146 Y. Wang et al. / Theoretical Computer Science 595 (2015) 143–158
which are proposed based on decisional linear assumption, the sizes of signatures increase linearly with the complexity of
the access structure. Another scheme for threshold case due to Herranz et al. [25] is constant-size, yet based on a non-static
assumption. More recently, a more general primitive, i.e., policy-based signature, was presented by Bellare and Fuchsbauer [5],
which allows a signer to generate signature on some message that fits in some policy, and simultaneously preserves the
privacy of the policy. Different from distributed signatures, a policy-based signature is produced by an individual signer
when the given message and the signer’s secret key meet the corresponding policy.
2. Definitions and security requirements
2.1. Secret sharing and monotone span program
A secret sharing scheme [7,42,30] allows a dealer to distribute the shares of some secret information to participants in
such a way that the secret can be recovered when a qualified set of participants pool their shares together. The collection
of all qualified sets is called an access structure, which satisfies the monotone increasing property, i.e., any set that contains a
qualified set is also qualified. This paper only involves perfect secret sharing schemes where unqualified sets cannot get any
information about the secret.
Notations. For a group of participants P ={p
1
, ···, p
n
}, let be a monotone access structure defined on P. It is easy to
see that there exists a collection of minimal qualified sets which denoted by min, such that their proper subsets are not
qualified, i.e., ∀A ∈ min and ∀B A, we have B /∈ . Also, we let = 2
P
\ be the collection of all unqualified participant
sets, where 2
P
is the power set of P . Clearly, is monotone decreasing, i.e., ∀A ∈ and ∀B ⊆ A, B ∈ . Similarly, let max
be the collection of all the maximal unqualified participant sets which are not contained in any other unqualified ones.
Definition 1 (Perfect secret sharing [2]). Let be a monotone access structure defined on the participant set P. A secret
sharing scheme S realizing is perfect if the following two properties are satisfied:
• for any qualified set A ∈ , Pr[Rect(S
A
(k)) = k] = 1for every k ∈ F;
• for any unqualified set B ∈ , Pr[S
B
(k
1
) = (s
i
)
p
i
∈B
] = Pr[S
B
(k
2
) = (s
i
)
p
i
∈B
] for any two distinct secrets k
1
, k
2
∈ F, and a
list of any possible shares (s
i
)
p
i
∈B
;
where Rect(·) denotes the reconstruction algorithm of S and S
A
(k) denotes the secret shares of k that are distributed to the
participants in A.
In
this paper, we are only interested in the linear secret sharing schemes (LSSS), where the shares are generated using a
linear mapping and the secret information can be linearly represented by a qualified set of shares. In the upcoming sections,
LSSS are modelled by monotone span programs (MSP). MSP was introduced by Karchmer and Wigderson [31], while it was
implicitly used in constructing vector space secret sharing schemes by Brickell [9].
Definition
2 (Monotone span program [31,2]). Let P be a participant set, a and b be two positive integers. A monotone span
program is defined by a quadruple M = (F,
τ , M, ρ), where F is a field,
τ is a target vector in F
b
, M is an a × b matrix
over F and ρ :{1, 2, ···, a} → P labels each row of M by a participant in P. For any set P ⊆ P , there is a corresponding
sub-matrix M
P
of M that comprises all the rows labeled by the participants in P . If
τ can be spanned by the rows in M
P
,
then the set P is accepted by M. An access structure defined on P is accepted by M if and only if M accepts all the
elements P ∈ .
It
can be seen from the above definition that M not only defines a linear mapping from the matrix M to the participants
in P, but also defines a linear relationship between
τ and each sub-matrix M
P
(P ∈ ) because
τ can be spanned by the
rows of M
P
. As we known, each monotone access structure can be realized by an LSSS [45,26] and MSP is also equivalent
to LSSS [31,2]. This implies that every monotone access structure can be realized by an MSP. Note that there may be several
rows of M labeled to one participant of P. However, for simplicity of exposition in the upcoming sections, we assume there
is a one to one correspondence between the rows of M and the participants in P .
As
we described in a multipartite access structure, the participant set P can be divided into u disjoint groups G
i
(i ∈
[
1, u]), i.e., P =∪
u
i
=1
G
i
and G
i
∩ G
j
= ∅ if i = j. In addition, all the participants in the same group G
i
have the same power,
that is, if a participant p ∈ G
i
is in a qualified set P ∈ , then p can be replaced by any other participant in the same group
p
∈ G
i
\ P and the resultant set is also qualified.
It
is well-known that in a plain LSSS, the dealer is assumed to be honest and thus will not deviate from the system.
However, in complicated scenarios, the dealer may misbehave and deceive the other involved participants. Thus, the verifi-
able
secret sharing schemes (VSS) [19,37] are introduced, which allow the participants to verify the received information from
the dealer in such a way that the misbehavior of the dealer can be detected by the participants.
For
other details on secret sharing, the readers can refer to [2].