315 research outputs found
B. V. Sreekantan: A versatile and humane scientist
‘It is the duty of people like us to stay in
our own country and build up outstanding
schools of research such as
some other countries are fortunate to
possess.’
That was the vision of Homi Bhabha,
the founder director of Tata Institute of
Fundamental Research (TIFR). Set up on
1 June 1945 by Bhabha with the support
of Sir Dorabji Tata Trust, TIFR’s mission
was to undertake front-ranking
research in basic sciences in India
X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)
The galactic superluminal transient X-ray source GRS 1915+105 was observed
with the pointed proportional counters (PPCs) onboard the Indian satellite
IRS-P3 during 1996 July 23-27. We report here details of the behavior of this
source during the relatively quiet and low luminosity state. Large intensity
variations by a factor of 2 to 3, generally seen in black-hole candidates, are
observed at a time scale of 100 ms to few seconds. No significant variation is
detected over larger time scale of minute or more. The intensity variations are
described as sum of shots in the light curve, and the number distribution of
the shots are found to be exponential function of the fluence and duration of
the shots. The cross correlation spectrum between 6-18 keV and 2-6 keV X-rays
is found to have asymmetry signifying a delay of the hard X-rays by about 0.2
to 0.4 sec. This supports the idea of hard X-rays being generated by Compton
up-scattering from high energy clouds near the source of soft X-rays. Very
strong and narrow quasi periodic oscillations in the frequency range 0.62 to
0.82 Hz are observed. We discuss about a model which explains a gradual change
in the QPO frequencies with corresponding changes in the mass accretion rate of
the disk.Comment: 14 pages including 6 figures. To appear in Astronomy and Astrophysics
Supplement Serie
On the automatic construction of indistinguishable operations
An increasingly important design constraint for software running
on ubiquitous computing devices is security, particularly against
physical methods such as side-channel attack. One well studied methodology
for defending against such attacks is the concept of indistinguishable
functions which leak no information about program control
flow since all execution paths are computationally identical. However,
constructing such functions by hand becomes laborious and error prone
as their complexity increases. We investigate techniques for automating
this process and find that effective solutions can be constructed with
only minor amounts of computational effort.Fundação para a Ciência e Tecnologia - SFRH/BPD/20528/2004
Practical Improvements of Profiled Side-Channel Attacks on a Hardware Crypto-Accelerator
Abstract. This article investigates the relevance of the theoretical frame-work on profiled side-channel attacks presented by F.-X. Standaert et al. at Eurocrypt 2009. The analyses consist in a case-study based on side-channel measurements acquired experimentally from a hardwired crypto-graphic accelerator. Therefore, with respect to previous formal analyses carried out on software measurements or on simulated data, the inves-tigations we describe are more complex, due to the underlying chip’s architecture and to the large amount of algorithmic noise. In this dif-ficult context, we show however that with an engineer’s mindset, two techniques can greatly improve both the off-line profiling and the on-line attack. First, we explore the appropriateness of different choices for the sensitive variables. We show that a skilled attacker aware of the regis-ter transfers occurring during the cryptographic operations can select the most adequate distinguisher, thus increasing its success rate. Sec-ond, we introduce a method based on the thresholding of leakage data to accelerate the profiling or the matching stages. Indeed, leveraging on an engineer’s common sense, it is possible to visually foresee the shape of some eigenvectors thereby anticipating their estimation towards their asymptotic value by authoritatively zeroing weak components containing mainly non-informational noise. This method empowers an attacker, in that it saves traces when converging towards correct values of the secret. Concretely, we demonstrate a 5 times speed-up in the on-line phase of the attack.
Algorithmic Tamper-Proof (ATP) Security: Theoretical Foundations for Security against Hardware Tampering
Abstract. Traditionally, secure cryptographic algorithms provide security against an adversary who has only black-box access to the secret information of honest parties. However, such models are not always adequate. In particular, the security of these algorithms may completely break under (feasible) attacks that tamper with the secret key. In this paper we propose a theoretical framework to investigate the algorithmic aspects related to tamper-proof security. In particular, we define a model of security against an adversary who is allowed to apply arbitrary feasible functions f to the secret key sk, and obtain the result of the cryptographic algorithms using the new secret key f(sk). We prove that in the most general setting it is impossible to achieve this strong notion of security. We then show minimal additions to the model, which are needed in order to obtain provable security. We prove that these additions are necessary and also sufficient for most common cryptographic primitives, such as encryption and signature schemes. We discuss the applications to portable devices protected by PINs and show how to integrate PIN security into the generic security design. Finally we investigate restrictions of the model in which the tampering powers of the adversary are limited. These restrictions model realistic attacks (like differential fault analysis) that have been demonstrated in practice. In these settings we show security solutions that work even without the additions mentioned above
‘It Takes Two Hands to Clap’: How Gaddi Shepherds in the Indian Himalayas Negotiate Access to Grazing
This article examines the effects of state intervention on the workings of informal institutions that coordinate the communal use and management of natural resources. Specifically it focuses on the case of the nomadic Gaddi
shepherds and official attempts to regulate their access to grazing pastures in the Indian Himalayas. It is often predicted that the increased presence of the modern state critically undermines locally appropriate and community-based resource management arrangements. Drawing on the work of Pauline Peters and Francis Cleaver, I identify key instances of socially embedded ‘common’ management institutions and explain the evolution of these arrangements
through dynamic interactions between individuals, communities and the agents of the state. Through describing the ‘living space’ of Gaddi shepherds across the annual cycle of nomadic migration with their flocks I explore the
ways in which they have been able to creatively reinterpret external interventions, and suggest how contemporary arrangements for accessing pasture at different moments of the annual cycle involve complex combinations of the
formal and the informal, the ‘traditional’ and the ‘modern’
A Model of User Preferences for Semantic Services Discovery and Ranking
Current proposals on Semantic Web Services discovery and
ranking are based on user preferences descriptions that often come with
insufficient expressiveness, consequently making more difficult or even
preventing the description of complex user desires. There is a lack of a
general and comprehensive preference model, so discovery and ranking
proposals have to provide ad hoc preference descriptions whose expressiveness
depends on the facilities provided by the corresponding technique,
resulting in user preferences that are tightly coupled with the
underlying formalism being used by each concrete solution. In order to
overcome these problems, in this paper an abstract and sufficiently expressive
model for defining preferences is presented, so that they may be
described in an intuitively and user-friendly manner. The proposed model
is based on a well-known query preference model from database systems,
which provides highly expressive constructors to describe and compose
user preferences semantically. Furthermore, the presented proposal is independent
from the concrete discovery and ranking engines selected, and
may be used to extend current Semantic Web Service frameworks, such
as wsmo, sawsdl, or owl-s. In this paper, the presented model is also
validated against a complex discovery and ranking scenario, and a concrete
implementation of the model in wsmo is outlined.Comisión Interministerial de Ciencia y Tecnología TIN2006-00472Comisión Interministerial de Ciencia y Tecnología TIN2009-07366Junta de Andalucía TIC-253
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