Paper 2025/364

Traitor Tracing in Multi-sender Setting ($\textsf{TMCFE}$: Traceable Multi-client Functional Encryption)

Abstract

Traitor tracing is a traditional cryptographic primitive designed for scenarios with multiple legitimate receivers. When the plaintext - that is, the output of decryption - is leaked and more than one legitimate receiver exists, it becomes imperative to identify the source of the leakage, a need that has motivated the development of traitor tracing techniques. Recent advances in standard encryption have enabled decryption outcomes to be defined in a fine-grained manner through the introduction of Functional Encryption (FE). Constructing FE schemes is intriguing, and achieving the tracing property adds an additional layer of complexity. Traitor tracing techniques have been actively developed for more than three decades, yet they have always remained within the same framework - a single sender responsible for encrypting all the data. However, fine-grained decryption is particularly useful when data originates from multiple sources, allowing for joint computation on personal data. This leads to the concept of multi-client functional encryption (MCFE), where multiple concurrent senders independently encrypt their data while agreeing on the decryption of a specific function (e.g., a statistical measure) computed on the aggregated data, without revealing any additional information. In the era of cloud computing and big data, privacy-preserving joint computation is crucial, and tracing the source of any breach by dishonest participants becomes essential. Thus, in this paper we take the first step toward addressing the tracing problem in the general context of joint computation with multiple senders. Our contributions are twofold: - $\textbf{Conceptually:}$ We propose the first tracing model in the context of multi-sender encryption, namely $\textit{Traceable Multi-Client Functional Encryption}$ ($\textsf{TMCFE}$), which allows a pirate to extract secret information from both receivers and senders. Our model supports strong and naturally admissible decoders, removing artificial restrictions on the pirate decoder and thus addressing the shortcomings of existing traceable functional encryption schemes designed for the single-sender setting. - $\textbf{Technically:}$ To achieve our conceptual objective, we build upon the recently introduced notion of strong admissibility for MCFE. Our main technical contribution is a generic compiler that transforms a large class of MCFE schemes with weak admissibility into schemes with strong admissibility. This compiler not only helps overcome existing challenges but may also be of general interest within the functional encryption domain. Finally, we present a concrete lattice-based scheme $\textsf{TMCFE}$ for inner-product functionalities that achieves post-quantum security under standard assumptions.