Researchers Say New Quantum Encryption Method Can’t Be Copied

Insider BriefResearchers have presented what they describe as the first efficient, information-theoretically secure unclonable encryption scheme with classical keys in the standard cryptographic model. In plain terms, it’s a result that could advance a longstanding goal in quantum cryptography by showing how encrypted quantum information can remain resistant to copying even after an encryption key is revealed.The study, posted on arXiv by researchers Prabhanjan Ananth, the University of California, Santa Barbara (UCSB) and Amit Sahai, the University of California, Los Angeles (UCLA), describes a one-time private-key encryption scheme for one-bit messages that relies on a fundamental property of quantum mechanics: quantum states cannot, in general, be copied perfectly.Although it’s early, theoretical work, the research could be a step toward secure communications and digital rights management, among other use cases.The researchers report that the protocol achieves perfect correctness while limiting an attacker’s probability of successfully producing two usable copies of an encrypted message to no better than one-half plus an exponentially small advantage. They write that the construction uses only single-qubit Clifford gates during encryption and local Pauli measurements during decryption, making it efficient to implement compared with previous theoretical approaches.The work addresses a problem that has remained unresolved despite several years of rapid progress in quantum cryptography.Conventional encryption assumes encrypted files can be copied freely without affecting security. Unclonable encryption takes a different approach by exploiting the quantum no-cloning theorem, which indicates that unknown quantum states cannot be perfectly copied. The idea is to create encrypted quantum messages that cannot be duplicated into two independently useful copies, so that even if an attacker later learns the decryption key, two separated recipients cannot both reliably recover the original message.According to the researchers, previous constructions either required quantum decryption keys, depended on idealized models such as quantum random oracles, or achieved optimal security only through computationally impractical algorithms.The study focuses on what cryptographers call indistinguishability security, regarded as the stronger and more practical security notion for unclonable encryption.Under this model, an attacker receives a single quantum ciphertext — which is the encrypted version of a message or file — while the secret key remains hidden. Before the key is revealed, the attacker must divide the ciphertext into two separate systems destined for two collaborators who are not allowed to communicate afterward. Once both collaborators receive the key, each independently attempts to identify which message was encrypted.If both can correctly identify the message, the attack succeeds.The baseline success probability is already 50% because the two recipients can simply agree in advance to make the same random guess. The challenge is therefore to prevent any strategy that significantly improves on that probability.The researchers report that their protocol bounds the attacker’s success probability at one-half plus an exponentially shrinking term, meaning any improvement over random guessing rapidly disappears as the security parameter increases.The result is information-theoretic rather than computational. In other words, the security does not depend on assumptions about limited computing power or the hardness of mathematical problems. Instead, it follows directly from the laws of quantum mechanics.According to the paper, earlier approaches commonly relied on encoding information using BB84 quantum states, one of the foundational techniques in quantum cryptography.The researchers instead replace that approach with what they describe as a random tensor Pauli construction.Rather than choosing between only two measurement bases, the encryption process randomly selects from the broader family of Pauli operators while encoding the message in the combined parity of measurement outcomes across multiple qubits.The encryption key consists entirely of classical information. During encryption, single-qubit Clifford gates prepare the quantum ciphertext. Once the legitimate recipient later learns the key, each qubit is measured locally, and the overall parity of those measurements reconstructs the original one-bit message with perfect accuracy.The researchers report that the underlying construction itself was first proposed in earlier work by Pierre Botteron and colleagues. Their contribution is to provide what they describe as a proof that the construction satisfies indistinguishability security with negligible adversarial advantage while remaining computationally efficient.Much of the paper develops the mathematical proof supporting the security claim.The researchers first transform the attacker’s strategy into an equivalent mathematical object using a standard technique from quantum information theory known as the Choi-Jamiołkowski representation. That transformation allows the attacker’s success probability to be expressed as the spectral properties of a particular operator.The remainder of the proof derives a bound on that operator by exploiting the orthogonality of Pauli operators and introducing what the researchers describe as a filtered-overlap argument.The resulting analysis leads directly to the exponentially small upper bound on an attacker’s advantage.The paper also compares its proof with recent work published earlier this year. Rather than relying on an unproven conjecture about a related operator, the researchers derive an explicit mathematical bound that establishes the desired security guarantee.The study includes an unusual disclosure regarding artificial intelligence.“The human authors take full responsibility for the claims and proofs contained in this paper, and have carefully refined and verified them,” the researchers write.“The construction and main ideas of the proof were generated entirely by Codex using GPT 5.6 Sol Ultra, using harness ideas generated by the authors based on the UCLA Moonshot Harness.”The statement reflects a growing trend in theoretical computer science and mathematics toward AI-assisted research while explicitly assigning responsibility for verification to the human researchers.Although highly theoretical, unclonable encryption represents one of the distinctive capabilities offered by quantum information science.Conventional digital information can be copied perfectly, making encryption schemes responsible for protecting confidentiality while assuming unlimited duplication of encrypted data.Quantum information behaves differently because of the no-cloning theorem, which prohibits creating perfect copies of unknown quantum states. Unclonable encryption attempts to convert that physical limitation into a practical security feature.If such protocols eventually become practical, they could enable applications where encrypted information self-limits duplication, potentially benefiting secure communications, digital rights management and other settings where preventing copying is as important as preventing unauthorized reading.The researchers report, however, that the new protocol remains limited in scope.The construction encrypts only a single bit of information in the one-time setting, meaning each encryption key is intended for only one use. Extending the techniques to longer messages, reusable keys or broader cryptographic applications remains an open research problem.For a deeper, more technical dive, please review the paper on arXiv. It’s important to note that arXiv is a pre-print server, which allows researchers to receive quick feedback on their work. However, it is not — nor is this article, itself — official peer-review publications. Peer-review is an important step in the scientific process to verify results.TopicsShare Get the latest research, company news, and market intelligence every week. MENTIONED IN THE ARTICLEThe University of California, Santa Barbara, is a public research university located in Santa Barbara, California, United States. It operates within the University of California system. Originating from an independent teachers' college in 1891, UCSB became part of the California State system in the 1920s and later joined the University of California system in 1944. It holds the distinction of being the third-oldest undergraduate campus within the system, following UC Berkeley and UCLA.The University of California, Los Angeles, commonly referred to as UCLA, is a public research university located in Los Angeles, California. Its origins date back to 1881 when it started as a normal school called the southern branch of the California State Normal School. It later became part of the University of California system in 1919 and was officially established as UCLA, making it the second-oldest campus among the ten in the University of California system.More in Research