Optimal Security Notion for Decentralized Multi-Client Functional Encryption

Ky Nguyen (1, 2) , Duong Hieu Phan (3, 4, 5, 6) , David Pointcheval (1, 2)

1 CASCADE - Construction and Analysis of Systems for Confidentiality and Authenticity of Data and Entities
2 DI-ENS - Département d'informatique - ENS Paris
3 C2 - Cybersécurité et Cryptographie
4 INFRES - Département Informatique et Réseaux
5 LTCI - Laboratoire Traitement et Communication de l'Information
6 IP Paris - Institut Polytechnique de Paris

Research on (Decentralized) Multi-Client Functional Encryption (or (D)MCFE) is very active, with interesting constructions, especially for the class of inner products. However, the security notions have been evolving over the time. While the target of the adversary in distinguishing ciphertexts is clear, legitimate scenarios that do not consist of trivial attacks on the functionality are less obvious. In this paper, we wonder whether only trivial attacks are excluded from previous security games. And, unfortunately, this was not the case. We then propose a stronger security notion, with a large definition of admissible attacks, and prove it is optimal: any extension of the set of admissible attacks is actually a trivial attack on the functionality, and not against the specific scheme. In addition, we show that all the previous constructions are insecure w.r.t. this new security notion. Eventually, we propose new DMCFE schemes for the class of inner products that provide the new features and achieve this stronger security notion.

Keywords: Functional Encryption; Corruptions; Security Notions

Domains: Cryptography and Security [cs.CR]

Ky Nguyen, Duong Hieu Phan, David Pointcheval. Optimal Security Notion for Decentralized Multi-Client Functional Encryption. Applied Cryptography and Network Security. ACNS 2023, Jun 2023, Kyoto, Japan. pp.336-365, ⟨10.1007/978-3-031-33491-7_13⟩. ⟨hal-04194145⟩