The safety problem in access matrix models determines whether a given subject can eventually obtain access privilege to a given object. Generally speaking, the safety problem is, unfortunately undecidable. Not much is known about protection systems for which the safety problem is decidable, except for strongly constrained systems (e.g., monotonic systems). Therefore, we propose the Dynamic-Typed Access Matrix (DTAM) Model, which extends the Typed Access Matrix model of Sandhu by allowing the type of an object to change dynamically. The DTAM model has an advantage that it can describe non-monotonic protection systems for which the safety problem is decidable. In particular, with further restrictions, we can show that the problem becomes NP-hard. In this paper, we formally define the DTAM model and then discuss various aspects of it thoroughly.

In 1989, Ahlswede and Dueck introduced a new formulation of Shannon theory called identification via channels. This paper presents a simple construction of codes for identification via channels when the probability of false identification is measured by its average. The proposed code achieves the identification capacity, and its construction does not require any knowledge of coding theory.

In satellite/terrestrial integrated mobile communication systems (STICSs), a user terminal directly connects both terrestrial and satellite base stations. STICS enables expansion of service areas and provides a robust communication service for large disasters. However, the cell radius of the satellite system is large (approximately 100km), and thus a capacity enhancement of the satellite subsystem for accommodating many users is needed. Therefore, in this paper, we propose an application of two methods — multiple-input multiple-output (MIMO) transmission using multi-satellites and non-orthogonal multiple access (NOMA) for STICS — to realize the performance improvement in terms of system capacity and user fairness. Through numerical simulations, we show that system capacity and user fairness are increased by the proposed scheme that applies the two methods.

In this paper a new type of digital proxy signature is proposed. The proxy signature allows a designated person, called a proxy signer, to sign on behalf of an original signer. Classification of the proxy signatures is shown from the point of view of the degree of delegation, and the necessary conditions of a proxy signature are clarified. The proposed proxy signature scheme is based on either the discrete logarithm problem or the problem of taking the square root modulo of a composite number. Compared to the consecutive execution of the ordinary digital signature schemes, it has a direct from, and a verifier does not need a public key of a user other than the original signer in the verification stage. Moreover, it requires less computational work than the consecutive execution of the signature schemes. Due to this efficiency together with the delegation property, an organization, e.g. a software company, can very efficiently create many signatures of its own by delegating its signing power to multiple employees. Another attractive feature is that the proxy signature based on the discrete logarithm problem is highly applicable to other ordinary signature schemes based on the same problem, For instance, designated confirmer proxy signatures can be constructed. As a stronger form of proxy signature for partial delegation, another type of proxy signature scheme is proposed in which even an original signer cannot create a proxy signature. Furthermore, using a proposed on-line proxy updating protocol, the orignal signer can revoke proxies of dishonest proxy signers.

A key distribution system is a system in which users securely generate a common key. One kind of identity-based key distribution system was proposed by E. Okamoto. Its security depends on the difficulty of factoring a composite number of two large primes like RSA public-key cryptosystem. Another kind of identity-based key distribution system was proposed by K. Nyberg, R. A. Rueppel. Its security depends on the difficulty of the discrete logarithm problem. On the other hand, Koblitz and Miller described how a group of points on an elliptic curve over a finite field can be used to construct a public key cryptosystem. In 1997, we proposed an ID-based key distribution system over an elliptic curve, as well as those over the ring Z/nZ. Its security depends on the difficulty of factoring a composite number of two large primes. We showed that this system over an elliptic curve is more suitable for the implementation than those over the ring Z/nZ. In this paper, we apply the Nyberg-Rueppel ID-based key distribution system to an elliptic curve. It provides relatively small block size and high security. This public key distribution system can be efficiently implemented. However the Nyberg-Rueppel's scheme requires relatively large data transmission. As a solution to this problem, we improve the scheme. This improved scheme is very efficient since data transferred for the common key generation is reduced to half of those in the Nyberg-Rueppel's scheme.

Physical layer security is effective in wireless communications because it makes a transmission secure from the beginning of protocols. We have proposed a chaos multiple-input multiple-output (C-MIMO) transmission scheme that achieves both physical layer security and channel coding gain using chaos signals. C-MIMO is a type of encryption modulation and it obtains the coding gain in conjunction with encryption without a decrease in the transmission efficiency. Thus, the error rate performance is improved in C-MIMO. However, decoding complexity increases exponentially with code length because of the use of maximum likelihood sequence estimation (MLSE), which restricts the code length of C-MIMO and thus the channel coding gain. Therefore, in this paper, we consider outer channel code concatenation instead of code length expansion for C-MIMO, and propose an iterative turbo decoding scheme for performance improvement by introducing a log-likelihood ratio (LLR) into C-MIMO and by utilizing turbo principle. The improved performances of the proposed scheme, compared to the conventional scheme when the outer channel codes are convolutional code and low-density parity check (LDPC) code, are shown by computer simulations.

Recently, Kuwakado and Tanaka proposed a transitive signature scheme for directed trees. In this letter, we show that Kuwakado-Tanaka scheme is insecure against a forgery attack, in which an attacker is able to forge edge signatures by composing edge signatures provided by a signer.

In fifth-generation mobile communications systems (5G), grant-free non-orthogonal multiple access (NOMA) schemes have been considered as a way to accommodate the many wireless connections required for Internet of Things (IoT) devices. In NOMA schemes, both system capacity enhancement and transmission protocol simplification are achieved, and an overload test of more than one hundred percent of the transmission samples over conducted. Multi-user shared multiple access (MUSA) has been proposed as a representative scheme for NOMA. However, the performance of MUSA has not been fully analyzed nor compared to other NOMA or orthogonal multiple access schemes. Therefore, in this study, we theoretically and numerically analyze the performance of MUSA in uplink fading environments and compare it with orthogonal frequency division multiple access (OFDMA), space division multiple access-based OFDMA, low-density signature, and sparse code multiple access. The characteristics and superiority of MUSA are then clarified.

Wavelet packet modulation (WPM) using the discrete wavelet transform is a multiplexing transmission method in which data is assigned to wavelet subbands having different time and frequency resolutions. The WPM keeps data transmission throughput even in tone and impulse interference environments that cannot be achieved with conventional multiplexing methods such as TDM (Time division multiplexing) or OFDM (Orthogonal frequency division multiplexing). In this paper, we propose an effective multicarrier transmission method of WPM for wireless mobile communications. First, the transmission characteristics of WPM in fading environments are minutely investigated. Then, taking the advantage of the WPM and the OFDM that has an equalizing technique in multipath fading environments, we propose a multimode transmission method using them. The adaptive transmission in those fading and interference environments is achieved by using the multimode transmission. Their transmission performances are evaluated by computer simulations.

We implemented a scalar multiplication method over elliptic curves using division polynomials. We adapt an algorithm for computing elliptic nets proposed by Stange. According to our experimental results, the scalar multiplication method using division polynomials is faster than the binary method in an affine coordinate system.

This paper investigates the error correcting capabilities of concatenated codes employing algebraic geometry codes as outer codes and time-varying randomly selected inner codes, used on discrete memoryless channels with maximum likelihood decoding. It is proved that Gallager's random coding error exponent can be obtained for all rates by such codes. Further, it is clarified that the error exponent arbitrarily close to Gallager's can be obtained for almost all random selections of inner codes with a properly chosen code length, provided that the length of the outer code is sufficiently large. For a class of regular channels, the result is also valid for linear concatenated codes, and Gallager's expurgated error exponent can be asymptotically obtained for all rates.

In this paper, we propose an efficient channel estimation scheme in bi-directional wireless orthogonal frequency division multiplexing (OFDM) relay systems applying analog network coding (ANC). In the relay systems applying ANC, channel separation is needed to estimate each of the bi-directional channels simultaneously from the combined received signal. In the conventional channel estimation schemes, relatively higher-ratio pilots are needed to obtain accurate channels. In contrast, we propose a channel estimation scheme with sparse pilots, while maintaining high accuracy for channel estimation. In the proposed scheme, Walsh codes are inserted as the pilot symbols at both end nodes, and the individual channels are obtained by correlation processing from the combined signals. The improved bit error rate (BER) and throughput performances of the proposed scheme are shown through computer simulations.

With popularity of radio communication, voice scrambling has received increased attention for communication privacy. It has been known that pre-emphasis at the transmit side and de-emphasis at the receive side improves the received signal to noise power ratio (SNR) both for scrambled and nonscrambled signals. In this paper, optimum pre- and de-emphasis are theoretically investigated for FM and PM transmission including frequency-domain scrambling for voice security. Using a linear system model, the optimum frequency-characteristics for pre- and de-emphasis filter are given as a function of a voice signal spectrum, demodulator output noise spectrum, and the scrambling method. Comparisons of the maximum attainable SNR are made for various case including plain voice (non scrambling) and secured voice scrambled by spectrum inversion, reverberation and FFT spectrum transformation, assuming simple voice and noise spectra.

It is known that, using just a deck of cards, an arbitrary number of parties with private inputs can securely compute the output of any function of their inputs. In 2009, Mizuki and Sone constructed a six-card COPY protocol, a four-card XOR protocol, and a six-card AND protocol, based on a commonly used encoding scheme in which each input bit is encoded using two cards. However, up until now, there are no known results to construct a set of COPY, XOR, and AND protocols based on a two-card-per-bit encoding scheme, which all can be implemented using only four cards. In this paper, we show that it is possible to construct four-card COPY, XOR, and AND protocols using polarizing plates as cards and a corresponding two-card-per-bit encoding scheme. Our protocols use a minimum number of cards in the setting of two-card-per-bit encoding schemes since four cards are always required to encode the inputs. Moreover, we show that it is possible to construct two-card COPY, two-card XOR, and three-card AND protocols based on a one-card-per-bit encoding scheme using a common reference polarizer which is a polarizing material accessible to all parties.

This paper considers the normalization of Miller functions for computing “point-evaluation” pairings on an elliptic curve E over a finite field Fq, where the characteristic of Fq is neither 2 nor 3. It is shown that the normalized Miller functions for computing point-evaluation pairings on G2G1 when (i) the embedding degree k is even, or (ii) 3|k and E/Fq(q ≡ (1 mod 3)) is a curve of the form Y2=X3+b. Thus, there is no need to consider the normalization for computing pairings on many pairing-friendly elliptic curves.

We describe a field experiment of flexible modulation format adaptation on a real-time 400Gbit/s/ch DSP-LSI. This real-time DSP-LSI features OSNR estimation, practical simplified back propagation, and high gain soft-decision forward error correction. With these techniques, we have successfully demonstrated modulation format allocation and transmission of 56-channel 400Gbit/s-2SC-PDM-16QAM and 200Gbit/s-2SC-PDM-QPSK signals in 216km and 3246km standard single mode fiber, respectively.

Polynomials are called permutation polynomials if they induce bijective functions. This paper investigates algebraic properties of permutation polynomials over a finite field, especially properties associated with permutation cycles. A permutation polynomial has a simple structure but good randomness properties suitable for applications. The cycle structure of permutations are considered to be related to randomness. We investigate the algebraic structure from the viewpoint of randomness. First we show the relationship between polynomials and permutations using a matrix equation. Then, we give a general form of a permutation polynomial corresponding to a product C1C2Ck of pairwise disjoint cycles. Finally, permutation polynomials with fixed points -or with 2, 3 and 4-cycles -and their compositions are given together with distribution of degree of the permutation polynomials.

In this paper, we investigate two receiver structures for spatially multiplexed transmission on MIMO frequency selective fading channels. Those receivers compensate the IAI (Inter-Antenna Interference) and ISI (Inter-Symbol Interference) in the time domain. We first propose the MIMO sequential equalizer in which the block of several receives symbols is processed symbol by symbol by MLD (Maximum Likelihood Detection). Next we investigate the MIMO MLSE (Maximum Likelihood Sequence Estimation) receiver in which the terminated block trellis is decoded by the Viterbi algorithm. The bit error rates of two time domain receivers are examined through computer simulations and we also compare their BER characteristics to those of the conventional MIMO SC-FDE.

In indoor localization using sensor networks, performance improvements are required for non-line-of-sight (NLOS) environments in which the estimation error is high. NLOS mitigation schemes involve the detection and elimination of the NLOS measurements. The iterative minimum residual (IMR) scheme, which is often applied to the localization scheme using the time of arrival (TOA), is commonly employed for this purpose. The IMR scheme is a low-complexity scheme and its NLOS detection performance is relatively high. However, when there are many NLOS nodes in a sensor field, the NLOS detection error of the IMR scheme increases and the estimation accuracy deteriorates. Therefore, we propose a new scheme that exploits coarse NLOS detection based on stochastic characteristics prior to the application of the IMR scheme to improve the localization accuracy. Improved performances were confirmed in two NLOS channel models by performing numerical simulations.