Abstract As a variant of the signcryption scheme, threshold unsigncryption operates among multiple receiver members through a (t, n) threshold process during the unsigncryption phase to improve security and robustness, and has been widely deployed in various applications such as cloud computing, blockchain, and electronic auctions. However, existing schemes only consider security under a limited static corruption model, where the adversary selects which parties to corrupt before the protocol begins. As a result, these schemes lack the flexibility to address more complex attack forms posed by real-world adversaries. To solve this issue, it is worth investigating threshold unsigncryption schemes with adaptive security. In this paper, we propose the first provably secure (t, n) threshold unsigncryption scheme in the standard model against adaptive corruption adversaries who can corrupt parties at any time during protocol execution. The main contributions include: First, we provide the formal syntactic definition and security model of threshold unsigncryption schemes under the adaptive corruption model. Second, we employ the dual system technique to formally prove that the pr
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