uni-directionality does not add security, we show that this rather stems from a defective security model and in an adequate model uni-directionality is indeed stronger. Irrespective of this fact, even uni-directional UE schemes still do not capture the intuitive security requirements expected from UE. To overcome this leakage problem and obtain natural security guarantees, UE schemes with so-called no-directional key updates are necessary, i.e., where tokens can solely update ciphertexts and only in one direction. However, it stayed unclear whether such UE schemes can be constructed and this tasks is presented as a challenging open problem in both aforementioned works.

In this work, we resolve these issues and present the first UE constructions with uni- and even no-directional key updates. We show that such UE schemes can be constructed in the standard model via the notion of dual system groups from the standard d-Lin assumption in prime-order bilinear groups. Our approach of constructing UE significantly departs from previous ones and in particular views UE from the perspective of puncturable encryption (Green and Miers, S&P 2015). Towards constructing UE, as an stepping stone, we introduce a variant of puncturable encryption that additionally support puncturing of ciphertexts. This turns out to be a useful abstraction on our way to construct UE and may be of independent interest.

Category / Keywords: public-key cryptography / Updatable Encryption, Puncturable Encryption, Dual System Encryption

Date: received 3 Mar 2021

Contact author: Christoph Striecks at ait ac at,Daniel Slamanig@ait ac at

Available format(s): PDF | BibTeX Citation

Version: 20210303:195236 (All versions of this report)

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