Recently, Yeh, Shen and Hwang proposed an one-time password authentication scheme, which enhances the S/KEY scheme to resist server spoofing attacks, preplay attacks and off-line dictionary attacks. In this letter, the weaknesses and inconveniences of their scheme are demonstrated.

Bandpass sampling algorithm is effectively adopted to obtain the digital signal with significantly reduced sampling rate for a single radio frequency(RF) signal. In order to apply the concept to multiple RF signals, we propose bandpass sampling algorithms with the normal and the inverse placements since we are interested in uniform order of the spectrum in digital domain after bandpass sampling. In addition, we verify the propose algorithms with generalized equation forms for the multiple RF signals.

To enable the use of passive transmission lines (PTLs) for the interconnection of single-flux-quantum (SFQ) circuits, we have implemented a driver and a receiver and have developed a method for designing SFQ circuits with passive interconnections. Basic components and properties of passive interconnections, such as the frequency characteristics of the driver and receiver, the PTL delay, and the crosstalk between PTLs, have been experimentally verified. Our developed components and design method have been applied to actual SFQ circuits, such as a 44 switch having block-to-block passive interconnections and a 22 switch having gate-to-gate passive interconnections. We have also shown the advantages of PTLs over Josephson transmission lines (JTLs). We also discuss the prospects of SFQ circuits having passive interconnections.

A high temperature superconductor (HTS) filter is designed and measured at 1.93 GHz, using microstrip half-wavelength spiral resonators. Resonant and coupling characteristics of miniaturized microstrip spiral resonators are investigated first. Then a 4-pole Chebyshev bandpass filter with a very narrow passband (4.1 MHz) is designed and realized using microstrip spiral resonators. The filter is fabricated using HTS YBCO films deposited on a LaAlO3 substrate. The measured frequency response of the filter agrees reasonably with the specifications, and shows that the filter owns excellent property of spurious resonance rejection over a wide frequency range.

In this paper, we propose a new algorithm for calculating the exact poles of the admittance matrix of RLCG interconnects. After choosing dominant poles and corresponding residues, each element of the exact admittance matrix is approximated by partial fraction. A procedure to obtain the residues that guarantee the passivity is also provided, based on experimental studies. In the procedure the residues are calculated by using the least squares method so that the partial fraction matches each element of the exact admittance matrix in the frequency-domain. From the partial fraction representation, the asymptotic equivalent circuit models which can be easily simulated with SPICE are synthesized. It is shown that an efficient model-order reduction is possible for short-length interconnects.

At frequencies currently used by mobile communications, many of the microstrip half-wavelength resonators are too large to realize miniaturized filters. For this reason, very small-sized microstrip spiral resonators and filters, using high-temperature superconductors (HTS), have been studied recently. In this paper, the resonant and coupling characteristics of microstrip G-type and S-type spiral resonators are investigated first by using an electromagnetic simulator. Then small-sized 4-pole, 8-pole, and 16-pole Chebyshev bandpass filters using S-type spirals are designed, respectively, with a midband frequency f0 = 1.93 GHz. The frequency responses of the filters satisfy well the desired specifications, and the measured frequency response of the 8-pole HTS filter agrees well with the theoretical prediction.

A miniaturized rectangular resonator made of a novel anisotropic artificial dielectric material is investigated which has advantages of small size and big separation of the higher-modes. To obtain a property of anisotropic permittivity, artificial dielectric material is fabricated by lamination of rectangular metal strips etched on a printed circuit board. Artificial dielectric rectangular resonators are designed to excite TE10δ mode selectively, aligning the rectangular metal patterns along the direction of the mode electric field line. The resonant frequencies and coupling coefficient of artificial dielectric rectangular resonators encapsulated in a metal waveguide are analyzed theoretically, and compared with the experimental result. As a microwave application, a high selectivity TE10δ mode bandpass filter (BPF) using two artificial dielectric rectangular resonators is demonstrated. A two-stage BPF with the center frequency of 1.718 GHz, bandwidth of 78 MHz, and insertion loss of 1.3 dB is successfully realized in a rectangular waveguide.

This paper focuses on the characteristics of tunable half-wavelength resonators and their applications to bandpass filters (BPFs). First, the resonance characteristics of various tunable half-wavelength resonators are examined for the tunabilities of transmission zeros and the center frequency of the proposed BPFs. We examine four types of tunable half-wavelength resonators, namely, an end-coupling resonator and three types of tap-coupling resonators. Secondly, the proposition and design of two types of BPFs using acquired resonators are carried out. The fabrication and experimental application of the resonators and designed BPFs are also performed based on coplanar waveguide (CPW) technologies. Their calculated and measured results are compared with each other. The results show that tunabilities of the transmission zero and the center frequency of the proposed BPF are obtained as expected.

This paper presents a design procedure of a directional coupler consisting of a twofold symmetric four-port circuit with four identical matching networks at each port. The intrinsic power-split ratio and the equivalent admittance of the directional coupler are formularized in terms of the eigenadmittances of the original four-port without the matching networks. These formulas are useful for judgment on the realizability of a directional coupler in a given circuit structure and for design of the matching networks. Actually, the present procedure is applied to designing various quadrature hybrids and directional couplers, and its practical usefulness as well as several new circuit structures are demonstrated.

In 2003, Shen et al. proposed an improvement on Yang-Shieh's timestamp-based password authentication scheme using smart cards. Then they claimed that their scheme cannot withstand a forged login attack, but also eliminate a problem of Yang-Shieh's. However, their scheme is still susceptible to forged login attack. In this letter, we show how the forged login attack can be worked out on Shen et al.'s scheme.

Novel microstrip lowpass filters are developed with reduced size and significantly improved stopband characteristics. After introducing quarter-wavelength open stubs, we get one or two transmission zeros in the stopband. By folding the high impedance microstrip lines, we reduce the size of the filter. Three-pole and five-pole lowpass filters are designed, and their measured frequency responses agree well with theoretical predictions.

There is an emerging requirement for real-time flow-based traffic monitoring, which is vital to detecting and/or tracing DoS attacks as well as troubleshooting and traffic engineering in the ISP networks. We propose the architecture for a scalable real-time flow measurement tool in order to allow operators to flexibly define "the targeted flows" on-demand, to obtain various statistics on those flows, and to visualize them in a real-time manner. A traffic distribution device and multiple traffic capture devices processing packets in parallel are included in the architecture, in which the former device copies traffic and distributes it to the latter devices. We evaluate the performance of a proto-type implementation on PC-UNIX in testbed experiments to demonstrate the scalability of our architecture. The evaluation shows that the performance increases in proportion to the number of the capture devices and the maximum performance reaches 80 K pps with six capture devices. Finally we also show applications of our tool, which indicate the advantage of flexible fine-grained flow measurements.

Performance measurements are indispensable for managing the Internet. Among the performance measurement techniques known, passive measurement is attractive because of its accuracy; user traffic is observed without inserting additional test traffic. However, the technique is handicapped by its large storage and bandwidth costs. This paper proposes a passive packet loss measurement technique that effectively avoids the difficulty of the conventional passive measurement approaches. Its key advance is utilizing the packet feature computed by a hash function. Since the feature can identify a packet with a short length of data, it becomes possible to greatly decrease the storage and bandwidth costs of passive measurements. The paper details the measurement procedure and assesses the design parameters used in the method. In addition, the validity of the proposed method is confirmed through experiments. The experiments also show the advantage of the method over the conventional active measurement.

This paper proposes a novel dual-mode ring bandpass filter (BPF) using defect ground structure (DGS). The proposed filter provides wide stopband characteristic resulted from the bandgap characteristic of DGS for suppressing spurious response of the dual-mode ring BPF. The H shaped DGS cell is modeled as a parallel LC resonator and the equivalent circuit parameters are extracted. The relationship between bandgap characteristic and design parameters of DGS dimension is discussed and the bandgap characteristic of DGS on the filter performance is also investigated. The novel proposed filter has the frequency characteristics with a central frequency f0 = 7.7 GHz, a 3-dB bandwidth of 4.5% and wider stopband from 9 to 15.5 GHz at the level of -35 GHz. Measured results of experimental filter has good agreement with the theoretical simulation results.

Photoacoustic (PA) spectra on the 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) films deposited by the vacuum evaporation were measured. The films have layered structures constructed from the perylene molecule plane structures. The crystal quality depended on the deposited substrate and the photoacoustic spectroscopy (PAS) seems to be the very useful tools to evaluate these properties from the non-radiative features. The films deposited on the three different substrate had the almost same PL spectra, but the films deposited on the glass substrate had the large non-radiative peaks in the PA spectra contrary to the films deposited on the alumina or crystal Si (100) those had the non-radiative peaks only observed at the short wavelength region.

To implement a fast and reliable question-answering system in Korean, we propose a two-pass answer indexer using co-occurrence information between answer candidates and adjacent content words. The two-pass indexer scans documents twice for obtaining local scores and global scores. Then, the two-pass indexer calculates the degrees of association between answer candidates and co-occurring content words. Using this technique, the proposed QA system shortens the response time and enhances the precision.

We will propose a key-agreement-type three-party password-authenticated key exchange protocol. The proposed protocol is quite efficient and, among the same type of protocols, is the first to be formally proven to be secure. A three-party formal model for security proof is proposed based on [25] and [26]. We construct a simulator in this model to show that our proposed protocol is secure under reasonable and well-defined cryptographic primitives.

A new circuit technique based on pass-gate logic with dynamic supply-voltage and clock-frequency control is proposed for a low-power motion-vector detection VLSI processor. Since the pass-gate logic style has potential advantages that have small equivalent stray capacitance and small number of short-circuit paths, its circuit implementation makes it possible to reduce the power dissipation with maintaining high-speed switching capability. In case the calculation result is obtained on the way of calculation steps, additional power saving is also achieved by combining the pass-gate logic circuitry with a mechanism that dynamically scales down the supply voltage and the clock frequency while maintaining the calculation throughput. As a typical example, a sum of absolute differences (SAD) unit in a motion-vector detection VLSI processor is implemented and its efficiency in power saving is demonstrated.

In 2002, Hwang and Yeh showed that Peyravian-Zunic's password authentication schemes are not secure and proposed an improvement by using the server public key. Since applying the server public key results in the additional burden, we propose secure password authentication schemes without using the server public key in this paper.

In many cryptosystems incorporating human beings, the users' limited memories and their indifference to keeping the systems secure may cause some severe vulnerability of the whole systems. Thus we need more studies on personal entropy, from an information theoretical point of view, to capture the characteristics of human beings as special information sources for cryptosystems. In this paper, we discuss and analyze the use of personal entropy for generating cryptographic keys. In such a case, it is crucially important to precisely evaluate the amount of personal entropy that indicates the actual key length. We propose an advanced key generation scheme based on the conventional graphical passwords proposed in [12]. We improve them to make the most of the secret information extracted in one drawing, i.e., we incorporate the on-line pen pressure and pen inclination information in addition to utilize more secret information. We call the scheme dynamic graphical passwords, and propose a practical construction of them. We also show a precise way of quantifying their entropy, and finally, as an experimental result, we can generate a key of over 110-bit long, using the data of a single drawing. When quantifying their entropy, we need to precisely evaluate the entropy of graphical passwords as well as that of the on-line information of pen movements. We need to precisely evaluate the entropy of graphical passwords by considering the users' biased choices of their graphical passwords. It is expected that they tend to choose their passwords that are memorable as easily as possible, thus we quantify the burden of memorizing each graphical password by the length of its description using a special language based on [12]. We improve the approach in [12] by more directly reflecting how easily each graphical password can be memorized.