ZERO KNOWLEDGE PROTOCOL: CHALLENGES AND OPPORTUNITIES

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 present— with 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.