Paper 2025/1765

Untelegraphable Encryption and its Applications

Abstract

We initiate the study of untelegraphable encryption (UTE), founded on the no-telegraphing principle, which allows an encryptor to encrypt a message such that a binary string representation of the ciphertext cannot be decrypted by a user with the secret key, a task that is classically impossible. This is a natural relaxation of unclonable encryption (UE), inspired by the recent work of Nehoran and Zhandry (ITCS 2024), who showed a computational separation between the no-cloning and no-telegraphing principles. In this work, we define and construct UTE information-theoretically in the plain model. Building off this, we give several applications of UTE and study the interplay of UTE with UE and well-studied tasks in quantum state learning, yielding the following contributions: - A construction of collusion-resistant UTE from standard secret-key encryption (SKE). We additionally show that hyper-efficient shadow tomography (HEST) is impossible assuming collusion-resistant UTE exists. By considering a relaxation of collusion-resistant UTE, we are able to show the impossibility of HEST assuming only pseudorandom state generators (which may not imply one-way functions). This almost unconditionally answers an open inquiry of Aaronson (STOC 2018). - A construction of UTE from a quasi-polynomially secure one-shot message authentication code (OSMAC) in the classical oracle model, such that there is an explicit attack that breaks UE security for an unbounded polynomial number of decryptors. - A construction of everlasting secure collusion-resistant UTE, where the decryptor adversary can run in unbounded time, in the quantum random oracle model (QROM), and formal evidence that a construction in the plain model is a challenging task. We additionally show that HEST with unbounded post-processing time (which we call weakly-efficient shadow tomography) is impossible assuming everlasting secure collusion-resistant UTE exists. - A construction of secret sharing for all polynomial-size policies that is resilient to joint and unbounded classical leakage from collusion-resistant UTE and classical secret sharing for all policies. - A construction (and definition) of collusion-resistant untelegraphable secret-key functional encryption (UTSKFE) from single-decryptor functional encryption and plain secret-key functional encryption, and a construction of collusion-resistant untelegraphable public-key functional encryption from UTSKFE, plain SKE, and plain public-key functional encryption.