Threshcon: Multi-Party Threshold Ascon

“Preview Talk” (by Team Threshcon) @ TCPT2, in reply to the NIST Threshold Call

Abstract: In this talk, we present the planned package submission for Threshcon, a protocol family developed to securely evaluate the Ascon lightweight symmetric cryptography suite within a multi-party threshold context. Our submission covers the Ascon-AEAD128 authenticated encryption and decryption scheme, as well as the Ascon-Hash256 and Ascon-CXOF128 modes, and guarantees strict functional equivalence with the standard NIST primitives. Threshcon is set in the preprocessing model and is designed for an n-party setting with an honest supermajority, providing information-theoretic security against active malicious adversaries that corrupt up to t < n/3 parties. Beyond Shamir secret sharing, we rely on Reverse Multiplication-Friendly Embeddings (RMFEs) to align the multi-party evaluation more closely with Ascon's parallelized bitwise design. We will discuss secret-sharing semantics applied to the required inputs and outputs. Finally, we will provide an overview of the ongoing implementation efforts for the NIST multi-party threshold call.

Joint work: Peter Schwarz, Aysajan Abidin

[Slides] Suggested readings:

• Preview Writeup (PDF): Threshcon: Multi-Party Threshold Ascon
• Evaluating Ascon in Secure Multi-Party Computation using Reverse Multiplication-Friendly Embeddings (ia.cr/2025/1538)