Physical Cryptographic Warhead Verification
Nations have been working to reduce the size of Cold War nuclear arsenals for more than thirty-five years. Because parity in force capability remains a central factor in strategic stability, reductions are possible only with intricately negotiated verification provisions that ensure neither side is cheating. Historically, verification was limited to delivery vehicles such as missiles and aircraft; it has not included checks on nuclear warheads directly. The limited approach allowed meaningful reductions in the level of deployed nuclear forces while avoiding the difficult challenge of having foreign governments inspect highly classified nuclear warheads. Ultimately, however, direct verification of warheads will be needed to facilitate warhead dismantlement, needed to eliminate the possibility of rapid rearmament, and to reduce the risk that “loose nukes” fall into the hands of terrorists. Despite decades of work, no protocol for direct warhead verification has been developed that is able simultaneously to provide high assurance that a purported warhead is real and protect the secrets of a warhead's design.
Made from the Right Stuff
Most people are familiar with the reddish glow of a neon sign, the orange of a sodium street lamp, or violet of a mercury black light. The colors of these lights are signatures of the elements emitting the light, each color unique to each element and determined by the energies of the electrons around that element's nucleus. In an analogous way, the protons and neutrons inside the nucleus are associated with another set of energies, and these too can glow with a unique set of colors, except the light from the nucleus is x-rays instead of visible light. By observing the spectrum of these x-rays, we can determine not only the elements, but also isotopes from which an object is made. This is called Nuclear Resonance Fluorescence (NRF), and it lets us determine the exact composition of the nuclear warhead. Unlike other nucler measurements, it is impossible to substitute one isotope or element for another.
All In the Right Place
Not only do we need to check all the materials used to make the warhead, it's also necessary to determine that the warhead has all those materials in all the correct locations. By measuring the NRF signature of the test object from many different angles, we in effect create a tomographic map of the distribution of isotopes within the object.
Keeping Secrets Secret
The challenge of warhead verification comes in protecting secret information. A straightforward measurement of the warhead's composition and geometry would reveal far too much information to the inspector. One possibility long pursued is to use software or electronics to analyze the measured data and hide it from inspectors, giving only a pass/fail result. Unfortunately, nobody has yet figured out how to make that approch perfectly secure against hacking or cheating. Instead, we use use a physical instantiation of one-time-pad encryption to protect information. The process is based on a special single-pixel tomographic transform and the use of a physical secret key for each single-pixel measurement. The physical secret key is simply a thin foil, the composition of which is known only to the weapon owner. The x-rays leaving the weapon are convolved with the foil-key. The result is the equivalent to an enrypted bit representing the line-integrated density of one isotope in the warhead. By taking multiple projections, corresponding to multiple lines of interrogation through the warhead, the equivalent of a digital hash is built up. By comparing the hash values for a real and test warhead, one can confirm if the test warhead is a match or not.
The NRF measurement starts with an electron accelerator. LNSP currently uses a 2.6 MeV Van de Graaff at 65 μA. The electron beam impinges on a tantalum foil target to create bremsstrahlung, continuous-energy x-rays. Although the NRF cross sections are extremely sharp and rapidly depleted the photon inventory in a narrow ~1 eV wide notch at each resonant energy of each isotope present, tests have confirmed that there are sufficient photons to interrogate at least 80 g/cm2 of uranium despite NRF, Compton, and pair-production effects, which is adequate for nuclear weapons.
The transmitted beam consists of a hardened continuum spectrum with numerous, very narrow notches. Those notches encode all the information needed to verify the warhead, but they are never directly observed, nor could they be because no real-world detector is able to resolve 1 eV notches against the continuum background. However, when the transmitted beam impinges on a foil composed of isotopes of interest, the rate at which the isotopes in the foil fluoresce is inversely proportional to the depth of each isotope's corresponding notches in the transmitted beam and proportional to the amount of that isotope in the foil, which is unknown to the inspector. The foil's fluorescence in 4π steradians is measured using four high-purity germanium detectors.
There is mild backscatter of the Compton continuum from the foil, which contains structure from phenomena such as x-ray fluorescence, k, l, and m-edges from the foil. This structure can be reduced far below the sensitivity of the detector using shielding. However, even absent shielding any detectable features in the continuum can be rendered information-free by placing additional, unknown quantities of the elements of interest in the path between the foil and detector.
Peaks in spectra corresponding to the isotopes of interest automatically authenticate the foil as being composed of the appropriate isotopes. The height of each peak constitutes the signature of the weapon-foil convolution. The spectrum for the authentic warhead can be statistically checked against the spectrum for the test warhead to determine if there is a discrepancy or if the test object is a match.
- R. Scott Kemp, Areg Danagoulian, Ruaridh Macdonald, and Jayson Vavrek, "Physical Cryptographic Verification of Nuclear Warheads," Proceedings of the National Academy of Sciences USA, www.pnas.org/cgi/doi/10.1073/pnas.1603916113.
The NRF Zero Knowledge Project is supported by the U.S. Department of Energy's National Nuclear Security Administration through a $25M five-year Consortium for Verification Technology. Additional support comes from the Carnegie Corporation of New York.