(2003) to enable single keyword search. Later, Wang et al. (2012)
proposed rank search scheme on single keyword utilizing the relevance
score and order-preserving encryption techniques. Liu et al. (2014)
classified the search queries into multiple ranks to reduce the query
overhead incurred in the cloud. Yu et al. (2013) employed vector space
model and homomorphic encryption methodologies to realize multi-
keyword rank search. Li et al. (2015) concealed the user's access
pattern adopting the blind storage system. The relevance score and k-
nearest neighbor techniques are developed to return the ranked result.
Besides these private searchable encryption schemes, Boneh et al.
(2004) proposed the concept of public key encryption with keyword
search (PEKS) to search on data that is encrypted using the public key
mechanism. Waters et al. (2004) indicated that the PEKS schemes can
be used to construct searchable secure auditing logs. Some research
results (Fang et al., 2013; Guo and Yau, 2015; Chen et al., 2016) tried
to remove secure channel in PEKS systems. The proxy re-encryption
technology is also introduced to PEKS system to realize authority proxy
(Yang and Ma, 2016a, 2016b).
1.2.3. ABE
The first attribute based encryption (ABE) scheme was introduced
by Sahai and Waters (2005) in 2005, in which both the secret key and
ciphertext are associated with a set of attributes. If the user's attribute
matches the attributes embedded in ciphertext, the message can be
recovered from the ciphertext. If the access policy is associated with the
secret key, the ABE scheme will be denoted as KP-ABE (Shi et al., 2015;
Wang and Lang, 2016; Touati and Challal, 2016). If the ciphertext
contains the access policy, the scheme would be called CP-ABE (Odelu
and Das, 2016; Kitagawa et al., 2015; Zhou et al., 2015).
In the seminal work of Sahai and Waters (2005), an open problem
is left to construct a multi-authority ABE system in order to reduce the
burden of the central authority. To deal with this problem, Chase
(2007) proposed a multi-authority ABE system, in which each user has
an identity and interact with the authority using pseudonyms. Han
et al. (2012) introduced a decentralized ABE scheme such that each
authority can issue secret keys independently without knowing any-
thing about the global identifier. Unfortunately, Han's scheme (Han
et al., 2012) was then proved to be insecure by Ge et al. (2013). Later,
Han et al. (2015) proposed another privacy-preserving multi-authority
ABE scheme, in which both the identifiers and the attributes are
protected from the authorities. Zhang et al. (2015) constructed multi-
authority ABE scheme based on the LWE (learning with errors)
problem on lattice to resist quantum attack.
2. Preliminaries
In this section, we present the preliminaries related to bilinear
paring, hardness assumption and notations that are used in our
scheme.
2.1. Bilinear groups
Let
p
be an algorithm that on input the security parameter λ .It
outputs the parameters of a prime order bilinear map as
pgGG e(, , , ,
T
, where G and G
T
are multiplicative cyclic groups of
prime order p and g is a random generator of G. The mapping
eG G G:×→
is a bilinear map. The bilinear map e has three
properties: (1) bilinearity:
uv
∀, ∈
and
ab Z,∈
p
, we have
eu v euv(, )=()
ab ab
. (2) non-degeneracy:
eg g(, )≠
. (3) computability:
e can be efficiently comnputed.
2.2. Hardness assumptions
Assumption 1 (DBDH: decisional bilinear Diffie-Hellman
assumption). . Let G be a bilinear group of prime order p and g be a
generator of G. Let
βγ Z,, ∈
p
be chosen at random. If an adversary
is given
ygggg
=( , , ,
αβγ
, it is hard for the attacker
to distinguish
eg g G(, ) ∈
αβγ
T
from an element Z that is randomly chosen from G
T
.
2.3. Notations
The main notations used in this paper are listed in Table 1.
3. System and security model
3.1. System architecture
The system architecture of LDAC-KS for distributed health IoT
system is shown in Fig. 1, which involves the following entities.
•
health Io
(
data owner
). The heterogenous health IoT network con-
tains various subsystems, such as the body sensor network (BSN),
home smart rehabilitation system and hospitals. The smart devices
in these subsystems are connected and communicated with each
other to collect and transmit data. For instance, the tiny sensors in
BSN are used to detect heart rate, the pulse and oxygen in blood,
body temperature, electrocardiogram (ECG) and pumped insulin
etc. The vital physiological information is detected and aggregated to
be patient's EHR. These sensitive medical information will be sent to
the cloud server through internet. For the sake of privacy protectoin,
the EHR has to be encrypted before outsourcing. In order to support
efficient keyword search on huge health documents, the keywords
has to be extracted and encrypted into keyword index to facilitate
the data retrieval.
•
Attribute Authoritie
. There exists multiple attribute authorities (AA)
in the distributed IoT system. They are responsible to authenticate
the users and then create the attribute public and private keys for
the users in their management domain.
•
Cloud Serve
. The cloud server in the architecture is responsible for
the health data storage and respond on the keyword search query. It
also possesses almost infinite storage capability and computation
resources (Chang et al., 2016b; Chang, 2014). In order to accelerate
the search speed on the big data, it is important to reduce the
computation overhead in test algorithm to find the match docu-
ments rapidly. Meanwhile, the cloud server is regarded as honest-
but-curious, who is honest to operate the designated calculations
and curious to the plaintext in the encrypted EHR and the searched
keyword trapdoor.
•
Data user. There are multiple kinds of users in this architecture,
including the heath care providers, the friends and family of the
patients as well as third party organizations (for instance, insurance
company). Each user in the system will be endowed a set of
attributes to describe his characteristics. In order to issue keyword
Table 1
Notations.
Notation Descryption
PK M
/
public key/secret key of the system
PK SK/
u
i
u
public key/secret key of user u
i
SK
u
i
out
outsourced secret key of user u
i
PK SK/
A
i
A
public key/secret key of authority A
i
PK SK/
attr
i
attr
public key/secret key of attribute attr
i
K
u
i
keyword query key for user u
i
CT ciphertext of message
CT
out
outsourced ciphertext
CT
transf
transformed ciphertext
I
KW
encrypted keyword index
WT/
K
keyword/keyword trapdoor
T
KW
out
outsourced keyword trapdoor
/attr access policy/attribute
S
u
attribute set of user u
i
Y. Yang et al.
Journal of Network and Computer Applications 89 (2017) 26–37
28