International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Kohtaro Tadaki

Affiliation: Chuo University

Publications

Year
Venue
Title
2010
EPRINT
Proposal of a Signature Scheme based on STS Trapdoor
A New digital signature scheme based on Stepwise Triangular Scheme (STS) is proposed. The proposed trapdoor has resolved the vulnerability of STS and secure against both Gröbner Bases and Rank Attacks. In addition, as a basic trapdoor, it is more efficient than the existing systems. With the efficient implementation, the Multivariate Public Key Cryptosystems (MPKC) signature public key has the signature longer than the message by less than 25 %, for example.
2009
EPRINT
Security Enhancement of Various MPKCs by 2-layer Nonlinear Piece In Hand Method
Following the last proposal of the nonlinear Piece in Hand method, which has 3-layer structure, 2-layer nonlinear Piece in Hand method is proposed. Both of them aim at enhancing the security of existing and future multivariate public key cryptosystems. The new nonlinear Piece in Hand is compared with the 3-layer method and PMI+, which was proposed by Ding, et al.
2008
EPRINT
Nonlinear Piece In Hand Matrix Method for Enhancing Security of Multivariate Public Key Cryptosystems
Shigeo Tsujii Kohtaro Tadaki Ryou Fujita
It is widely believed to take exponential time to find a solution of a system of random multivariate polynomials because of the NP-completeness of such a task. On the other hand, in most of multivariate public key cryptosystems proposed so far, the computational complexity of cryptanalysis is apt to be polynomial time due to the trapdoor structure. In this paper, we develop the concept, piece in hand matrix (PH matrix, for short), which aims to bring the computational complexity of cryptanalysis of multivariate public key cryptosystems close to exponential time by adding random polynomial terms to original cryptosystems. This is a general concept which can be applicable to any reasonable type of multivariate public key cryptosystems for the purpose of enhancing their security. There are two types of the PH matrices: a linear matrix whose elements are constants and a nonlinear matrix whose elements are polynomial functions of the plain text or random numbers. In the present paper, we focus our thought on the nonlinear PH matrix method and develop the framework of it. The nonlinear PH matrix method is obtained by generalizing the linear PH matrix method, and the nonlinearity may bring an additional randomization to the original linear PH matrix method. Thus, the nonlinear PH matrix method may enhance the security of the original multivariate public key cryptosystem more than the linear PH matrix method. We show, in an experimental manner, that this actually holds in the enhancement of the security of the Matsumoto-Imai cryptosystem and RSE(2)PKC against the Gr\"obner basis attack.
2008
EPRINT
Nonlinear Piece In Hand Perturbation Vector Method for Enhancing Security of Multivariate Public Key Cryptosystems
Ryou Fujita Kohtaro Tadaki Shigeo Tsujii
Abstract. The piece in hand (PH) is a general scheme which is applicable to any reasonable type of multivariate public key cryptosystems for the purpose of enhancing their security. In this paper, we propose a new class PH method called NLPHPV (NonLinear Piece in Hand Perturbation Vector) method. Although our NLPHPV uses similar perturbation vectors as is used for the previously known internal perturbation method, this new method can avoid redundant repetitions in decryption process. With properly chosen parameter sizes, NLPHPV achieves an observable gain in security from the original multivariate public key cryptosystem. We demonstrate these by both theoretical analyses and computer simulations against major known attacks and provides the concrete sizes of security parameters, with which we even expect the grater security against potential quantum attacks.
2006
EPRINT
Proposal for Piece In Hand Matrix Ver.2: General Concept for Enhancing Security of Multivariate Public Key Cryptosystems
Shigeo Tsujii Kohtaro Tadaki Ryou Fujita
We proposed the concept, piece in hand (soldiers in hand) matrix and have developed the framework based on the concept so far. The piece in hand matrix is a general concept which can be applicable to any type of multivariate public key cryptosystems to enhance their security. In this paper, we make improvements in the PH matrix method as follows. (i) In the PH matrix method, an arbitrary number of additional variables can be introduced to the random polynomial term in the public key, which is eliminated by the multiplication of the PH matrix to the public key in the decryption. Thus these additional variables enables the public key to have more than one solution, and therefore increases the difficulty to solve the public key. We show, in an experimental manner, that the PH matrix method improved in this way is secure even against the Gr\"obner basis attack. (ii) In the nonlinear PH matrix method proposed previously, the degree of polynomials in the public key is more than two, and this may cause an undesirable increase in the length of the public key. In this paper, we propose a nonlinear PH matrix method, where the degree of the public key is kept the same as the degree of the public key of the original cryptosystem, which is normally two.
2004
EPRINT
Piece In Hand Concept for Enhancing the Security of Multivariate Type Public Key Cryptosystems: Public Key Without Containing All the Information of Secret Key
Shigeo Tsujii Kohtaro Tadaki Ryou Fujita
We propose a new concept, named piece in hand, which can be applicable to a wide class of multivariate type public key cryptosystems to enhance their security. The piece in hand provides such cryptosystems with a new type of trapdoor which is compatible with the trapdoor originally equipped in them. The piece in hand concept is based on a new paradigm for public key cryptosystem in general. On the one hand, in most traditional public key cryptosystems such as the RSA and ElGamal schemes, the public key contains all the information of the secret key. On the other hand, in our scheme, the piece in hand, which is a part of the secret key, is not contained in the public key but is taken from outside of the public key to plug in during the decryption. In this paper, we illustrate how to apply the piece in hand concept to enhance the security of multivariate type public key cryptosystems, by presenting the general theory for the use of the concept.