Paper 2026/275

PhantomCrypt: Second-Order Deniable Encryption with Post-Quantum Security

Stefan Rass, LIT Secure and Correct Systems Lab, Johannes Kepler University, Linz, Austria

Zahra Seyedi, Department of Electronics, Information and Bioengineering, Polytechnic University of Milan, Milan, Italy

Abstract

Traditional deniable encryption focuses on denying the $content$ of secret communications by allowing plausible alternative plaintexts under coercion. However, the recognizable use of deniable encryption may itself defeat its purpose: any revealed plaintext becomes suspicious if the coercer detects that a non-standard encryption tool was used. We introduce $second-order~deniability$: the property that the use of a deniability mechanism is itself deniable. We formalize this notion via game-based security definitions for $content~deniability$ (CD) and $existence~deniability$ (ED), prove that CD and ED are logically independent, and show that their conjunction suffices to achieve second-order deniability. We present PhantomCrypt, a concrete construction composing False-Bottom Encryption (for CD) with Invisible Encryption (for ED) under a hybrid encryption envelope, and prove that this composition preserves both security properties with an explicit security bound. A proof-of-concept implementation demonstrates practical feasibility, achieving encryption of a 32-byte message with three decoys in under 10ms on commodity hardware.

Note: PhantomCrypt v2: 20.04.2026

@misc{cryptoeprint:2026/275,
      author = {Shahzad Ahmad and Stefan Rass and Zahra Seyedi},
      title = {{PhantomCrypt}: Second-Order Deniable Encryption with Post-Quantum Security},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/275},
      year = {2026},
      url = {https://eprint.iacr.org/2026/275}
}