| Summary: | The recent advancements of multimedia signal processing techniques enable us to
conveniently edit/modify multimedia contents, which are usually available in the compressed
form for transmission and storage purposes. Among the compression standards,
JPEG for still image and H.264/AVC for video are the common formats that we handle
in our daily life. As the application of compressed image/video widens, practical
security tools for compressed image/video also become increasingly important. In that
regard, encryption is the process designed to transform a content into an unintelligible
form. Classical encryption approaches (e.g., DES, AES, RSA) handle the bitstream of a
compressed image/video directly, but this approach invalidates many features of the compressed
image/video, including hierarchical decoding and transcoding. In addition, the
computational time of conventional encryption is also high. To overcome the aforementioned
problems, the selective-encryption approach, which encrypts a subset of a compressed
content, has been pursued by many researchers. Although many were proven to
be robust against traditional cryptanalysis, the conventional selective-encryption methods
are vulnerable to non-traditional forms of attack called sketch attack. In this thesis, five
sketch attacks, i.e., four of them are based on DCT coefficients while another one exploits
the statistical features of a coding block, are proposed. Their performances are evaluated
and then compared. Furthermore, as a mean to evaluate the sketch image, an objective
evaluation metric called outline clearness assessment (OCA) is put forward. Moreover,
a selective encryption method for JPEG compressed image is put forward to address the
problem of bitstream size overhead while being robust against both the traditional and
non-traditional attacks. As an application, sketch attack is utilized as a feature extraction
process in H.264/AVC compressed video for text detection purpose. Finally, contributions
and limitations of this study are summarized, and future works are discussed.
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