VerSum: Verifiable Computations over Large Public Logs

VerSum allows lightweight clients to outsource expensive computations over large and frequently changing data structures, such as the Bitcoin or Namecoin blockchains, or a Certificate Transparency log. VerSum clients ensure that the output is correct by comparing the outputs from multiple servers. VerSum assumes that at least one server is honest, and crucially, when servers disagree, VerSum uses an efficient conflict resolution protocol to determine which server(s) made a mistake and thus obtain the correct output. VerSum's contribution lies in achieving low server-side overhead for both incremental re-computation and conflict resolution, using three key ideas: (1) representing the computation as a functional program, which allows memoization of previous results; (2) recording the evaluation trace of the functional program in a carefully designed computation history to help clients determine which server made a mistake; and (3) introducing a new authenticated data structure for sequences, called SeqHash, that makes it efficient for servers to construct summaries of computation histories in the presence of incremental re-computation. Experimental results with an implementation of VerSum show that VerSum can be used for a variety of computations, that it can support many clients, and that it can easily keep up with Bitcoin's rate of new blocks with transactions.United States. Defense Advanced Research Projects Agency. Clean-slate design of Resilient, Adaptive, Secure Hosts (CRASH) Program (Contract N66001-10-2-4089)National Science Foundation (U.S.) (Award CNS-1053143)National Science Foundation (U.S.) (Award CNS-1413920

CFD modeling of microwave electrothermal thrusters

Microwave-heated plasmas in convergent nozzles are analyzed using a coupled Maxwell and Navier-Stokes solver to examine relevant issues associated with microwave thermal propulsion. Parametric studies are conducted to understand the effect of power, pressure, and plasma location with respect to the nozzle throat. For nozzles in the 0.5 to 3 N range with helium flow, results show that specific impulses up to 550-650 seconds are possible, with further increases being limited by severe wall-heating. Coupling efficiencies of over 90 percent are consistently obtained, with overall efficiencies ranging from 40 percent to 80 percent. Size scale-up studies-done by scaling the frequency from 2.45 GHz to 0.91 GHz-indicate that plasma migration toward the walls occurs more frequently for the lower frequency. Increasing the cavity aspect ratio and detuning the cavity are found to be effective ways of keeping the plasma on axis

Microprojectile-Mediated Genetic Transformation of Embryogenic Sweetgum Cultures

With its rapid biomass accumulation, sweetgum (Liquidambar styraciflua) may be particularly suitable for phytoremediation purposes, especially if it can be engineered with genes that enhance tolerance and/or accumulation of heavy metals. Sweetgum has been transformed using Agrobacterium tumefaciens-mediated transformation of shoot explants or nodule cultures, followed by adventitious shoot regeneration, and by microprojectile bombardment of nodule cultures. We tested microprojectile bombardment of embryogenic cultures as an alternative means to produce transgenic sweetgum trees, with the long-term goal of engineering the species with genes conferring heavy metal resistance. Embryogenic cultures were obtained by culturing immature seeds on an induction/maintenance medium (IMM) with 2,4-D and BA. Cultures proliferated as proembryogenic masses (PEMs) with monthly transfer to fresh medium and formed suspensions following transfer to liquid IMM. Bombardment parameters were optimized using transient GUS expression. Different osmotic conditioning and selection agents and concentrations were tested to determine suitable levels of these agents for sweetgum PEMS. PEMs representing 4 different genotypes were sizefractionated and bombarded with pBI426, which contains a translational fusion between the GUS and the NPTII coding region, under the control of a double 35S promoter. Following selection on proliferation medium supplemented with 50 mg/l of kanamycin, 8 kanamycin-resistant sweetgum lines were recovered, all from one of the bombarded genotypes. While all lines remaining kanamycin-resistant were GUS- and PCR-positive, and Southern analysis indicated stable integration of the transgenes, neither the transclones nor the untransformed control line from which they were derived were capable of producing somatic embryos, having apparently lost this potential over time. Sweetgum transclones cryostored for several months maintained GUS expression.Papers and abstracts from the 27th Southern Forest Tree Improvement Conference held at Oklahoma State University in Stillwater, Oklahoma on June 24-27, 2003

Progress with American Chestnut Somatic Embryogenesis

American chestnut (Castanea dentata), once the dominant forest species of the Appalachian Mountains in the eastern United States, was devastated during the early twentieth century by the introduction of the chestnut blight fungus. As part of an effort to restore the species to the forest, we have been working with embryogenic cultures of the species, aiming to establish a reliable somatic embryogenesis system for mass clonal propagation, as well as for genetic transformation with potential anti-fungal genes. While initiation of embryogenic cultures from immature ovules of American chestnut has become routine, bottlenecks still remain for embryo maturation, germination and conversion. Effects of cold stratification, gellan gum concentration and activated charcoal on somatic seedling production were investigated. Studies of other variables, such as the effects of light quality on germination and conversion, are underway. Using five genotypes, clusters of proembryogenic masses maintained on WPM with 2 mg/l 2,4-D were transferred to basal WPM for somatic embryo development. Individual cotyledonary-stage embryos (2-4 mm) were cultured for 10 days on basal medium prior to storage at 4o C for 12 weeks in the dark. These embryos germinated at an average frequency of 23% following transfer to WPM basal medium in GA7 vessels in the light. Embryos that did not receive cold treatment or were stored for only 6 weeks failed to germinate. Embryos on WPM with 5 g/l activated charcoal (AC) germinated at the same frequencies as those cultured without AC, but AC prevented darkening of taproots. Embryos cultured on 3 or 6 g/l Phytagel germinated at higher frequencies following 12 weeks cold storage than did those cultured on 10 g/l Phytagel. To date, over 30 somatic seedlings representing 3 genotypes have been transferred successfully to greenhouse conditions.Papers and abstracts from the 27th Southern Forest Tree Improvement Conference held at Oklahoma State University in Stillwater, Oklahoma on June 24-27, 2003

Distributed Management of Massive Data: an Efficient Fine-Grain Data Access Scheme

This paper addresses the problem of efficiently storing and accessing massive data blocks in a large-scale distributed environment, while providing efficient fine-grain access to data subsets. This issue is crucial in the context of applications in the field of databases, data mining and multimedia. We propose a data sharing service based on distributed, RAM-based storage of data, while leveraging a DHT-based, natively parallel metadata management scheme. As opposed to the most commonly used grid storage infrastructures that provide mechanisms for explicit data localization and transfer, we provide a transparent access model, where data are accessed through global identifiers. Our proposal has been validated through a prototype implementation whose preliminary evaluation provides promising results

Scalable Verification for Outsourced Dynamic Databases

Query answers from servers operated by third parties need to be verified, as the third parties may not be trusted or their servers may be compromised. Most of the existing authentication methods construct validity proofs based on the Merkle hash tree (MHT). The MHT, however, imposes severe concurrency constraints that slow down data updates. We introduce a protocol, built upon signature aggregation, for checking the authenticity, completeness and freshness of query answers. The protocol offers the important property of allowing new data to be disseminated immediately, while ensuring that outdated values beyond a pre-set age can be detected. We also propose an efficient verification technique for ad-hoc equijoins, for which no practical solution existed. In addition, for servers that need to process heavy query workloads, we introduce a mechanism that significantly reduces the proof construction time by caching just a small number of strategically chosen aggregate signatures. The efficiency and efficacy of our proposed mechanisms are confirmed through extensive experiments. 1

Chosen-ciphertext security from subset sum

We construct a public-key encryption (PKE) scheme whose security is polynomial-time equivalent to the hardness of the Subset Sum problem. Our scheme achieves the standard notion of indistinguishability against chosen-ciphertext attacks (IND-CCA) and can be used to encrypt messages of arbitrary polynomial length, improving upon a previous construction by Lyubashevsky, Palacio, and Segev (TCC 2010) which achieved only the weaker notion of semantic security (IND-CPA) and whose concrete security decreases with the length of the message being encrypted. At the core of our construction is a trapdoor technique which originates in the work of Micciancio and Peikert (Eurocrypt 2012

Generalized conductance sum rule in atomic break junctions

When an atomic-size break junction is mechanically stretched, the total conductance of the contact remains approximately constant over a wide range of elongations, although at the same time the transmissions of the individual channels (valence orbitals of the junction atom) undergo strong variations. We propose a microscopic explanation of this phenomenon, based on Coulomb correlation effects between electrons in valence orbitals of the junction atom. The resulting approximate conductance quantization is closely related to the Friedel sum rule.Comment: 4 pages, 1 figure, appears in Proceedings of the NATO Advanced Research Workshop ``Size dependent magnetic scattering'', Pecs, Hungary, May 28 - June 1, 200