A Hybrid Chaotic Image Encryption Scheme Bas~d on S-Box and Ciphertext Feedback

The fascinating developments in digital image processing and network communications
during the past decade have created a great demand for real-time secure image
transmission over the internet and through wireless networks. Due to some intrinsic
features of images, such as bulk data capacity and high correlation among pixels,
traditional encryption algorithms such as IDEA, DES and AES are not suitable for
practical image encryption, especially for real time applications. In order to meet these
challenges, a number of schemes have been proposed for encryption of digital images,
making use of chaotic dynamical systems.
The objective of the work undertaken in this thesis is two-fold - firstly to evaluate the
security of a few representative chaotic ciphers by performing the cryptanalysis on them
and secondly, to design an appropriate cipher that would fulfill the needs for both security
and speed.
The cryptanalysis is performed on two recently proposed chaotic ciphers by Pareek eta!. in
[Pareek et a!., 2005] and [Pareek et a!., 2006]. The first cipher is a generic chaotic block
cipher. It is shown that the proposed cipher is insecure against differential and knownplaintext
attacks. We also show that the key space size of the proposed cipher is less than
what is claimed by the authors. The second cipher of Pareek et a!. is a complete image
encryption scheme. This scheme is also shown insecure against the differential attack in the
thesis. It is also shown suffering from a few security defects and, therefore, is not suitable
for real time secure encryption of digital images.
In this work, a complete image encryption scheme - Hybrid Chaotic Image Encryption
Scheme (HyChiES) is designed. HyChiES is based on a cryptosystem consisting of
multiple piecewise linear chaotic maps (m-PLCMs), a generalized logistic map, AES S-box
and ciphertext feedback. The analysis of the HyChiES shows that it is extremely sensitive
to changes in pixels and, therefore, has an avalanche effect - a highly desirable property
for any cipher. As a result, HyChiES randomizes plain images very effectively

In this thesis, an AES like 128-bit block cipher is also designed, named as Hybrid-Chaotic
Encryption Scheme (H-CES). The heart ofHyChiES and H-CES is the same cryptosystem
that consists of AES S-box, generalized logistic map and ciphertext feedback. In order to
analyze the differential characteristic probability of this cryptosystem, we consider it as a
hybrid S-box. Based on the maximum differential probability of this hybrid S-box,
differential characteristic probability for two rounds of H-CES is calculated and it is shown
that H-CES is secure against differential cryptanalysis.