Natanael

See also discussion here; https://reddit.com/comments/1jv572r

Cryptology ePrint Archive
Paper 2025/585
Adaptively-Secure Big-Key Identity-Based Encryption
Jeffrey Champion, The University of Texas at Austin
Brent Waters, The University of Texas at Austin, NTT Research
David J. Wu, The University of Texas at Austin

Abstract
Key-exfiltration attacks on cryptographic keys are a significant threat to computer security. One proposed defense against such attacks is big-key cryptography which seeks to make cryptographic secrets so large that it is infeasible for an adversary to exfiltrate the key (without being detected). However, this also introduces an inconvenience to the user who must now store the large key on all of their different devices. The work of Döttling, Garg, Sekar and Wang (TCC 2022) introduces an elegant solution to this problem in the form of big-key identity-based encryption (IBE). Here, there is a large master secret key, but very short identity keys. The user can now store the large master secret key as her long-term key, and can provision each of her devices with short ephemeral identity keys (say, corresponding to the current date). In this way, the long-term secret key is protected by conventional big-key cryptography, while the user only needs to distribute short ephemeral keys to their different devices. Döttling et al. introduce and construct big-key IBE from standard pairing-based assumptions. However, their scheme only satisfies selective security where the adversary has to declare its challenge set of identities at the beginning of the security game. The more natural notion of security is adaptive security where the user can adaptively choose which identities it wants to challenge after seeing the public parameters (and part of the master secret key).

In this work, we give the first adaptively-secure construction of big-key IBE from standard cryptographic assumptions. Our first construction relies on indistinguishability obfuscation (and one-way functions), while our second construction relies on witness encryption for NP together with standard pairing-based assumptions (i.e., the SXDH assumption). To prove adaptive security, we show how to implement the classic dual-system methodology with indistinguishability obfuscation as well as witness encryption.

Abstract;

In this paper, we present the first practical algorithm to compute an effective group action of the class group of any imaginary quadratic order O on a set of supersingular elliptic curves primitively oriented by O. Effective means that we can act with any element of the class group directly, and are not restricted to acting by products of ideals of small norm, as for instance in CSIDH. Such restricted effective group actions often hamper cryptographic constructions, e.g. in signature or MPC protocols.

Our algorithm is a refinement of the Clapoti approach by Page and Robert, and uses 4-dimensional isogenies. As such, it runs in polynomial time, does not require the computation of the structure of the class group, nor expensive lattice reductions, and our refinements allows it to be instantiated with the orientation given by the Frobenius endomorphism. This makes the algorithm practical even at security levels as high as CSIDH-4096. Our implementation in SageMath takes 1.5s to compute a group action at the CSIDH-512 security level, 21s at CSIDH-2048 level and around 2 minutes at the CSIDH-4096 level. This marks the first instantiation of an effective cryptographic group action at such high security levels. For comparison, the recent KLaPoTi approach requires around 200s at the CSIDH-512 level in SageMath and 2.5s in Rust.

See also; https://bsky.app/profile/andreavbasso.bsky.social/post/3ljkh4wmnqk2c

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See also;

https://talkingpointsmemo.com/edblog/another-update-on-the-situation-at-indiana-university

Via; https://bsky.app/profile/kennyog.bsky.social/post/3ll7ulyayts2d

Via; https://bsky.app/profile/nicksullivan.org/post/3ll7galasrc2z

CFRG process documentation has been updated.

From: https://mastodon.social/@fj/114171907451597856

Interesting paper co-authored by Airbus cryptographer Erik-Oliver Blass on using zero-knowledge proofs in flight control systems.

Sensors would authenticate their measurements, the control unit provides in each iteration control outputs together with a proof of output correctness (reducing the need in some cases for redundant computations), and actuators verify that outputs have been correctly computed