Interleaving Command Sequences: a Threat to Secure Smartcard Interoperability

The increasingly widespread use of smartcards for a variety of sensitive applications, including digital signatures, creates the need to ensure and possibly certify the secure interoperability of these devices. Standard certification criteria, in particular the Common Criteria, define security requirements but do not sufficiently address the problem of interoperability. Here we consider the interoperability problem which arises when various applications interact with different smartcards through a middleware. In such a situation it is possible that a smartcard of type S receives commands that were supposed to be executed on a different smartcard of type S'. Such "external commands" can interleave with the commands that were supposed to be executed on S. We experimentally demonstrate this problem with a Common Criteria certified digital signature process on a commercially available smartcard. Importantly, in some of these cases the digital signature processes terminate without generating an error message or warning to the user.Comment: 6 pages; published in the 10th WSEAS International Conference on Information Security and Privacy (ISP 2011

Pairing and superfluidity in strongly interacting Fermi gases

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 164-173).This thesis describes experiments with superfluid spin mixtures of ultracold fermionic 6Li atoms. The properties of the strongly interacting gas are studied in the crossover regime between Bose-Einstein condensation (BEC) of two-body bound molecules and a Bardeen-Cooper-Schrieffer (BCS) superfluid of pairs bound by many-body interactions. We obtain the homogeneous phase diagram of the two -component gas with resonant interactions. As a function of temperature and spin polarization the phase diagram shows first and second order phase transitions that merge at a tricritical point. At zero temperature a first order phase transition from a superfluid with equal spin populations to a mixed normal phase is observed at a critical spin polarization known as the Chandrasekhar-Clogston limit of superfluidity. Pairing correlations in the superfluid and normal phase are studied with radio-frequency (rf) spectroscopy. A signature of strong correlations is observed above the critical temperature but also at spin polarizations where superfluidity is quenched even at zero temperature. Significant limitations for the interpretation of these experiments due to final state interactions are overcome by the creation of new superfluid spin mixtures. The asymmetric rf dissociation spectra of the new mixture allow us to determine the spectroscopic pair size in the crossover regime. The size of the resonantly interacting pairs is found to be on the order of, but smaller than the interparticle spacing. Rf spectra of the majority component in an imbalanced system show a signature of thermally excited quasiparticles and by comparison to the minority spectra reveal changes in the nature of the binding as a function of spin polarization.by Christian H. Schunck.Ph.D

Realization of a strongly interacting Bose-Fermi mixture from a two-component Fermi gas

We show the emergence of a strongly interacting Bose-Fermi mixture from a two-component Fermi mixture with population imbalance. By analyzing in situ density profiles of $^6$Li atoms in the BCS-BEC crossover regime we identify a critical interaction strength, beyond which all minority atoms pair up with majority atoms, and form a Bose condensate. This is the regime where the system can be effectively described as a boson-fermion mixture. We determine the dimer-fermion and dimer-dimer scattering lengths and beyond-mean-field contributions. Our study realizes a Gedanken experiment of bosons immersed in a Fermi sea of one of their constituents, revealing the composite nature of the bosons.Comment: 5 pages, 5 figure

Determination of the Superfluid Gap in Atomic Fermi Gases by Quasiparticle Spectroscopy

We present majority and minority radiofrequency (RF) spectra of strongly interacting imbalanced Fermi gases of Li-6. We observed a smooth evolution in the nature of pairing correlations from pairing in the superfluid region to polaron binding in the highly polarized normal region. The imbalance induces quasiparticles in the superfluid region even at very low temperature. This leads to a local bimodal spectral response, which allows us to determine the superfluid gap \Delta and the Hartree energy U.Comment: 5 pages, 4 figures (appendix: 3 pages, 5 figures

Determination of the Fermion Pair Size in a Resonantly Interacting Superfluid

Fermionic superfluidity requires the formation of pairs. The actual size of these fermion pairs varies by orders of magnitude from the femtometer scale in neutron stars and nuclei to the micrometer range in conventional superconductors. Many properties of the superfluid depend on the pair size relative to the interparticle spacing. This is expressed in BCS-BEC crossover theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS) type superfluid of loosely bound and large Cooper pairs to Bose-Einstein condensation (BEC) of tightly bound molecules. Such a crossover superfluid has been realized in ultracold atomic gases where high temperature superfluidity has been observed. The microscopic properties of the fermion pairs can be probed with radio-frequency (rf) spectroscopy. Previous work was difficult to interpret due to strong and not well understood final state interactions. Here we realize a new superfluid spin mixture where such interactions have negligible influence and present fermion-pair dissociation spectra that reveal the underlying pairing correlations. This allows us to determine the spectroscopic pair size in the resonantly interacting gas to be 2.6(2)/kF (kF is the Fermi wave number). The pairs are therefore smaller than the interparticle spacing and the smallest pairs observed in fermionic superfluids. This finding highlights the importance of small fermion pairs for superfluidity at high critical temperatures. We have also identified transitions from fermion pairs into bound molecular states and into many-body bound states in the case of strong final state interactions.Comment: 8 pages, 7 figures; Figures updated; New Figures added; Updated discussion of fit function

Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases

Water freezes into ice, atomic spins spontaneously align in a magnet, liquid helium becomes superfluid: Phase transitions are dramatic phenomena. However, despite the drastic change in the system's behaviour, observing the transition can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and superfluidity in trapped, weakly interacting Bose gases is the sudden appearance of a dense central core inside a thermal cloud. In strongly interacting gases, such as the recently observed fermionic superfluids, this clear separation between the superfluid and the normal parts of the cloud is no longer given. Condensates of fermion pairs could be detected only using magnetic field sweeps into the weakly interacting regime. The quantitative description of these sweeps presents a major theoretical challenge. Here we demonstrate that the superfluid phase transition can be directly observed by sudden changes in the shape of the clouds, in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the non-interacting wings of excess atoms serve as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions which drive the normal-to-superfluid transition at the critical population imbalance of 70(5)%.Comment: 16 pages (incl. Supplemental Material), 5 figure

Facets of diazotrophy in the oxygen minimum zone waters off Peru

Nitrogen fixation, the biological reduction of dinitrogen gas (N2) to ammonium (NH4+), is quantitatively the most important external source of new nitrogen (N) to the open ocean. Classically, the ecological niche of oceanic N2 fixers (diazotrophs) is ascribed to tropical oligotrophic surface waters, often depleted in fixed N, with a diazotrophic community dominated by cyanobacteria. Although this applies for large areas of the ocean, biogeochemical models and phylogenetic studies suggest that the oceanic diazotrophic niche may be much broader than previously considered, resulting in major implications for the global N-budget. Here, we report on the composition, distribution and abundance of nifH, the functional gene marker for N2 fixation. Our results show the presence of eight clades of diazotrophs in the oxygen minimum zone (OMZ) off Peru. Although proteobacterial clades dominated overall, two clusters affiliated to spirochaeta and archaea were identified. N2 fixation was detected within OMZ waters and was stimulated by the addition of organic carbon sources supporting the view that non-phototrophic diazotrophs were actively fixing dinitrogen. The observed co-occurrence of key functional genes for N2 fixation, nitrification, anammox and denitrification suggests that a close spatial coupling of N-input and N-loss processes exists in the OMZ off Peru. The wide distribution of diazotrophs throughout the water column adds to the emerging view that the habitat of marine diazotrophs can be extended to low oxygen/high nitrate areas. Furthermore, our statistical analysis suggests that NO2− and PO43− are the major factors affecting diazotrophic distribution throughout the OMZ. In view of the predicted increase in ocean deoxygenation resulting from global warming, our findings indicate that the importance of OMZs as niches for N2 fixation may increase in the futu

The Crowdsourced Replication Initiative: Investigating Immigration and Social Policy Preferences. Executive Report.

In an era of mass migration, social scientists, populist parties and social movements raise concerns over the future of immigration-destination societies. What impacts does this have on policy and social solidarity? Comparative cross-national research, relying mostly on secondary data, has findings in different directions. There is a threat of selective model reporting and lack of replicability. The heterogeneity of countries obscures attempts to clearly define data-generating models. P-hacking and HARKing lurk among standard research practices in this area.This project employs crowdsourcing to address these issues. It draws on replication, deliberation, meta-analysis and harnessing the power of many minds at once. The Crowdsourced Replication Initiative carries two main goals, (a) to better investigate the linkage between immigration and social policy preferences across countries, and (b) to develop crowdsourcing as a social science method. The Executive Report provides short reviews of the area of social policy preferences and immigration, and the methods and impetus behind crowdsourcing plus a description of the entire project. Three main areas of findings will appear in three papers, that are registered as PAPs or in process