Aim: The aim of the paper is to analyse the security strength of image encryption
schemes which are based on pixel rotation and inversion functions. The key independent image decryption
methodologies are presented to obtain original images with intelligible contents from encrypted
images using neighbourhood similarity characteristics and divide-and-conquer attack.
Background: The efficiency and security strength of secure communication of sensitive data depend
on the computing resources and cryptographic strength of encryption schemes. An encryption
scheme is cryptographically strong if it does not leave any weakness, vulnerability or pattern which could
be exploited by cryptanalyst to obtain the original image from an encrypted image. Prior to the use of any
image encryption scheme for multimedia security applications, it should be analysed for its security
strength to ensure the safety of information so that an adversary could not extract intelligible information
from encrypted image data. A number of encryption schemes developed for image security applications
and claimed highly secure but some of these are cryptanalysed successfully and found insecure.
Objective: The analysis of image ciphers which encrypt plain images by transforming its pixels using
circular rotation or inversion function in a random fashion is carried out for decrypting encrypted images
to obtain original images. The encryption schemes, namely ‘Chaotic Image Encryption (CIE)’ and
‘Graphical Image Encryption (GIE)’, were reported secure but we find that these schemes are insecure
which can be exploited to obtain meaningful information from the ciphered images. We apply the similarity
characteristics of images to mount cryptanalytic attacks on these ciphers and obtain original images
without any knowledge of the encryption/decryption keys. These encryption schemes encrypting the specified
region-of-interest (ROI) are also analysed to decrypt ROI encrypted images.
Methods: The methodology of decryption is key independent and based on divide-and-conquer strategy
to obtain original images from the given encrypted images. It utilizes the neighbourhood similarity of pixels
in an image which is measured in terms of pixel-to-pixel difference between adjacent pixels for pixel
inversion based image cipher (GIE) and line-to-line correlation between adjacent lines for pixel rotation
based image cipher (CIE). The ROI encrypted and masked encrypted images are also decrypted.
Results: Experimental test results show that the decrypted images obtained are quite intelligible and
one can understand the contents of decrypted images. It is also seen that an image cipher encrypting
the ROI can be decrypted by utilizing unencrypted region surrounding encrypted ROI part of an image.
Conclusion: It has been shown that CIE, GIE, ROI and masked encryption schemes reported for image
security applications are insecure and not providing adequate security. Such encrypted images can be
decrypted successfully without any key knowledge with high intelligibility by considering image similarity
characteristics of neighbouring pixels and applying divide-and-conquer attack strategy.
Future work: The key independent decryption methodology can be considered to cryptanalyse the
encryption schemes under noise attack scenario as future work to see the applicability of decryption
methods with respect to increase in noise in encrypted images. Moreover, other modern encryption
schemes based on pixel inversion and rotation functions can be analysed for their security strength.
Keywords: Image encryption, circular rotation, pixel inversion, ROI encryption, cryptanalysis, decryption, neighbourhood similarity,
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