Received 10 November 2014; revised 1 December 2014; accepted 7 December 2014

ABSTRACT

A secret sharing scheme is one of cryptographies. A threshold scheme, which is introduced by Shamir in 1979, is very famous as a secret sharing scheme. We can consider that this scheme is based on Lagrange's interpolation formula. A secret sharing scheme has one key. On the other hand, a multi-secret sharing scheme has more than one keys; that is, a multi-secret sharing sche- me has keys. Dealers distribute shares of keys among participants. Gathering participants, keys can be reconstructed. In this paper, we give a scheme of a multi-secret sharing based on Hermite interpolation, in the case of.

Keywords:

Secret Sharing Scheme, Multi Secret Sharing Scheme, Hermite Interpolation

1. Introduction

A secret sharing scheme is one of cryptographies. A secret sharing scheme was introduced by Shamir in 1979 [1] and Blakley in 1979 [2] independently. A secret sharing scheme has been studied by many scientists until today. Now, a secret sharing scheme has some important application to several areas of the information security.

The secret sharing scheme is a method to distribute shares of a secret value―we call it a key, too―among a set of participants such a way that only the qualified subsets of are able to reconstruct the value of from their shares. In 1979, Shamir [1] introduced the secret sharing scheme which was based on Lagrange’s interpolation formula. This scheme is called Shamir’s threshold scheme. In 1987, Feldman [3] studied a verifiable scheme in distributing system. In 1992, Pedersen [4] applied a verifiable secret sharing scheme to Shamir’s threshold scheme.

A secret sharing scheme has one key. On the other hand, a multi-secret sharing scheme has more than one keys; that is, a multi-secret sharing scheme has keys. Dealers distribute shares of keys

among participants. Let a set of participants. Gathering participants, keys can be reconstructed.

Various schemes are proposed about a multi-secret sharing scheme. In 1994, Jackson et al. [5] studied a multi-secret sharing scheme for a matroid. A multi-secret sharing scheme by utilizing a one-way function is studied by He and Dawson [6] in 1994, and by Harn [7] in 1995. A multi-secret sharing scheme by utilizing a two variables one-way function is studied by He and Dawson [8] in 1995. A multi-secret sharing scheme by utilizing a linear block code is studied by Chien et al. [9] in 2000, and by Pang and Wang [10] in 2005. A multi- secret sharing scheme based on Lagrange’s interpolation is studied by Yang et al. [11] in 2004, and by Pang and Wang [10] in 2005. Recently, Adachi [12] studies a secret sharing scheme with two keys based on Hermite interpolation.

In this paper, we give a scheme of a multi-secret sharing based on Hermite interpolation, in the case of. Here, is the number of keys, is the number of participants, and is the number of gathering participants for secret reconstruction. In a multi-secret sharing, we need to consider separately the cases where is greater than, equal to, or less than. In this paper, we give new scheme in the case of. The goal of this paper is 1) to find system parameters, 2) to construct secret distribution, and 3) to complete secret reconstruction, for a multi-secret sharing, by utilizing Hermite interpolation.

2. Lagrange’s Interpolation and Hermite Interpolation

In this section, we describe two famous interpolation formula, that is, Lagrange’s interpolation and Hermite interpolation. In numerical analysis, Lagrange’s interpolation and Hermite interpolation is a method of interpolating data points as a polynomial function.

2.1. Lagrange’s Interpolation

Suppose that a function is defined on a closed interval. Given data points, (,for), and values, , , , , we want to find an dimensional polynomial such that satisfies, , , ,. In other words, I want to get an approximation of, for any variable except data points, by calculating the value of an dimensional polynomial.

Here, we can get an dimensional polynomial by the following equation.

where an dimensional polynomial satisfies

Let, we can decide an unique dimensional polynomial. This equation is called Lagrange’s interpolation formula.

2.2. Hermite Interpolation

Hermite interpolation is an extension of Lagrange’s interpolation. When using divided differences to calculate the Hermite polynomial of a function.

Suppose that a function is defined on a closed interval. Given data points, (,for), and values, , , , , and their differential values, , , , , we want to find a dimensional polynomial such that satisfies, , , , , and, , , ,. In other words, I want to get an approximation of, for any variable except data points, by calculating the value of a dimensional polynomial.

Here, it is known that we can get an unique dimensional polynomial by the following equation.

where two dimensional polynomial, satisfy

and

This is called Hermite interpolation.

3. A Multi-Secret Sharing Scheme Based on Lagrange’s Interpolation

In this section, we describe a multi-secret sharing scheme based on Lagrange’s interpolation, which is proposed by Yang et al. in 2004. We refer to [11] . We refer to the part of Yang’s scheme in [10] , too.

In Yang et al.’s scheme, for secret distribution, the secret are distributed in two separate cases, and. In Yang et al.’s scheme, also, for secret reconstruction, the secret are reconstructed in two separate cases, and. Since our scheme is treating in the case of, we describe only the case for secret distribution and for secret reconstruction, in Yang et al.’s scheme.

1) System parameters. Let be a two-variable one way function. Let be a large prime and all the numbers are element in the finite field. The trusted dealer randomly selects distinct integers, , as secret shadows of participants.

Here, we use to denote keys (secret values).

2) Secret distribution. In the case of, the secret dealer executes the following steps:

2a) Construct a -th degree polynomial , where are keys and are randomly chosen from.

2b) Randomly choose an integer and compute for.

2c) Publish in any authenticated manner such as those in digital signature scheme.

3) Secret reconstruction. In the case of, at least participants pool their pseudo shadows. For example, participants pool their pseudo shadows. By Lagrange’s interpolation polynomial, with the knowledge of pairs of, the -th degree polynomial can be uniquely determined. From the obtained polynomial, we can easily get the keys.

4. Our Scheme: A Multi-Secret Sharing Scheme Based on Hermite Interpolation

In this section, we describe our new scheme, that is, a multi-secret sharing scheme based on Hermite interpolation in the case, where is the number of keys (secrets), and is the number of necessary participants who can reconstruct keys (secrets).

In the case of, the following theorem is known.

Theorem 1. [12] Suppose that we have two keys (secrets), is the number of participants, and is the number of necessary participants who can reconstruct two keys (secrets). We can propose a scheme of a secret sharing scheme with two keys based on Hermite interpolation.

We expand this theorem. In our scheme, at first, we prepare system parameters which we need. Secondly, we describe secret distribution. Finally, we describe secret reconstruction.

1) System parameters. Let be a two-variable one way function. Let be a large prime and all the numbers are element in the finite field. The trusted dealer randomly selects distinct integers, , as secret shadows of participants. The trusted dealer randomly selects an integer, calculates, , ,.

Here, we use to denote keys (secret values).

2) Secret distribution. In the case of, the secret dealer executes the following steps:

2a) He constructs a -th degree polynomial as follows, where are keys and are randomly chosen from.

2b) He computes and for.

2c) He publishes in any authenticated manner such as those in digital signature scheme.

3) Secret reconstruction. In the case of, at least participants execute the following steps:

3a) They pool their pseudo shadows. For example, participants pool their pseudo shadows.

3b) They compute and for.

3c) They compute and for by utilizing the solution of (3b).

3d) By Hermite interpolation polynomial, with the knowledge of triplets of, the -th degree polynomial can be uniquely determined as follows.

From the obtained polynomial , we can easily get the keys, , ,.

As stated above, we obtain the following theorem.

Theorem 2. Suppose that is the number of keys (secrets), is the number of participants, and is the number of necessary participants who can reconstruct keys (secrets). In the case, we can propose a scheme of a multi-secret sharing scheme with many keys based on Hermite interpolation.

Corollary 1. Suppose that is the number of participants, and is the number of necessary participants who can reconstruct keys (secrets). Theorem 2 contains a scheme of a secret sharing with one key based on Lagrange’s interpolation, that is, Shamir’s threshold scheme.

Proof. In the case of Theorem 2, we obtain Corollary 1.

(Q.E.D.)

Corollary 2. Theorem 2 contains Theorem 1.

Proof. In the case of Theorem 2, we obtain Corollary 2.

(Q.E.D.)

5. Computational Complexity

In this section, we compare computational complexity of our scheme which we describe in Section 4, and that of Yang et al.’s scheme which we describe in Section 3.

As regards phase 1) system parameters, the both schemes have the same amount of parameters.

As regards phase 2) secret distribution, computational complexity of our scheme is twice of that of Yang et al.’s scheme. Since, in our scheme, there are for.

As regards phase 3) secret reconstruction, computational complexity of our scheme is twice of that of Yang et al.’s scheme. Since, in 3b) of our scheme, there are for. In 3c) of our scheme, there

are not only but also for. In 3d) of our scheme, there are not only but also.

Hence, computational complexity of our scheme is twice of that of Yang et al.’s scheme. This is suitable, since computational complexity of Hermite interpolation is twice of that of Lagrange’s interpolation.

6. Conclusion

We can propose a new scheme of a multi-secret sharing scheme with many keys based on Hermite interpolation. Hermite interpolation is a higher precision analysis and needs more complex computation than Lagrange’s interpolation. The merit on our scheme is that we can use many keys with fine distinctions. On the other hand, the demerit on our scheme is that its computation is complex for participants.

Acknowledgements

We thank the editor and the referee for their comments.

References