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21498 Towards Provably Secure Efficiently Searchable Encryption
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Sedghi, S. (2012) Towards Provably Secure Efficiently Searchable Encryption. PhD thesis, University of Twente. CTIT Ph.D.-thesis series No. 12-219 ISBN 978-90-365-3333-1

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Traditional encryption systems are designed in such a way that either the whole data
is decrypted, if the encryption and decryption keys match, or nothing is decrypted
otherwise. However, there are applications that require a more flexible encryption
system which supports decrypting data partially. Searchable encryption is a technique
that provides functionalities to decrypt data partially by searching encrypted
In searchable encryption, each message of data is associated with a set of keywords.
Searchable encryption transforms both, the message and the associated keywords,
to an encrypted form, in such a way that the encrypted keywords can be
queried later. This allows a client to retrieve or decrypt only the messages of the
data that contain a particular keyword without decrypting the data.
Searchable encryption can be based on either symmetric key or public key cryptography.
In the symmetric key setting, only the client who stores the data on the
server can search the encrypted data. This setting is appropriate for situations where
the client stores encrypted data on an honest but curious server, in such a way the
the encrypted data can be retrieved selectively. Using symmetric key searchable encryption,
the server learns as little information as possible after the storage and the
search. In public key searchable encryption, anyone can encrypt data using a public
key, while only the owner of the corresponding private key can query encrypted data.
Public key searchable encryption allows a client to delegate a decryption key to other
users, in such a way that the delegated decryption key can decrypt parts of data only.
The main aspects of searchable encryption are security and efficiency. The efficiency
of a scheme is evaluated by the complexity of the scheme. The security of a
scheme shows the ability of the scheme in hiding both, the message and the associated
keywords, from adversaries. To define the security formally, security models are
proposed where each model defines certain computational resources and restrictions
for the adversary. Since security is never free, there is a trade off between efficiency
and security. Searchable encryption schemes that achieve security in a security model
with a more powerful adversary have a higher complexity. The best trade off is
achieved when the scheme achieves certain level of security with the lowest possible
The main contributions of this thesis are the efficient and provably secure searchable
encryption schemes which have a lower complexity compared to existing
schemes. Our focus in this thesis is the complexity of the search, which is the main
functionality of searchable encryption. In this thesis we propose:
• A searchable encryption scheme in the symmetric key setting which is secure
in the symmetric key searchable encryption model. This security model is the
only security model proposed in the symmetric key setting. Our scheme, called
SES, has a lower complexity for the search compared to existing symmetric key
searchable encryption schemes.
• A public key searchable encryption scheme which is secure in the random oracle
model. A ransom oracle is a function that maps an input value to a true random
output value. In the random oracle model anyone including the adversary has
access to a random oracle. Our scheme, called SEPE, allows searching and
enforcing an access control policy, while revealing as little as possible about the
data and the policy. The SEPE scheme has a lower complexity to perform the
search and enforce an access control policy compared to existing schemes.
• A public key searchable encryption scheme which is secure in the selective
security model. The adversary in the selective security model must inform in
advance of the attack which keyword is intended to be attacked. Our scheme,
called SEPS, supports wildcards in the queried keyword. The SEPS scheme
is more efficient that existing schemes which allow searching keywords with
wildcards on encrypted data.
• A public key searchable encryption scheme which is fully secure. The full
security model is the strongest proposed security model. Our scheme, called
SEPF, has a lower search complexity compared to existing fully secure schemes.

Item Type:PhD Thesis
Supervisors:Jonker, W. and Hartel, P.H.
Assistant Supervisors:Nikova, S.I.
Research Group:EWI-DB: Databases, EWI-DIES: Distributed and Embedded Security
Research Program:CTIT-ISTRICE: Integrated Security and Privacy in a Networked World
Research Project:SEDAN: Searchable Data Encryption
ID Code:21498
Deposited On:17 February 2012
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