1,722 research outputs found
GOTCHA Password Hackers!
We introduce GOTCHAs (Generating panOptic Turing Tests to Tell Computers and
Humans Apart) as a way of preventing automated offline dictionary attacks
against user selected passwords. A GOTCHA is a randomized puzzle generation
protocol, which involves interaction between a computer and a human.
Informally, a GOTCHA should satisfy two key properties: (1) The puzzles are
easy for the human to solve. (2) The puzzles are hard for a computer to solve
even if it has the random bits used by the computer to generate the final
puzzle --- unlike a CAPTCHA. Our main theorem demonstrates that GOTCHAs can be
used to mitigate the threat of offline dictionary attacks against passwords by
ensuring that a password cracker must receive constant feedback from a human
being while mounting an attack. Finally, we provide a candidate construction of
GOTCHAs based on Inkblot images. Our construction relies on the usability
assumption that users can recognize the phrases that they originally used to
describe each Inkblot image --- a much weaker usability assumption than
previous password systems based on Inkblots which required users to recall
their phrase exactly. We conduct a user study to evaluate the usability of our
GOTCHA construction. We also generate a GOTCHA challenge where we encourage
artificial intelligence and security researchers to try to crack several
passwords protected with our scheme.Comment: 2013 ACM Workshop on Artificial Intelligence and Security (AISec
Sliding Density-Wave in Sr_{14}Cu_{24}O_{41} Ladder Compounds
We used transport and Raman scattering measurements to identify the
insulating state of self-doped spin 1/2 two-leg ladders of Sr_{14}Cu_{24}O_{41}
as a weakly pinned, sliding density wave with non-linear conductivity and a
giant dielectric response that persists to remarkably high temperatures
Examining the Connections within the Startup Ecosystem: A Case Study of St. Louis
This paper documents the resurgence of entrepreneurial activity in St. Louis by reporting on the collaboration and local learning within the startup community. This activity is happening both between entrepreneurs and between organizations that provide support, such as mentoring and funding, to entrepreneurs. As these connections deepen, the strength of the entrepreneurial ecosystem grows. Another finding from the research is that activity-based events, where entrepreneurs have the chance to use and practice the skills needed to grow their businesses, are most useful. St. Louis provides a multitude of these activities, such as Startup Weekend, 1 Million Cups, Code Until Dawn, StartLouis, and GlobalHack. Some of these are St. Louis specific, but others have nationwide or global operations, providing important implications for other cities
Possible manifestation of spin fluctuations in the temperature behavior of resistivity in Sm_{1.85}Ce_{0.15}CuO_4 thin films
A pronounced step-like (kink) behavior in the temperature dependence of
resistivity is observed in the optimally-doped
thin films around and attributed to
manifestation of strong spin fluctuations induced by moments with the
energy . In addition to fluctuation
induced contribution due to thermal broadening effects (of the
width ), the experimental data are found to be well fitted
accounting for residual (zero-temperature) , electron-phonon and electron-electron contributions. The
best fits produced , ,
, and for estimates of the plasmon frequency, the
impurity scattering rate, electron-phonon coupling constant, and the Fermi
energy, respectively.Comment: 6 pages (REVTEX4), 2 EPS figures; accepted for publication in JETP
Letter
Modulation of p27/Cdk2 complex formation through 4D5-mediated inhibition of HER2 receptor signaling
The molecular mechanisms mediating the anti-proliferative effects of the murine anti-HER2 monoclonal antibody (4D5) were investigated in HER2-overexpressing human carcinoma cell lines. Treatment with 4D5 resulted in a dramatic accumulation of BT-474 breast carcinoma cells in Gl; concomitant with reduced expression of proteins involved in sequestration of the cyclin E/Cdk2 inhibitor protein p27, increased association of p27 with Cdk2 complexes and Cdk2 inactivation. No equivalent effects were observed in BT-474 cells treated with a control, non-inhibitory HER2 monoclonal antibody (FRP5) or in a HER2-overexpressing cell line insensitive to 4D5 treatment (MKN7 gastric carcinoma cells), confirming the relationship between these molecular changes and 4D5-mediated inhibition of proliferation. Increased p27 expression was also observed in 4D5-treated BT-474 cells; however an antisense approach demonstrated that this increase was not required for Cdk2 inactivation or establishment of the Gl block. These data suggest that 4D5 interferes with HER2 receptor signaling, resulting in downregulation of proteins involved in p27 sequestration. This causes release of p27, allowing binding and inhibition of cyclin E/Cdk2 complexes and inhibition of Gl/S progression. This model was confirmed using a second 4D5-sensitive, HER2-overexpressing breast tumor line (SKBR3), and suggests that the dependency of a given tumor cell on elevated HER2-receptor signaling for the maintenance of p27 sequestration proteins may determine the clinical response to treatment with the humanized anti-HER2 monoclonal antibody Herceptin® (trastuzumab
Electrical Resistivity and Thermal Expansion Measurements of URu2Si2 under Pressure
We carried out simultaneous measurements of electrical resistivity and
thermal expansion of the heavy-fermion compound URu2Si2 under pressure using a
single crystal. We observed a phase transition anomaly between hidden (HO) and
antiferromagnetic (AFM) ordered states at TM in the temperature dependence of
both measurements. For the electrical resistivity, the anomaly at TM was very
small compared with the distinct hump anomaly at the phase transition
temperature T0 between the paramagnetic state (PM) and HO, and exhibited only a
slight increase and decrease for the I // a-axis and c-axis, respectively. We
estimated each excitation gap of HO, Delta_HO, and AFM, Delta_AFM, from the
temperature dependence of electrical resistivity; Delta_HO and Delta_AFM have
different pressure dependences from each other. On the other hand, the
temperature dependence of thermal expansion exhibited a small anomaly at T0 and
a large anomaly at TM. The pressure dependence of the phase boundaries of T0
and TM indicates that there is no critical end point and the two phase
boundaries meet at the critical point.Comment: 4 pages, 4 figure
Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator
We report strong instantaneous photoinduced absorption (PA) in the
quasi-one-dimensional Mott insulator in the IR spectral
region. The observed PA is to an even-parity two-photon state that occurs
immediately above the absorption edge. Theoretical calculations based on a
two-band extended Hubbard model explains the experimental features and
indicates that the strong two-photon absorption is due to a very large
dipole-coupling between nearly degenerate one- and two-photon states. Room
temperature picosecond recovery of the optical transparency suggests the strong
potential of for all-optical switching.Comment: 10 pages, 4 figure
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