Greenpass RADIUS Tools for Delegated Authorization in Wireless Networks

Dartmouth\u27s Greenpass project extends how public key cryptography can be used to secure the wireless LAN with a RADIUS (Remote Authentication Dial In User Service) server that is responsible for handling authentication requests from clients (called supplicants in the 802.1x authentication model). This thesis describes the design and implementation of the authentication process of Greenpass, specifically what decisions are made in determining who is granted access and how a small modification of already existing protocols can be used to provide guest access in a way that better reflects how delegation of authority works in the real world. Greenpass takes advantage of the existing PKI to authenticate local Dartmouth users via X.509 identity certificates using EAP-TLS. We use the flexibility of SPKI/SDSI (Simple Public Key Infrastructure/Simple Distributed Security Infrastructure) authorization certificates to distribute the responsibility of delegating access to guests to certain authorized delegators, avoiding some of the necessary steps and paperwork associated with having a large centralized entity responsible for the entire institution. This thesis also discusses how our solution can be adapted to support different methods of guest delegation and investigates the possibility of eliminating the cumbersome central entity and administrative overhead traditionally associated with public key cryptography

Stretching-induced conductance variations as fingerprints of contact configurations in single-molecule junctions

Molecule-electrode contact atomic structures are a critical factor that characterizes molecular devices, but their precise understanding and control still remain elusive. Based on combined first-principles calculations and single-molecule break junction experiments, we herein establish that the conductance of alkanedithiolate junctions can both increase and decrease with mechanical stretching and the specific trend is determined by the S-Au linkage coordination number (CN) or the molecule-electrode contact atomic structure. Specifically, we find that the mechanical pulling results in the conductance increase for the junctions based on S-Au CN two and CN three contacts, while the conductance is minimally affected by stretching for junctions with the CN one contact and decreases upon the formation of Au monoatomic chains. Detailed analysis unravels the mechanisms involving the competition between the stretching-induced upshift of the highest occupied molecular orbital-related states toward the Fermi level of electrodes and the deterioration of molecule-electrode electronic couplings in different contact CN cases. Moreover, we experimentally find a higher chance to observe the conductance enhancement mode under a faster elongation speed, which is explained by ab initio molecular dynamics simulations that reveal an important role of thermal fluctuations in aiding deformations of contacts into low-coordination configurations that include monoatomic Au chains. Pointing out the insufficiency in previous notions of associating peak values in conductance histograms with specific contact atomic structures, this work resolves the controversy on the origins of ubiquitous multiple conductance peaks in S-Au-based single-molecule junctions.Comment: 11 pages, 4 figures; to be published in J. Am. Chem. So