5 research outputs found
Stamp \& Extend -- Instant but Undeniable Timestamping based on Lazy Trees
We present a Stamp\&Extend time-stamping scheme based on linking via modified creation of Schnorr signatures.
The scheme is based on lazy construction of a tree of signatures.
Stamp\&Extend returns a timestamp immediately after the request, unlike the schemes based on the concept of timestamping rounds.
Despite the fact that all timestamps are linearly linked, verification of a timestamp requires a logarithmic number of steps with respect to the chain length.
An extra feature of the scheme is that any attempt to forge a timestamp by the Time Stamping Authority (TSA) results in revealing its secret key, providing an undeniable cryptographic evidence of misbehavior of TSA.
Breaking Stamp\&Extend requires not only breaking Schnorr signatures,
but to some extend also breaking Pedersen commitments
Techniques for Transparent Parallelization of Discrete Event Simulation Models
Simulation is a powerful technique to represent the evolution of real-world phenomena
or systems over time. It has been extensively used in different research
fields (from medicine to biology, to economy, and to disaster rescue) to study
the behaviour of complex systems during their evolution (symbiotic simulation)
or before their actual realization (what-if analysis).
A traditional way to achieve high performance simulations is the employment
of Parallel Discrete Event Simulation (PDES) techniques, which are based
on the partitioning of the simulation model into Logical Processes (LPs) that
can execute events in parallel on different CPUs and/or different CPU cores,
and rely on synchronization mechanisms to achieve causally consistent execution
of simulation events. As it is well recognized, the optimistic synchronization
approach, namely the Time Warp protocol, which is based on rollback for recovering
possible timestamp-order violations due to the absence of block-until-safe
policies for event processing, is likely to favour speedup in general application/
architectural contexts.
However, the optimistic PDES paradigm implicitly relies on a programming
model that shifts from traditional sequential-style programming, given
that there is no notion of global address space (fully accessible while processing
events at any LP). Furthermore, there is the underlying assumption that the
code associated with event handlers cannot execute unrecoverable operations
given their speculative processing nature. Nevertheless, even though no unrecoverable
action is ever executed by event handlers, a means to actually undo
the action if requested needs to be devised and implemented within the software
stack.
On the other hand, sequential-style programming is an easy paradigm for
the development of simulation code, given that it does not require the programmer
to reason about memory partitioning (and therefore message passing) and
speculative (concurrent) processing of the application.
In this thesis, we present methodological and technical innovations which
will show how it is possible, by developing innovative runtime mechanisms, to
allow a programmer to implement its simulation model in a fully sequential way,
and have the underlying simulation framework to execute it in parallel according
to speculative processing techniques. Some of the approaches we provide show
applicability in either shared- or distributed-memory systems, while others will
be specifically tailored to multi/many-core architectures.
We will clearly show, during the development of these supports, what is the
effect on performance of these solutions, which will nevertheless be negligible,
allowing a fruitful exploitation of the available computing power. In the end,
we will highlight which are the clear benefits on the programming model tha