Yodai Watanabe

Security of audio secret sharing scheme encrypting audio secrets with bounded shares

Secret sharing is a method of encrypting a secret into multiple pieces called shares so that only qualified sets of shares can be employed to reconstruct the secret. Audio secret sharing (ASS) is an example of secret sharing whose decryption can be performed by human ears. This paper examines the security of an audio secret sharing scheme encrypting audio secrets with bounded shares, and optimizes the security with respect to the probability distribution used in its encryption.

Formulation of visual secret sharing schemes encrypting multiple images

Secret sharing is a method of generating multiple shares from secret information so that only a qualified set of shares can be employed to recover this secret information. Visual secret sharing (VSS) is an example of secret sharing; its decryption can be performed by using human eyes without a computer. This paper provides a formulation of encryption for multiple secret images, which is a generalization of the existing ones, and also a general method of constructing VSS schemes encrypting multiple secret images.

Visual secret sharing schemes encrypting multiple images

The aim of this paper is to maximize the range of the access control of visual secret sharing (VSS) schemes encrypting multiple images. First, the formulation of access structures for a single secret is generalized to that for multiple secrets. This generalization is maximal in the sense that the generalized formulation makes no restrictions on access structures; in particular, it includes the existing ones as special cases. Next, a sufficient condition to be satisfied by the encryption of VSS schemes realizing an access structure for multiple secrets of the most general form is introduced, and two constructions of VSS schemes with encryption satisfying this condition are provided. Each of the two constructions has its advantage against the other; one is more general and can generate VSS schemes with strictly better contrast and pixel expansion than the other, while the other has a straightforward implementation. Moreover, for threshold access structures, the pixel expansions of VSS schemes generated by the latter construction are estimated and turn out to be the same as those of the existing schemes called the threshold multiple-secret visual cryptographic schemes. Finally, the optimality of the former construction is examined, giving that there exist access structures for which it generates no optimal VSS schemes.

Security of (n, n)-threshold audio secret sharing schemes encrypting audio secrets

Secret sharing is a method of encrypting a secret into multiple pieces called shares so that only qualified sets of shares can be employed to reconstruct the secret. Audio secret sharing (ASS) is an example of secret sharing whose decryption can be performed by human ears. This paper examines the security of (n, n)-threshold ASS schemes encrypting audio secrets by estimating the mutual information between secret and shares.

A novel class of QPSK zero-correlation zone sequence sets

The present paper introduces the construction of Quadrature Phase Shift Keying (QPSK) modulation based sequences having a zero-correlation zone. For a zero-correlation zone sequence set of N sequences, each of length L, the cross-correlation function and the side lobe of the autocorrelation function of the proposed sequence set is zero for the phase shifts τ within the zero-correlation zone z, such that |τ| ≤ z (τ ≠ 0 for the autocorrelation function). The ratio N(z+1) over ℓ is theoretically limited to one. When the ratio of a sequence set is equal to one, the sequence set is called an optimal zero-correlation sequence set. The proposed zero-correlation zone sequence set can be generated from an arbitrary Hadamard matrix of order n. First, the proposed sequence set is generated as a set of 4n sequences of length 8n with the zero-correlation zone z = 1. The length of the proposed sequence set can be extended by sequence interleaving, where m times interleaving can generate the 4n sequences, each of length 2m+3n. The proposed sequence set is optimal for m = 0,1 and almost optimal for m > 1.