Nuclear physicists may have just invented a ‘zero-knowledge’ warhead inspection system

Alexander Glaser, Boaz Barak, R.J. Goldston

Peter Marleau, E. Brubaker, Nathan R. Hilton +4 more

Glaser, Barak, and Goldston recently described an approach for nuclear warhead verification based on the cryptographic concept of a zero-knowledge protocol (ZKP).The verification approach included both procedural elements and a physical implementation.A group of Sandia National Laboratories researchers, whose expertise include radiation instrumentation design and development, cryptography, and arms control verification implementation, jointly reviewed the paper and identified specific challenges to implementing the approach as well as some opportunities.This paper compares the ZKP concept as presented by Glaser et al., to other warhead verification concepts and approaches described in the literature.The paper also summarizes challenges and opportunities for elements of this (new) approach.We note that ZKP as used in cryptography is a useful model for the arms control verification problem, but find that the direct analogy to arms control breaks down quickly.For example, fault tolerance in cryptographic ZKP is achieved by brute force repetition of the challenge/response procedure; in arms control, repeated measurements can be expensive.The ZKP methodology fits within the general class of template-based verification techniques, where template in this case refers to a reference measurement that is used for comparison.Template methods in general confirm that a given object is like another object that has already been accepted as a warhead by some other means.This can be a powerful verification approach, but requires independent means to trust the authenticity of the reference warhead -a standard that may be difficult to achieve.Note that this use of template differs from that of Glaser et al., who use the term to describe the accepted reference warhead.In the ZKP approach as described, the reference warhead must be presentwith inspector confidence in its provenance-at each subsequent confirmation measurement.This imposes significant additional constraints on maintaining continuity of knowledge of the reference between inspections.Despite some technical challenges, the concept of last-minute selection of the pre-loads and equipment could be a valuable component of a verification regime.In addition, the neutron transmission image using a bubble detector array is an interesting idea that can be considered for template measurements independently of ZKP..

Sébastien Philippe

Future nuclear arms-control agreements may call for reductions of the total number of nuclear weapons and warheads in the world arsenals. Such agreements would require new trusted verification mechanisms to confirm that items presented to inspectors are nuclear warheads and not spoofs. Proliferation and national security concerns require, however, that inspectors gain no warhead design information through this process. To address this paradox, considerable efforts have been directed towards the development of "information barriers." These barriers consist of automated measurement systems that process sensitive information but only display the results of internal analysis in a binary valid/invalid manner. These systems are, by their nature, at risk of electronic tampering and snooping – and their trusted implementation has so far proved extremely difficult to realize. This thesis takes radically different directions to address this challenge. It demonstrates new approaches to information protection and trusted instruments for nuclear warhead verification that are based on the cryptographic concepts of zero-knowledge proofs and physically unclonable functions. To the author's knowledge, this thesis provides the first demonstration of a zero-knowledge physical measurement technique. Using fast neutron differential radiography and superheated emulsion detectors, such a technique can show two objects have identical geometry and opacity to 14-MeV neutrons without revealing what these properties are. This zero-knowledge feature no longer holds when the objects compared are significantly different. Such a technique could form the basis of a template-matching verification system that could confirm the authenticity of nuclear weapons without sharing any secret design information. The thesis then introduces and demonstrates key elements of an optical physical unclonable function sensitive to neutrons and based on superheated emulsions. Such sensors are unique objects that cannot be cloned or simulated. The data they produce are a function of both their internal disordered structure and the physical quantity they measure. Due to their sensitivity against any structural variation, including through neutron irradiation, it is possible to show that they – or the data they have recorded -- have not been tampered with. Such sensors could be used by adversarial parties in sensitive facilities without the risk of being compromised.

Sébastien Philippe, Boaz Barak, Alexander Glaser

Gabriel V. Turturica, Violeta Iancu

Disarmament treaties have been the driving force towards reducing the large nuclear stockpile assembled during the Cold War. Further efforts are built around verification protocols capable of authenticating nuclear warheads while preventing the disclosure of confidential information. This type of problem falls under the scope of zero-knowledge protocols, which aim at multiple parties agreeing on a statement without conveying any information beyond the statement itself. A protocol capable of achieving all the authentication and security requirements is still not completely formulated. Here we propose a protocol that leverages the isotopic capabilities of NRF measurements and the classification abilities of neural networks. Two key elements guarantee the security of the protocol, the implementation of the template-based approach in the network's architecture and the use of homomorphic inference. Our results demonstrate the potential of developing zero-knowledge protocols for the verification of nuclear warheads using Siamese networks on encrypted spectral data.

Si Khaled MEHDI, Akila Frahi-Amroun