Paper 2026/1900 Original: eprint.iacr.org/2026/1900 ↗

Threshold Fully Deniable Interactive Encryption

Abstract

Deniable Encryption (Canetti et al., CRYPTO 1997) enables parties to produce fake internal states making any transcript consistent with any plaintext of their choice. Fully Deniable Encryption (FDE) (Canetti et al., CRYPTO 2020) achieves the strongest two-party guarantee since both sender and receiver can independently fabricate randomness, even with mutually inconsistent claimed plaintexts, and anyone can fake the receiver's side. However, FDE addresses only post-execution coercion and remains confined to the two-party setting, which leaves both adaptive coercion (where parties may be coerced at any round during execution) and threshold deniability (where secrets are distributed among $n$ parties) as open problems. In this work, we resolve the threshold gap and take a definitional step toward the adaptive one. We design Threshold Fully Deniable Interactive Encryption (TFDE) via FDE with a universal thresholdizer, equivocable commitments, and non-committing encryption via a four-layer equivocation pipeline so that any coalition of fewer than~$t_S$ senders and~$t_R$ receivers can produce per party fake states indistinguishable from honest executions, while satisfying all standard threshold public key encryption properties; like FDE itself, which is a feasibility result. Moreover, we formalize Strong Off-the-Record Deniability (SORD), a new notion that captures adaptive mid-execution coercion and consistent fake claims by both parties.