What Uncloneable Encryption Means for a Post-Quantum World
Unclonable encryption
The Perimeter Institute and University of Waterloo have shown that data may be encrypted in a way that is impossible to copy, a major advance in quantum information science. The “no-cloning” principle of quantum mechanics is used in uncloneable encryption to provide security that classical computers cannot match.
Digital security has relied on computational assumptions that some mathematical problems are too tough for modern computers to solve for decades. As quantum computing advances, many of these classical precautions are at risk. Archishna Bhattacharyya and Eric Culf's new study suggests a novel security system based on physics rather than computer processing power.
No More “Copy-Paste”
In classical times, digital communications could be copied accurately. An opponent who intercepts a ciphertext can copy and save the data for examination. However, quantum mechanics forbids this. The no-cloning principle states that an unknown quantum state cannot be copied independently and identically.
Uncloneable encryption goes further. Classical communications are encoded into quantum “ciphertext” to prevent two adversaries from decrypting them even if they get the encryption key. This is shown in a high-stakes security game with Alice as the referee and Bob and Charlie as cooperative but non-interacting "pirates."
Alice sends the pirates quantum state. Pirates then transfer quantum information via a “pirate channel”. Alice reveals the encryption key when they are separated and cannot communicate. Bob and Charlie should not be able to guess the original message if the encryption is uncloneable.
A “Plain Model” Innovation
Uncloneable encryption was previously established, but cryptographers have yet to prove its security. Most prior security proofs used the "quantum random oracle model," a heuristic. Other methods required specific, unproven quantum game conjectures.
First, uncloneable security in the “plain model” without computational assumptions was demonstrated. The researchers focused on “Haar-measure encryption of a bit”. Alice chooses a random basis from the Haar measure to determine a really random path in quantum space and prepares a state based on whether she wants to convey a “0” or a “1”.
Scientists call it information-theoretic security. Show that as the system's complexity (or “dimension”) increases, the probability of both pirates winning the game reduces to 50%, which is a random estimate.
Strength of “Decoupling”
Decoupling is the mathematical “secret sauce” of this proof. If Alice and Bob are substantially entangled, Charlie must be “decoupled” from them in quantum systems. This is called entanglement monogamy.
The researchers utilized the decoupling theorem to explain that if Bob understands the message, Charlie is “locked out” since his system becomes statistically independent of Alice's. A “one-shot” variation of this theorem showed that pirates cannot escape this fundamental physical restriction regardless of their method.
Authorities say this is a big success since it allows efficient buildings. Although a fully Haar-random system is difficult to build, the researchers showed that unitary 2-designs like the Clifford group, which can be implemented on quantum hardware, can provide the same security.
Considering the Future
The current proof only encrypts one bit, but the repercussions are immense. Any-length messages can be encrypted with its secure “uncloneable bit”.
However, work continues. The researchers call their current achievement “weak uncloneable security” since security increases at an inverse-polynomial rate as the system grows. The ultimate goal is "strong uncloneable security," where a successful assault is inconsequential.
“To achieve the full strength of uncloneable cryptography, this should be improved to a negligible scaling,” leaving the job to future quantum physicists. Currently, the world is one step closer to a future where data is physically impossible to steal.