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Image Processing & Compression Techniques
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IMAGE AUTHENTICATION TECHNIQUES
This paper explores the various techniques used to authenticate the visual data recorded by the automatic video surveillance system. Automatic video surveillance systems are used for continuous and effective monitoring and reliable control of remote and dangerous sites. Some practical issues must be taken in to account, in order to take full advantage of the potentiality of VS system. The validity of visual data acquired, processed and possibly stored by the VS system, as a proof in front of a court of law is one of such issues. But visual data can be modified using sophisticated processing tools without leaving any visible trace of the modification. So digital or image data have no value as legal proof, since doubt would always exist that they had been intentionally tampered with to incriminate or exculpate the defendant. Besides, the video data can be created artificially by computerized techniques such as morphing. Therefore the true origin of the data must be indicated to use them as legal proof. By data authentication we mean here a procedure capable of ensuring that data have not been tampered with and of indicating their true origin.
2. AUTOMATIC VISUAL SURVEILLANCE SYSTEM
Automatic Visual Surveillance system is a self monitoring system which consists of a video camera unit, central unit and transmission networks.(figure)
A pool of digital cameras is in charge of frame the scene of interest and sent corresponding video sequence to central unit. The central unit is in charge of analyzing the sequence and generating an alarm whenever a suspicious situation is detected. Central unit also transmits the video sequences to an intervention centre such as security service provider, the police department or a security guard unit. Somewhere in the system the video sequence or some part of it may be stored and when needed the stored sequence can be used as a proof in front of court of law. If the stored digital video sequences have to be legally credible, some means must be envisaged to detect content tampering and reliably trace back to the data origin
3. AUTHENTICATION TECHNIQUES
Authentication techniques are performed on visual data to indicate that the data is not a forgery; they should not damage visual quality of the video data. At the same time, these techniques must indicate the malicious modifications include removal or insertion of certain frames, change of faces of individual, time and background etc. Only a properly authenticated video data has got the value as legal proof. There are two major techniques for authenticating video data. They are as follows
1. Cryptographic Data Authentication
It is a straight forward way to provide video authentication, namely through the joint use of asymmetric key encryption and the digital Hash function.
Cameras calculate a digital summary (digest) of the video by means of hash function. Then they encrypt the digest with their private key, thus obtaining a signed digest which is transmitted to the central unit together with acquired sequences. This digest is used to prove data integrity or to trace back to their origin. Signed digest can only read by using public key of the camera.
2. Watermarking- based authentication
Watermarking data authentication is the modern approach to authenticate visual data by imperceptibly embedding a digital watermark signal on the data.
Digital watermarking is the art and science of embedding copyright information in the original files. The information embedded is called ‘watermarks ‘. Digital watermarks are difficult to remove without noticeably degrading the content and are a covert means in situation where copyright fails to provide robustness.
Mounting concern over the new threats to privacy and security has lead to wide spread adoption of cryptography. Cryptography is the science of transforming documents. It has mainly two functions
The purpose of encryption is to render a document unreadable by all except those who authorize to read it. Cryptographers refer to the content of the original document as plain text. Plain text is converted in to cipher form using an algorithm and a variable or key. The key is a randomly selected string of numbers.
Only after decoding the cipher text using the key the content of the document is revealed to the common people. Encryption schemes are classified in to
1. Symmetric encryption
In which the same key is used to both encode and decode the document.
2. Public key or asymmetric encryption
It requires a pair of keys: one for encrypting the plain text and the other for decrypting the cipher text. A file encrypted with one key of a pair can be decrypted with other key of the same pair.
5. CRYPTOGRAPHIC DATA AUTHENTICATION
To authenticate visual data each video camera is assigned a different public or private key pair, with private key hardwired within the cameras. A document encrypted with the private key of any particular camera can be decrypted with its own public key. This property is used to provide center authentication that is to trace back to the true origin of the data.
Before sending the video sequence to the central unit, cameras calculate a digital summary or digest of the video by means of a proper hash function. The digest is then encrypted with their private key. Encryption is done by considering the digitized value of the brightness of each pixel. Digital signal is a sequence of zeros and ones and it is encrypted with the private key using a proper algorithm. The signed digest thus obtained is then transmitted to the central unit together with the acquired visual sequence.
Later the signal digest is used to prove data integrity or to trace back to their origin. The signed digest is read using the public key of the camera which produce the video and check if it corresponds to the digest derived from the decrypted video content using the same hash function.
Any manipulation of the data will change the calculated image digest derived from the decrypted data. Any discrepancy between the decrypted digest and calculated image digest indicate that the data has been tampered, with identical digest indicates that the data is genuine.
Value of the visual data can be added by tying each frame to the particular label of the instant the frame has been produced yet. This can be achieved by printing date and time of creation of each frame. Any modification of either the date or time could be easily revealed since it would change the locally calculated image digest.
5.1 DRAWBACKS OF CRYPTOGRAPHIC AUTHENTICATION
Even though cryptographic data authentication is highly resistant to content tampering, it suffers from few drawbacks. They are as follows:
1. Knowledge of private key
If the manipulator knows the private key of the camera, he can change the digest to involve the modifications he had made on the actual sequence. But the possibility of such a thing is very small because the private key is hardwired within the camera.
2. Impossible to distinguish between malicious and innocuous modification
It is difficult to distinguish between malicious and innocuous manipulations if cryptography is used. Innocuous modifications include compression of the video sequences. These modifications are usually performed by the central unit whereas digest is calculated on the basis of the uncompressed data by the camera. So on compression the correspondence between the digest and data would be lost.
3. High requirements of video camera
To avoid the above discussed problem, the video camera should perform the compression of the video sequences prior to digest calculation. This requires the video camera to have high computation as storage requirements.
4. Delay in transmission
Digest calculations and encryption introduces a delay in transmission of video documents. This is harmful in system where the timely generation of alarm is critical.
5. Protecting privacy is difficult
A part of the program cannot be removed for privacy reasons since it will alter the calculated digest.
A digital watermark is a signal that is imperceptibly embedded within digital data. This signal can be detected or extracted by means of computations to make some assertions about the host data.
Digital watermark is a signal which added to a document to authenticate it and to prove the ownership. A commonly encountered digital watermark is the logo most television channels display on the top of the television screen. Not only does it advertise the channel but also provides the legal benefit of having a source signature persist during video recording. Watermark task consists of two main steps
1. Watermark casting:-in which the signal represented by the watermark is transmitted over the channel, that is in watermark casting an encoder function Є takes a host image ‘f’ and a watermark ‘w’ and generate a new image
Fw= Є (f, w)
2. Watermark detection:-in which the signal is received and extracted from possibly corrupted image.