Building Secure and Anonymous Communication Channel: Formal Model and its Prototype Implementation

Various techniques need to be combined to realize anonymously authenticated communication. Cryptographic tools enable anonymous user authentication while anonymous communication protocols hide users' IP addresses from service providers. One simple approach for realizing anonymously authenticated communication is their simple combination, but this gives rise to another issue; how to build a secure channel. The current public key infrastructure cannot be used since the user's public key identifies the user. To cope with this issue, we propose a protocol that uses identity-based encryption for packet encryption without sacrificing anonymity, and group signature for anonymous user authentication. Communications in the protocol take place through proxy entities that conceal users' IP addresses from service providers. The underlying group signature is customized to meet our objective and improve its efficiency. We also introduce a proof-of-concept implementation to demonstrate the protocol's feasibility. We compare its performance to SSL communication and demonstrate its practicality, and conclude that the protocol realizes secure, anonymous, and authenticated communication between users and service providers with practical performance.Comment: This is a preprint version of our paper presented in SAC'14, March 24-28, 2014, Gyeongju, Korea. ACMSAC 201

Spectral evolution of GRB 060904A observed with Swift and Suzaku -- Possibility of Inefficient Electron Acceleration

We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma = 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim t^{-4}$ while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.Comment: 19 pages, 9 figures, accepted for publication in PASJ (Suzaku 2nd Special Issue

Influence of particulate size on creep properties of TiB-reinforced orthorhombic Ti-22Al-27Nb

Intermetallic titanium alloys based on the orthorhombic phase Ti2-Al-Nb exhibit a higher ductility and fracture toughness compared to other intermetallic systems such as TiAl and super-2 (Ti3Al) [1]. These are under development for applications up to 700°C in aeronautic gasturbines. The reinforcement with titanium boride (TiB) particulates leads to an improvement of the mechanical properties. TiB is chemically and physically compatible with titanium alloys, especially with respect on the coefficient of thermal expansion. The effect of the reinforcement is influenced not only by the volume content but also by the size of particulates. Subject of this study are the creep properties of Ti-22Al-27Nb alloy reinforced with 6.5% TiB. It has been reported recently, that smaller particulates enhances the tensile strength as well as the fatigue strength [2]. Reasons for the additional reinforcing effects can be found in the micromechanisms of plastic deformation such as blocking of dislocations

Creep properties of TiB Particulate-Reinforced Ti-22Al-27Nb Composites

The Ti2AlNb intermetallic alloys, based on the ordered orthorhombic (O) phase, have a higher specific strength, better creep resistance and better workability than conventional titanium aluminides, such as TiAl and Ti3Al. Typical alloy system so far studied is a Ti-22Al-27Nb (mol pct) alloy, consisting primarily of the (O+B2 (ordered BCC)) two-phase microstructure. However, the mechanical properties of these alloys must be improved before commercial applications are feasible. To further improve the mechanical properties of this Ti2AlNb alloy, a TiB particulate-reinforced Ti-22Al-27Nb matrix composite was prepared using the gas-atomized powder metallurgy method. Because of the rapid solidification during the gas atomization process, the TiB particulates dispersed in the composite were extremely fine, with an average diameter of less than 1 μm and lengths ranging up to 5 μm. This composite (PM composite) showed better creep properties at high temperatures than both an unreinforced Ti-22Al-27Nb matrix alloy and a Ti-22Al-27Nb/TiB composite produced using a conventional ingot metallurgy method (IM composite) with relatively coarse (average diameter 5 μm and average length 40 μm) TiB particulates. We have also found that the size of TiB particulate in the PM composite can be controlled by heat treatment at high temperature (around 1473 K), and investigated the effect of the size of dispersed TiB particulates on the creep properties of Ti-22Al-27Nb/TiB composites

Effects of Mo segregation on Charpy absorbed energy in Ti-12Mo alloys

Beta phase stabilizing elements such as Mo have strong tendency to segregate. We have introduced swirly type segregation of Mo in Ti-12Mo (mass %) alloy through groove bar rolling. After solution treatment and low temperature aging, hard omega phase was precipitated heterogeneously, which improved the room temperature tensile elongation values without sacrificing tensile strength. In this study, the effect of Mo segregation and heterogeneous distribution of omega phase on Charpy absorbed energy was investigated in Ti-12Mo alloy. Samples with two types of segregation were prepared; namely, swirly segregation in bar rolled sample and layered segregation in plate rolled sample. For comparison, we have also prepared Ti-12Mo bar samples with lesser Mo segregation, through high temperature thermomechanical treatment. Charpy impact tests were carried out at room temperature, 373 K and 473 K, respectively, using the samples after aging to introduce isothermal omega-phase. The samples with the segregation exhibited higher Charpy absorbed energy, especially at higher temperature of 473 K, while the sample with the swirly segregation showed higher Charpy absorbed energy than that with the layered segregation. The sample with lesser Mo segregation exhibited brittle intergranular fracture surface after Charpy testing. On the contrary, samples with Mo segregation exhibited ductile transgranular fracture surfaces

Microstructure-twinning relations in beta-Ti alloys

We have investigated twinning-microstructure relations in β-Ti alloys by statistical analysis of the evolving twin structure upon deformation by in-situ SEM testing and electron backscattering diffraction (EBSD). In particular, we have analyzed the effects of crystallographic orientation, grain size and chemical gradient structure on the nucleation and propagation behavior of {332} twins in a β-Ti-15 Mo (wt.%) alloy and a multilayered β-Ti-10Mo-xFe (x: 1-3 wt.%). Microstructural parameters such as number of twins per grain and number of twins per grain boundary area were statistically analyzed

Deformation mechanisms and effect of oxygen addition on mechanical properties of Ti-7.5Mo alloy with α” martensite

Effect of oxygen content as an important interstitial solute on the microstructure and mechanical properties of Ti-7.5Mo alloy was investigated. With increasing the oxygen content, the yielding strength, ultimate tensile strength and Young’s modulus of Ti-7.5Mo-xO (x=0, 0.2, 0.3, 0.4, 0.5) alloys increased, while the elongation showing a decreasing tendency. Solid-solution strengthening by the oxygen atoms has been addressed as the main strengthening mechanism. Ti-7.5Mo-xO (x ≤ 0.3) alloys have been regarded with an excellent combination of high yield strength (~640 MPa) and elongation (~28%), as well as low Young’s modulus (~60 GPa). The deformation microstructure of orthorhombic-α” martensite in Ti-7.5Mo alloy was also investigated by tracking a change in the microstructure of a selected area upon tensile deformation. Deformation twins induced by 5% tensile straining was identified as {112}α”-type I twins, which had not been reported before in α”martensite in β-Ti alloys