Distinguishing Majorana bound states from localized Andreev bound states by interferometry

Experimental evidence for Majorana bound states (MBSs) is so far mainly based on the robustness of a zero-bias conductance peak. However, similar features can also arise due to Andreev bound states (ABSs) localized at the end of an island. We show that these two scenarios can be distinguished by an interferometry experiment based on embedding a Coulomb-blockaded island into an Aharonov-Bohm ring. For two ABSs, when the ground state is nearly degenerate, cotunneling can change the state of the island and interference is suppressed. By contrast, for two MBSs the ground state is nondegenerate and cotunneling has to preserve the island state, which leads to $h / e$-periodic conductance oscillations with magnetic flux. Such interference setups can be realized with semiconducting nanowires or two-dimensional electron gases with proximity-induced superconductivity and may also be a useful spectroscopic tool for parity-flip mechanisms

Coupling and braiding Majorana bound states in networks defined in proximitized two-dimensional electron gases

Two-dimensional electron gases with strong spin-orbit coupling covered by a superconducting layer offer a flexible and potentially scalable platform for Majorana networks. We predict Majorana bound states (MBSs) to appear for experimentally achievable parameters and realistic gate potentials in two designs: either underneath a narrow stripe of a superconducting layer (S-stripes) or where a narrow stripe has been removed from a uniform layer (N-stripes). The coupling of the MBSs can be tuned for both types in a wide range (10 $\mu$eV) using gates placed adjacent to the stripes. For both types, we numerically compute the local density of states for two parallel Majorana-stripe ends as well as Majorana trijunctions formed in a tuning-fork geometry. The MBS coupling between parallel Majorana stripes can be suppressed below 1 neV for potential barriers in the meV range for separations of about 200 nm. We further show that the MBS couplings in a trijunction can be gate-controlled in a range similar to the intra-stripe coupling while maintaining a sizable gap to the excited states (tens of $\mu$eV). Altogether, this suggests that braiding can carried out on a time scale of 10-100 ns

On the Design and Analysis of Stream Ciphers

This thesis presents new cryptanalysis results for several different stream cipher constructions. In addition, it also presents two new stream ciphers, both based on the same design principle. The first attack is a general attack targeting a nonlinear combiner. A new class of weak feedback polynomials for linear feedback shift registers is identified. By taking samples corresponding to the linear recurrence relation, it is shown that if the feedback polynomial has taps close together an adversary to take advantage of this by considering the samples in a vector form. Next, the self-shrinking generator and the bit-search generator are analyzed. Both designs are based on irregular decimation. For the self-shrinking generator, it is shown how to recover the internal state knowing only a few keystream bits. The complexity of the attack is similar to the previously best known but uses a negligible amount of memory. An attack requiring a large keystream segment is also presented. It is shown to be asymptotically better than all previously known attacks. For the bit-search generator, an algorithm that recovers the internal state is given as well as a distinguishing attack that can be very efficient if the feedback polynomial is not carefully chosen. Following this, two recently proposed stream cipher designs, Pomaranch and Achterbahn, are analyzed. Both stream ciphers are designed with small hardware complexity in mind. For Pomaranch Version 2, based on an improvement of previous analysis of the design idea, a key recovery attack is given. Also, for all three versions of Pomaranch, a distinguishing attack is given. For Achterbahn, it is shown how to recover the key of the latest version, known as Achterbahn-128/80. The last part of the thesis introduces two new stream cipher designs, namely Grain and Grain-128. The ciphers are designed to be very small in hardware. They also have the distinguishing feature of allowing users to increase the speed of the ciphers by adding extra hardware

Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires

We numerically compute the low-energy spectrum of a gate-controlled superconducting topological nanowire segmented into two islands, each Josephson-coupled to a bulk superconductor. This device may host two pairs of Majorana bound states and could provide a platform for testing Majorana fusion rules. We analyze the crossover between (i) a charge-dominated regime utilizable for initialization and readout of Majorana bound states, (ii) a single-island regime for dominating inter-island Majorana coupling, (iii) a Josephson-plasmon regime for large coupling to the bulk superconductors, and (iv) a regime of four Majorana bound states allowing for topologically protected Majorana manipulations. From the energy spectrum, we derive conservative estimates for the time scales of a fusion-rule testing protocol proposed recently [arXiv:1511.05153]. We also analyze the steps needed for basic Majorana braiding operations in branched nanowire structures

Communicating Cybersecurity Vulnerability Information: A Producer-Acquirer Case Study

The increase in both the use of open-source software (OSS) and the number of new vulnerabilities reported in this software constitutes an increased threat to businesses, people, and our society. To mitigate this threat, vulnerability information must be efficiently handled in organizations. In addition, where e.g., IoT devices are integrated into systems, such information must be disseminated from producers, who are implementing patches and new firmware, to acquirers who are responsible for maintaining the systems. We conduct an exploratory case study with one producer of IoT devices and one acquirer of the same devices, where the acquirer integrates the devices into larger systems. Through this two-sided case study, we describe company roles, internal and inter-company communication, and the decisions that need to be made with regard to cybersecurity vulnerabilities. We also identify and discuss both challenges and opportunities for improvements, from the point of view of both the producer and acquirer

Cryptanalysis of the Stream cipher BEAN

BEAN is a recent stream cipher proposal that uses Feedback with Carry Shift Registers (FCSRs) and an output function. There is a sound motivation behind the use of FCSRs in BEAN as they provide several cryptographically interesting properties. In this paper, we show that the output function is not optimal. We give an efficient distinguisher and a key recovery attack that is slightly better than brute force, requiring no significant memory. We then show how this attack can be made better with access to more keystream. Already with access to 6 KiB, the 80-bit key is recovered in time 2^73

A Data Model and Processing Environment for Ocean-Wide Bathymetric Data Compilations

The compilation of ocean-wide digital bathymetric models (DBM) requires specific features of the bathymetric data storage and great flexibility of the data processing chain. In this article a solution based upon a spatial relational database management system and a Geographical Information System front end is introduced, which will eventually serve the compilation of a new DBM of the North Atlantic Ocean. As shown in a preliminary case study, the abundance of sounding data-both single beam and multibeam-available in that area to date bears an extremely high potential to derive a DBM with much greater accuracy and resolution than the DBMs commonly used today.La compilacion de modelos batimetricos digitales oceanicos requiere caracterfsticas especfficas de almacenamiento de datos batimetricos y una gran flexibilidad en la cadena de procesamiento de datos. En este articulo se presenta una solucion basada en un sistema de administracion de una base de datos relacionales espaciales y se introduce un Sistema de Informacion Geografica, que servira finalmente para la compilacion de un nuevo Modelo Batimetrico Digital del Oceano Atlantica Norte. Tal y como se muestra en un estudio de un caso preliminar, la abundancia de datos de sondeos - tanto multihaz como monohaz - disponibles actualmente en esa zona ofrecen un potencial extremadamente alto para alcanzar un Modelo Batimetrico Digital con mayor precision y resolucion que los modelos utilizados comunmente hoy en dia.La compilation des modeles bathymetriques numeriques (DBM) des oceans necessite des elements specifiques du stockage des donnees bathymetriques et une grande flexibilite de la chaine de traitement des donnees. Dans cet article, une solution reposant sur le systeme de gestion de la base de donnees relationnelle et un systeme d'information geographique frontal sont introduits, ce qui servira en fin de compte a la compilation d'un nouveau systeme DBM de l'ocean atlantique nord septentrional. Comme indique dans l'etude de cas preliminaire, le grand nombre de donnees de sondage, a la fois monofaisceau et multifaisceaux, disponible dans cette zone, constitue a ce jour un potentiet tres eteve pour la mise au point d'un DBM avec une exactitude et une resolution bien superieures a celle aujourd'hui des DBMs en service dans ces jours

Security Evaluation of Stream Cipher Enocoro-128v2

Abstract in Undetermined This report presents a security evaluation of the Enocoro-128v2 stream cipher. Enocoro-128v2 was proposed in 2010 and is a member of the Enocoro family of stream ciphers. This evaluation examines several different attacks applied to the Enocoro-128v2 design. No attack better than exhaustive key search has been found

Evaluation of Some Algorithms for Hardware-Oriented Message Authentication

In this technical report, we consider ultra light-weight constructions of message authentication in hardware applications. We examine several known constructions and evaluate details around their hardware implementations. These constructions are all based on the framework of universal hash functions

Four-Majorana qubit with charge readout: dynamics and decoherence

We present a theoretical analysis of a Majorana-based qubit consisting of two topological superconducting islands connected via a Josephson junction. The qubit is operated by electrostatic gates which control the coupling of two of the four Majorana zero modes. At the end of the operation, readout is performed in the charge basis. Even though the operations are not topologically protected, the proposed experiment can potentially shed light on the coherence of the parity degree of freedom in Majorana devices and serve as a first step towards topological Majorana qubits. We discuss in detail the charge-stability diagram and its use for characterizing the parameters of the devices, including the overlap of the Majorana edge states. We describe the multi-level spectral properties of the system and present a detailed study of its controlled coherent oscillations, as well as decoherence resulting from coupling to a non-Markovian environment. In particular, we study a gate-controlled protocol where conversion between Coulomb-blockade and transmon regimes generates coherent oscillations of the qubit state due to the overlap of Majorana modes. We show that, in addition to fluctuations of the Majorana coupling, considerable measurement errors may be accumulated during the conversion intervals when electrostatic fluctuations in the superconducting islands are present. These results are also relevant for several proposed implementations of topological qubits which rely on readout based on charge detection