"Preview Talk" (by team Holocron) @ TCPT3, in reply to the NIST Threshold Call
Abstract: In this presentation, we introduce a framework for thresholdizing K-PKE, the public-key encryption (PKE) underlying ML-KEM, using fully homomorphic encryption (FHE). Our construction achieves full ciphertext interchangeability: threshold encryption outputs a K-PKE ciphertext that can be decrypted by standard K-PKE decryption, while threshold decryption decrypts ordinary K-PKE ciphertexts with equivalent functionality. For encryption, parties combine independently sampled random bits while encrypted in a discrete Cheon-Kim-Kim-Song (CKKS) ciphertext. By lifting the plaintext space utilizing CKKS short-integer bootstrapping, we exploit noise flooding, plaintext masking, and linearity to form the final ciphertext without revealing the message or randomness. For decryption, we embed the K-PKE phase computation into a Brakerski-Gentry-Vaikuntanathan (BGV) scheme, homomorphically evaluate the nonlinear compression/decompression step, and use modulus expansion and noise flooding to protect partial decryptions. We discuss correctness and active-static security with abort based on the commit-and-reveal technique and Buckler non-interactive zero-knowledge proofs (NIZKs).
Joint work: Jihoon Cho, Siwoo Eum, Kyoohyung Han, Intak Hwang, Jaejun Ko, Jihoon Kwon, Byeonghak Lee, Sangyub Lee, Seonhong Min, Hwajeong Seo, Yongsoo Song, Hyojin Yoon.
Suggested readings
- Preview Writeup (PDF): Holocron: Threshold K-PKE Encryption and Decryption from FHE
