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does not require the re-encryption of the outsourced
files. Rather, it only calls for an update of the access
policy of the data owner at the cloud server.
8 RELATED WORK
As opposed to the proposed solution, most of existing
word search mechanisms be them asymmetric (Bel-
lare et al., 2007; Boneh et al., 2004; Waters et al.,
2004) or symmetric (Curtmola et al., 2006; Kamara
et al., 2012; Song et al., 2000; Golle et al., 2004) seem
to guarantee query privacy partially: Indeed, in these
solutions, although the outsourced data and queries
are encrypted, the cloud can discover the response to
any encrypted query. Furthermore very few of current
solutions (Curtmola et al., 2006; Dong et al., 2008)
propose the ability to delegate the search operation;
unfortunately, these solutions provide the authorized
user with the data encryption key and therefore revo-
cation of a user requires the re-encryption of the en-
tirely outsourced data and the distribution of this new
key to the authorized users.
The first solution which transforms an original
PIR mechanism into a privacy preserving word-search
solution is proposed by Chor et. al. in (Chor et al.,
1997). Similarly to our solution, in (Chor et al.,
1997), the owner of the data constructs an index based
on all distinct words in the outsourced file. This index
is a hash-table that is filled according to the perfect
hashing algorithm of (Fredman et al., 1984). Our so-
lution outperforms the solution in (Chor et al., 1997)
thanks to the use of Cuckoo hashing instead of perfect
hashing. Namely, in the scheme of (Chor et al., 1997),
a word search query consists of three PIR queries,
whereas in our protocol it is composed of two PIR
queries. Additionally, the PIR queries in the case of
Cuckoo hashing are independent. This implies that
the server can execute the two PIR instances in paral-
lel to respond to the word search query.
Another solution that resembles the proposed so-
lution is PRISM (Blass et al., 2012) where the cloud
constructs some binary matrices in which each cell
represents one or more words without knowing their
content and the owner sends PIR requests to retrieve
the content of one of these cells. Thanks to the use of
Cuckoo hashing, our solution outperforms the origi-
nal PRISM mechanism without lowering the security
level. PRISM defines a matrix in which each cell cor-
responds to one or more words; therefore, two words
can turn out to be represented by the same cell. In
order to decrease the probability of such collisions,
the data owner send multiple (q) queries for the same
word. In the newly proposed mechanism, the prob-
ability of collisions within the binary matrices is 0
and the data owner and/or the authorized user need
to send a single query for each word. Additionally,
PRISM does not offer any delegation capability and
a straightforward delegation operation would require
the distribution of the data encryption key to autho-
rized users which can increase privacy risks.
9 CONCLUSION
We introduced a protocol for privacy preserving del-
egated word search in the cloud. This protocol al-
lows a data owner to outsource its encrypted data to a
cloud server, while empowering the data owner with
the capability to delegate word search operations to
third parties. By employing keyed hash functions and
oblivious pseudo-random functions, we ensure that
authorized users only learn whether a given word is
in the outsourced files or not. In addition, we use pri-
vate information retrieval to make sure that the cloud
server cannot infer any information about the out-
sourced files from the execution of the word search
protocol. Furthermore, we combine attribute-based
encryption and oblivious pseudo-random functions to
accommodate efficient revocation. Finally, the data
owner in our protocol is only required to perform
symmetric operations, whereas the computationally
intensive computations are performed by the cloud
server, and they can easily be parallelized.
ACKNOWLEDGEMENT
This work was partially funded by the Cloud Ac-
countability project - A4Cloud (grant EC 317550).
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