From Farfalle to Megafono via Ciminion: The PRF Hydra for MPC Applications

Lorenzo Grassi, Morten Øygarden, Markus Schofnegger, Roman Walch*

*Korrespondierende/r Autor/-in für diese Arbeit

Publikation: Beitrag in Buch/Bericht/KonferenzbandBeitrag in einem KonferenzbandBegutachtung

Abstract

The area of multi-party computation (MPC) has recently increased in popularity and number of use cases.
At the current state of the art, Ciminion, a Farfalle-like cryptographic function, achieves the best performance in MPC applications involving symmetric primitives.
However, it has a critical weakness. Its security highly relies on the independence of its subkeys, which is achieved by using an expensive key schedule. Many MPC use cases involving symmetric pseudo-random functions (PRFs) rely on secretly shared symmetric keys, and hence the expensive key schedule must also be computed in MPC. As a result, Ciminion's performance is significantly reduced in these use cases.

In this paper we solve this problem. Following the approach introduced by Ciminion's designers, we present a novel primitive in symmetric cryptography called Megafono. Megafono is a keyed extendable PRF, expanding a fixed-length input to an arbitrary-length output. Similar to Farfalle, an initial keyed permutation is applied to the input, followed by an expansion layer, involving the parallel application of keyed ciphers. The main novelty regards the expansion of the intermediate/internal state for "free", by appending the sum of the internal states of the first permutation to its output. The combination of this and other modifications, together with the impossibility for the attacker to have access to the input state of the expansion layer, make Megafono very efficient in the target application.

As a concrete example, we present the PRF Hydra, an instance of Megafono based on the Hades strategy and on generalized versions of the Lai--Massey scheme. Based on an extensive security analysis, we implement Hydra in an MPC framework. The results show that it outperforms all MPC-friendly schemes currently published in the literature.
Originalspracheenglisch
TitelEUROCRYPT 2023
PublikationsstatusVeröffentlicht - 2023
Veranstaltung42nd Annual International Conference on the Theory and Applications of Cryptographic Techniques: EUROCRYPT 2023 - Lyon, Frankreich
Dauer: 23 Apr. 202327 Apr. 2023
Konferenznummer: 42
https://eurocrypt.iacr.org/2023/

Konferenz

Konferenz42nd Annual International Conference on the Theory and Applications of Cryptographic Techniques
KurztitelEUROCRYPT 2023
Land/GebietFrankreich
OrtLyon
Zeitraum23/04/2327/04/23
Internetadresse

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