Short and Adjustable Signatures

Motivated by the problem of one-time password generation with security against server breaches, we introduce the notion of {\em adjustable signature schemes} that allow the length of a signature to be adjusted---at the setup, signing or verification stages, depending on the application. Defining security for such schemes poses several challenges, such as: (i) different signature lengths should provide different levels of security, and (ii) the effort required for forging a very short signature (e.g., 6 bytes) should not be reusable for forging additional signatures. We provide security definitions that concretely capture the trade-off between signature length, number of forgeries and level of security provided by the scheme. The above requirements rule out all existing solutions for short signatures. In this paper, as a feasibility result, we provide the first instantiation of all variants of adjustable signatures based on indistinguishability obfuscation. Our starting point is the state-of-the-art construction by Ramchen and Waters [ACM CCS 2014]. We observe that their scheme fails to meet our requirements for an adjustable signature scheme, and enhance it to obtain adjustable signatures with {\em shorter} signatures, {\em faster} signing and {\em strong} unforgeability. We also employ new proof techniques in order toobtain the above-mentioned notions of security. For the simpler case where adversarial effort does not grow with the number of forgeries, we also provide a concrete construction based on the BLS signature scheme, by instantiating it using smaller group sizes that yield shorter signature lengths while providing reasonable security. We implement this scheme for various signature sizes an report on its efficiency

An On-line Diagnostic Method for Open-circuit Switch Faults in NPC Multilevel Converters

On-line condition monitoring is of paramount importance for multilevel converters used in safety-critical applications. A novel on-line diagnostic method for detecting open-circuit switch faults in neutral-point-clamped (NPC) multilevel converters is introduced in this paper. The principle of this method is based on monitoring the abnormal variation of the dc-bus neutral-point current in combination with the existing information on instantaneous switching states and phase currents. Advantages of this method include simpler implementation and faster detection speed compared to other existing diagnostic methods in the literature. In this method, only one additional current sensor is required for measuring the dc-bus neutral-point current, therefore the implementation cost is low. Simulation and experimental results based on a lab-scale 50 kVA adjustable speed drive (ASD) with a three-level NPC inverter validate the efficacy of this novel diagnostic method

Ultracold atom superfluidity induced by the Feshbach resonance

We discuss the possible signatures of superfluidity induced by the Feshbach resonance in ultracold gas of fermion atoms. Approaching the phase transition from above there appear various manifestations of the gradually emerging order parameter, but yet the long range coherence is not established due to strong quantum fluctuations. The single particle excitation spectrum becomes gapped while at the same time the pair excitations are characterized by the narrow quasiparticle peak surrounded by the incoherent background. This quasiparticle shows up certain collective features such as the remnant of the "first sound" which at Tc spreads down to low momenta. Presence of this Goldstone mode is the most unambiguous proof for appearance of the superfluid state. We discuss how such mode can be detected experimentally.Comment: 5 pages, 3 figure