07-05-2013, 04:57 PM
Survey: Image Encryption Using Salsa20
Survey Image.pdf (Size: 456.32 KB / Downloads: 53)
Abstract
In present times, multimedia protection is becoming increasingly
jeopardized. Therefore numerous ways of protecting information
are being utilized by individuals, businesses, and governments.
In this paper, we survey Salsa20 as a method for protecting the
distribution of digital images in an efficient and secure way. So,
we performed a series of tests and some comparisons to justify
salsa20 efficiency for image encryption. These tests included
visual testing, key space analysis, histogram analysis,
information entropy, encryption quality, correlation analysis,
differential analysis, sensitivity analysis and performance
analysis. Simulation experiment has validated the effectiveness
of the Salsa20 scheme for image encryption
Introduction
Along with the fast progression of data exchange in
electronic way, it is important to protect the confidentiality
of image data from unauthorized access. Security breaches
may affect user's privacy and reputation. So, data
encryption is widely used to confirm security in open
networks such as the internet. Due to the substantial
increase in digital data transmission via public channels,
the security of digital images has become more prominent
and attracted much attention in the digital world today.
The extension of multimedia technology in our society has
promoted digital images to play a more significant role
than the traditional texts, which demand serious protection
of users' privacy for all applications. Each type of data has
its own features; therefore, different techniques should be
used to protect confidential image data from unauthorized
access. Most of the available encryption algorithms are
used for text data. However, due to large data size and real
time requirement, it is not reasonable to use traditional
encryption methods.
Symmetric Cryptography
Symmetric encryption is the oldest branch in the field of
cryptology, and is still one of the most important ones
today. Symmetric cryptosystems rely on a shared secret
between communicating parties. This secret is used both
as an encryption key and as a decryption key. Generally,
symmetric encryption systems with secure key are divided
into two classes: stream ciphers and block ciphers. Stream
ciphers encrypt individual characters of a plaintext
message, using a time variant encryption function, as
opposed to block ciphers that encrypt groups of characters
of a plaintext message using a fixed encryption function
[6]. Nowadays, the boundaries between block ciphers and
stream ciphers are becoming blurred. So, it is difficult to
tell whether a symmetric cipher is a stream or block cipher.
Security and Performance Analysis
A fundamental issue of all kinds of ciphers is the security.
A strong cipher is capable of resisting any kind of
cryptanalytic attacks including brute-force attack,
statistical attack, known plain text attack and chosenplaintext
attack. Thus, a cipher of high key and plaintext
sensitivity with a large key space is desirable. Besides,
computational speed, size and quality of encrypted images
are other important issues as well since they always
include the feasibility of encryption schemes. In this
section we performed a series of test to justify and
compare the efficiency of the cryptosystem under study.
Differential Analysis
In general, a desirable property for an encrypted image is
being sensitive to the small changes in plain-image (e.g.,
modifying only one pixel). Opponent can create a small
change in the input image to observe changes in the result.
By this method, the meaningful relationship between
original image and encrypted image can be found. If one
small change in the plain-image can cause a significant
change in the cipher-image, with respect to diffusion and
confusion, then the differential attack actually loses its
efficiency and becomes practically useless. Three common
measures were used for differential analysis: MAE, NPCR
and UACI [15, 16]. MAE is mean absolute error. NPCR
means the number of pixels change rate of ciphered image
while one pixel of plain-image is changed. UACI which is
the unified average changing intensity, measures the
average intensity of the differences between the plainimage
and ciphered image.
Performance Analysis
Apart from the security consideration, some other issues
on image encryption are also important. This includes the
encryption speed for real-time processes. In general,
encryption speed is highly dependent on the CPU
structure, memory size, OS platform, the programming
language and also on the compiler options. So, it is
pointless to compare the encryption speeds of two ciphers
without using the same developing environment and
optimization techniques. Despite of the mentioned
difficulty, in order to show the effectiveness of the
proposed image encryption scheme over existing
algorithms, we have undertaken an analysis for the explicit
comparison between the encryption speeds of three
variants of Salsa20. We evaluated the performance of
encryption schemes with an un-optimized MATLAB code.
Performance was measured on a machine with Intel core 2
Duo 2.00 GHz CPU with 2 Gbytes of RAM running on
Windows XP. The average time used for
encryption/decryption on 256 gray-scale images of size
256×256 for Salsa20/8, Salsa20/12 and Salsa20/20 is
respectively about 1.3, 1.7 and 2.6 s (decryption and
encryption speed are the same).
Conclusion
In this paper, a successfully efficient implementation of
Salsa20 scheme is introduced for digital image encryption.
The encryption system has different variants according to
number of ciphering rounds. Salsa20 has a large key space
that is resistant to all kinds of brute-force attacks.
Theoretical and experimental Research results showed that
the scheme has resistance to statistical attacks. The
uniformity was justified by the chi-square test. It is shown
that Salsa20 hash function generates uniform cipherimages.
Information entropy test results indicate that the
cipher-image histogram distribution of the encryption
scheme is so even that the entropy measured is almost
equal to the ideal value. So, the surveyed encryption
system is secure upon the entropy attack. The measured
encryption quality showed that Salsa20/8 has a better
encryption quality than the other two variants. Correlation
analysis showed that correlation coefficients between
adjacent pixels in the plain-image are significantly
decreased after applying encryption function.