IMAGE AUTHENTICATION TECHNIQUES[Type the document subtitle]

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ABSTRACT

Automatic video surveillance (AVS) systems are used for continuous and effective

monitoring of dangerous and remote sites. Video data acquired by the automatic

video surveillance system can be recorded and presented as a proof in front of

court law. But digital video data lacks legal validity due to the ease to manipulate

them without leaving any trace of modification. Image authentication is the process

of giving a legal validity to the video data. By authentication technique content

tampering can be detected and we can indicate the true origin of the data. There

are

two types of authentication schemes, which are

1. Cryptographic data authentication.

2. Watermarking-based authentication.

In this presentation an attempt is made to present the basic features of the image

authentication techniques.

1. INTRODUCTION

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 SYSTEMAutomatic Visual Surveillance system is a self monitoring system which consists of a

video camera unit, central unit and transmission networks.(figure)

PLAIN TEXT CIPHER TEXT

ENCRYPTION ENCRYPTION DECRYPTION DECRYPTION

KEY ALGORITHM KEY ALGORITHM

CIPHER TEXT PLAIN TEXT

4.1 Encryption and Decryption

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

3.1 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.

3.2 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.

4. CRYPTOGRAPHY

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

¨ Encryption

¨ Decryption

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.

WATER MARKING

FRAGILE ROBUST

WATERMARKING WATERMARKING

INVISIBLE VISIBLE DUAL

WATERMARKING WATERMARKING WATERMARKING

6.1 Types of Watermarking

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.

6. WATERMARKING

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 _ takes a host

image

‘f’ and a watermark ‘w’ and generate a new image

Fw= _ (f, w)

2. Watermark detection:-in which the signal is received and extracted from possibly

corrupted image.

6.1 CLASSIFICATION OF WATERMARKINGAUTHENTICATION SCHEMS

The characteristics of watermarking system largely depend on its application

scenario.

For instant copy write protection application require that the watermark is robust

against

most common data manipulation,ie its presents can still be detected after

nondestructive

transformation of host document. Two approaches for watermarking data

authentication

are possible:

1.Fragile watermarking

2. Robust watermarking

Fragile watermarking refers to the case where watermark inserted within the data is

lost

or altered as soon as host data undergoes any modification. Watermark loss or

alternation

is taken as evidence that data has been tampered with, whereas the information

contained

within data used to demonstrate data origin

In case of robust watermarking a summary of the candidate frame or video

sequence is

computed and is inserted within the video sequence. Information about the data

origin is

also with the summary. To prove data integrity the information conveyed by the

watermark is recovered and compared with the actual content of the sequence.

Their

mismatch is taken as an evidence of data tampering. The capability to localize the

manipulation will depend on the summary of which is embedded in to the image.

6.1.1 ROBUST VERSUS FRAGILE WATERMARK

Semi fragile watermark is more mature than robust watermarking. Tamper

localization is

easier in fragile watermarking but it is difficult to distinguish between malicious and

innocuous manipulations.

Image authentication by means of robust watermarking is very promising with

regards to

the distinction between malicious and innocuous manipulations. The robustness of

such

technique depends on the number of bits that can be hidden in to the image.

6.2 REQIREMENTS OF WATERMARKING BASED VS DATAAUTHENTICATION

In order to highlight the peculiarities of VS data authentication. Let us consider the

most

common requirements for watermarking authentication techniques.

1. The authentication technique must not deteriorate the visual quality of data

quality.

2. The authentication technique should be able to identify any unauthorized

processing

acquired to visual data.

3. The authentication technique should not consider innocuous manipulation, e.g.,

image

compression and zooming, as valid authentication attacks.

4. It should be difficult for unauthorized person to forge an authenticated image.

5. The authentication checking procedure should be easily performed by authorized

persons.

6. The authentication checking procedure should localize data tampering.

By considering particular cases the requirement for the authentication of VS data

are

following.

6.2.1 THE INVISIBILITY CONSTRAINT

The requirement on the deterioration of the visual quality of authenticated data is

usually

referred to us as invisibility constraint.

The authentication technique must not deteriorate the visual quality of data. In this

VS

case however this is not crucial issue since VS do not exhibit a quality comparable

that of

visual data used in a media. VS data acquired by inexpensive, low quality devices.

Visual

analysis would possibly be carried out in a low court will focus on the semantic

content

of the image, rather than on their visual quality.

6.2.2 MALICIOUS VERSUS INNOCUOS MANIPULATIONS

The authentication technique should be able to identify any nonauthorized or

malicious

processing occurred to the visual data. Besides the authentication technique should

not

consider innocuous manipulation Eg: Image compression or Zooming, as valid

authentication techniques.

When data compression is done in central unit the authentication is performed

before

compression and must survive it. To satisfy privacy complaints, some processing is

done

on the authenticated video, before it is stored. E.g. for obscuring the faces of

persons

which are unimportant on the law point of view. This kind of processing as to be

considered as innocuous. On the other side, the same processing procedure as to

be

considered as malicious when information that is important for a court law is

removed.

A solution to this problem is offered by authentication techniques capable of

localizing

manipulations. Once the modification is precisely localized, it will be up to the court

law

to decide if it is malicious of innocuous.

7. WATERMARKING ALGORITHM

A watermarking algorithm for VS data authentication based on semi-fragile

watermarking of each frame of the video sequence is described in this section. The

various steps for the watermarking process are as follows.

7.1 WATERMARK GENERATION AND EMBEDDING

Watermark generation aims at producing a binary or ternary watermark W(X) using

a

digital key K and host image f(x). The watermark key corresponds to the image

owner or

camera that has captured the image. The block diagram of watermark generation

and

embedding is shown below.

In the proposed method as indicating in the figure the watermark generation by

using a

pseudo random number generator and appropriate thresholding.

The watermark key used for watermarking a specific frame in the sequence is

composed

of the camera id and frame number. Generating the watermark key using frame

number

provides the advantage of producing frame–dependent watermarks.

In this case, frame removal or frame substitution can be easily detected as non

authentic.

To perform authenticity check, the detection should first know the frame number in

the

sequence which is always zero.

Watermark embedding is performed by altering all the pixels of the original frame

according to the following formula.

Fw(x) = f(x) if w(x) = 0

g1 (f(x), n(x)) if w(x) =-1

g2 (f(x), n(x) if w(x) =1

Where g1g2 are suitably designed function based on x.

n(x) denotes a function that depends on neighborhood of x.

The function g1g2 are called embedding function and are selected so as to detect

the

inverse detection function. D (fw(x), n(x)). The detection function, when applied to

the

watermarked image fw(x), produces the watermark w(x).

D (fw(x), N(x)) = w(x)

7.2 WATERMARK DETECTION

In the watermark detection procedure, the detector generates first the water mark

for each

frame to be checked. To do so, the id number of camera that produce the sequence

and

the frame number are needed.

7.2 Watermark Detection

A detection function D is defined such that by applying the detection function to the

watermarked image a detection image d(x) is produced.

d(x) = D (fw(x), n(x))

Now we frame the false detection image given by

Ew(x) = 1if w(x) _0 and w(x) _d(x)

0 otherwise

The false detection image has value 1 if a watermarked pixel is falsely detected and

0

otherwise. The watermarked detection ratio is given by the ratio of the correctly

detected

pixel to the sum of the watermarked pixels in the image.

7.3 AUTHENTICATION CHECK

Authentication check is a two level process. A first level decision on image

authenticity

is taken by comparing the watermark detection of the text image with a pre

specified

threshold T.

If the first level decision test indicates that the image is somehow altered but

authentic, a

second level decision test should be performed. This test indicates whether the

alternations made on the image are concentrated in certain regions (Malicious

tampering)

or one spread on the image (innocuous alternations).

8. OTHER APPLICATIONS

1. To protect the intellectual property right of a music publisher who distributes

music scores over digital media.

Digital piracy is a serious concern to the musical industry.Customers receive

music in digital data format and such data can be pirated and redistributed very

easily. By using image score watermarking we can prevent this.

1. It can be used for everything from sending e-mail and storing medical records

and legal contracts to conducting on-line transactions.

9. ADVANTATGES

1. Robustness to high quality lossy image compression.

2. Automatic discrimination between malicious and innocuous manipulations.

3. Controllable visual deterioration of the VS sequence by varying the watermark

embedding power.

4. Watermark embedding and detection can be performed in real time for digital

data.

10. DISADVANTAGES

1. Frame independent watermark can be easily found by comparative analysis of all

image sequence frames and then could be easily added again to fake frames.

2. The detector should know the frame number in order to perform authenticity

check

11. CONCLUSION

In these modern eras, visual surveillance system finds application in almost all

fields,

ranging from commercial to defense. The video data acquired by VS system are

forming

vital evidence for several legal situations. So for such situations, the importance of

authenticating their content is very high. Cryptography and watermarking based

authenticating techniques are quite safe and efficient for this purpose and they are

likely

to remain for quite for some while.

TABLE OF CONTENTS

CHAPTER NO. TITLE

PAGE NO.

1. ABSTRCT

2. INTRODUCTION

3. AUTOMATIC VISUAL SURVEILLANCE SYSTEM

4. AUTHENTICATION TECHNIQUES

5. CRYPTOGRAPHY

6. CRYPTOGRAPHIC DATA AUTHENTICATION

6.1 DRAWBACKS OF CRYPTOGRAPHIC

AUTHENTICATION

7. WATERMARKING

7.1 CLASSIFICATION OF WATERMARKING

AUTHENTICATION SCHEMES

7.2 REQUIREMENTS OF WATERMARKING

BASED VS DATA AUTHENTICATION

8. WATERMARKING ALGORITHM

8.1 WATERMARK GENERATION AND

EMBEDDING

8.2 WATERMARK DETECTION

8.3 AUTHENTICATION CHECK

9. OTHER APPLICATIONS

10. ADVANTAGES

11. DISADVANTAGES

12. CONCLUSION

13. REFERENCE