Two computer scientists have designed an encryption method that relies on the laws of quantum mechanics, rather than on computing power alone, to guarantee that a copied message stays scrambled even after an attacker gets the decryption key.
The scheme, described in a paper on arXiv posted by Prabhanjan Ananth of the University of California, Santa Barbara, and Amit Sahai of the University of California, Los Angeles, addresses proving, without relying on unproven assumptions, that a piece of encrypted data cannot be secretly copied and later unlocked by two separate people. It’s a security goal researchers have investigated for years.
The technique falls into a field known as unclonable cryptography. It exploits a basic rule of quantum physics called the no-cloning principle, which holds that an unknown quantum state cannot be copied exactly. Classical data, the kind stored as ordinary bits on a hard drive, can always be duplicated. Data encoded in quantum bits, or qubits, cannot.
The paper reports a formula for calculating how easily two colluding attackers could beat the system. If a message is encoded across n qubits, the researchers calculate that two attackers working together can guess the message correctly no better than roughly 50% of the time, plus an extra sliver of probability that shrinks exponentially as the number of qubits grows. With even a modest number of qubits, that extra edge becomes vanishingly small, according to the paper.
That result matters because it is unconditional. Most encryption methods used today, including the systems that protect credit card numbers and emails, is secure only because certain math problems are assumed to be too hard for computers to solve quickly. Ananth and Sahai’s scheme instead draws its guarantee directly from quantum physics, which does not depend on assumptions about future computing power, including future quantum computers.
How Does The Encryption Method Work?
The scheme encrypts a single bit, either a 0 or a 1, using a secret key made of ordinary classical bits, not quantum information. To encrypt a message, the sender applies simple quantum operations, known as single-qubit Clifford gates, to a string of qubits, producing a quantum ciphertext. To decrypt it, the receiver measures each qubit individually, using a specific quantum measurement technique, and combines the results to recover the message.
An attacker who intercepts the ciphertext before the key is revealed can try to split it between two accomplices, hoping both can decode the message once the key becomes public later. Because the qubits cannot be copied without disturbing them, the researchers report, splitting the ciphertext degrades the information so much that both accomplices are very unlikely to recover the message at the same time.
The underlying idea was not new as the paper indicates the construction was first proposed last year by another research team, led by Pierre Botteron. That group’s own analysis of the scheme’s security was incomplete, and the group had verified it only for small cases. Ananth and Sahai’s contribution is a full mathematical proof showing the scheme is secure for any size, with the attackers’ advantage shrinking to a negligible amount as the number of qubits increases, according to the paper.
Limitations and Next Steps
The scheme is stripped down, as the paper indicates. It transmits only a single bit of information per use and cannot be reused, meaning a new key and new ciphertext are required for each message. Building it up to send full-length messages, such as documents or files, would require additional work the paper does not undertake. The system also requires generating and transmitting quantum states, which today requires specialized hardware not present in ordinary computers or phones.
The paper also discloses the encryption method and the core proof ideas were generated by an artificial intelligence coding tool, describing it as Codex running on a model called GPT 5.6 Sol Ultra, built on a research framework developed at UCLA. The human researchers say they take full responsibility for the claims and proofs contained in the paper, and add that they checked and refined the AI-generated work themselves.
Other research groups have pursued related but less complete approaches, some requiring quantum decryption keys instead of classical ones, others achieving strong security only in simplified theoretical models that do not fully reflect real-world conditions. A separate line of research had achieved similarly strong guarantees but only with encryption and decryption procedures too slow to be practical, according to the paper.
Ananth and Sahai’s scheme is both fast and unconditionally secure for one-bit messages, closing a gap between those two goals that researchers had been unable to bridge. Extending the approach to longer messages, and eventually building it into usable quantum communication systems, would be the focus of future research.
