638 research outputs found
Mitigating Branch-Shadowing Attacks on Intel SGX using Control Flow Randomization
Intel Software Guard Extensions (SGX) is a promising hardware-based
technology for protecting sensitive computations from potentially compromised
system software. However, recent research has shown that SGX is vulnerable to
branch-shadowing -- a side channel attack that leaks the fine-grained (branch
granularity) control flow of an enclave (SGX protected code), potentially
revealing sensitive data to the attacker. The previously-proposed defense
mechanism, called Zigzagger, attempted to hide the control flow, but has been
shown to be ineffective if the attacker can single-step through the enclave
using the recent SGX-Step framework.
Taking into account these stronger attacker capabilities, we propose a new
defense against branch-shadowing, based on control flow randomization. Our
scheme is inspired by Zigzagger, but provides quantifiable security guarantees
with respect to a tunable security parameter. Specifically, we eliminate
conditional branches and hide the targets of unconditional branches using a
combination of compile-time modifications and run-time code randomization.
We evaluated the performance of our approach by measuring the run-time
overhead of ten benchmark programs of SGX-Nbench in SGX environment
An Oort cloud origin of the Halley-type comets
The origin of the Halley-type comets (HTCs) is one of the last mysteries of
the dynamical evolution of the Solar System. Prior investigation into their
origin has focused on two source regions: the Oort cloud and the Scattered
Disc. From the former it has been difficult to reproduce the non-isotropic,
prograde skew in the inclination distribution of the observed HTCs without
invoking a multi-component Oort cloud model and specific fading of the comets.
The Scattered Disc origin fares better but suffers from needing an order of
magnitude more mass than is currently advocated by theory and observations.
Here we revisit the Oort cloud origin and include cometary fading. Our
observational sample stems from the JPL catalogue. We only keep comets
discovered and observed after 1950 but place no a priori restriction on the
maximum perihelion distance of observational completeness. We then numerically
evolve half a million comets from the Oort cloud through the realm of the giant
planets and keep track of their number of perihelion passages with perihelion
distance q<2.5AU, below which the activity is supposed to increase
considerably. We can simultaneously fit the HTC inclination and semi-major axis
distribution very well with a power law fading function of the form m^-k, where
m is the number of perihelion passages with q<2.5 AU and k is the fading index.
We match both the inclination and semi-major axis distributions when k~1 and
the maximum imposed perihelion distance of the observed sample is q~1.8AU. The
value of k is higher than the one obtained for the Long-Period Comets (LPCs),
with k~0.7. This increase in k is most likely the result of cometary surface
processes. We argue the HTC sample is now most likely complete for q<1.8AU. We
calculate that the steady-state number of active HTCs with diameter D>2.3km and
q<1.8AU is of the order of 100.Comment: Accepted for publication in Astronomy and Astrophysic
An Oort cloud origin for the high-inclination, high-perihelion Centaurs
We analyse the origin of three Centaurs with perihelia in the range 15 AU to
30 AU, inclinations above 70 deg and semi-major axes shorter than 100 AU. Based
on long-term numerical simulations we conclude that these objects most likely
originate from the Oort cloud rather than the Kuiper Belt or Scattered Disc. We
estimate that there are currently between 1 and 200 of these high-inclination,
high-perihelion Centaurs with absolute magnitude H<8.Comment: Accepted for publication in MNRA
How planetary growth outperforms migration
Planetary migration is a major challenge for planet formation theories. The
speed of Type I migration is proportional to the mass of a protoplanet, while
the final decade of growth of a pebble-accreting planetary core takes place at
a rate that scales with the mass to the two-thirds power. This results in
planetary growth tracks (i.e., the evolution of a protoplanet's mass versus its
distance from the star) that become increasingly horizontal
(migration-dominated) with rising mass of the protoplanet. It has been shown
recently that the migration torque on a protoplanet is reduced proportional to
the relative height of the gas gap carved by the growing planet. Here we show
from 1-D simulations of planet-disc interaction that the mass at which a planet
carves a 50% gap is approximately 2.3 times the pebble isolation mass. Our
measurements of the pebble isolation mass from 1-D simulations match published
3-D results relatively well, except at very low viscosities where the 3-D
pebble isolation mass is significantly higher, possibly due to gap edge
instabilities not captured in 1-D. The pebble isolation mass demarks the
transition from pebble accretion to gas accretion. Gas accretion to form
gas-giant planets therefore takes place over a few astronomical units of
migration after reaching first the pebble isolation mass and, shortly after,
the 50% gap mass. Our results demonstrate how planetary growth can outperform
migration, both during core accretion and during gas accretion, even when the
Stokes number of the pebbles is small, St~0.01, and the pebble-to-gas flux
ratio in the protoplanetary disc is in the nominal range of 0.01-0.02. We find
that planetary growth is very rapid in the first million years of the
protoplanetary disc and that the probability for forming gas-giant planets
increases with the initial size of the protoplanetary disc and with decreasing
turbulent diffusion.Comment: Accepted for publication in Astronomy & Astrophysic
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