Paper 2026/422

Threshold Traitor Tracing Revisited: Insider Attacks and Multi-Traitor Tracing

Abstract

Threshold traitor tracing [Boneh et al., Crypto’24] enhances threshold decryption with anti-collusion guarantees via a tracing mechanism. In this model, a set of colluders constructs a decryption box, called a decoder, and sells it to an external buyer. Collusion-resilience is guaranteed by a tracing algorithm that, given only black-box access to the decoder, can identify at least one of the colluders. Since its introduction, threshold traitor tracing has attracted significant attention, leading to a number of new constructions that have been proven secure within this model [Asiacrypt'25, CiC'25, CCS’25, Eurocrypt’26], focusing on optimizing parameter sizes and removing trusted authority. However, in this paper, we argue that the state-of-the-art definition and constructions are not resilient against natural attacks where the decoder is sold to an insider, i.e., a member of the decryption committee. Motivated by this gap, we introduce new definitions to model decoder boxes that are sold to insiders. We show that existing threshold traitor tracing techniques are inherently vulnerable to insider attacks. Then, we introduce a novel approach to achieve insider resilience through \emph{multi-traitor tracing}, i.e., identifying multiple traitors. We present compilers that amplify the number of traitors that can be found, thereby achieving insider-resilience with efficient parameters, in particular, sublinear ciphertext size. These compilers may also be of independent interest.