Image Authentication Techniques

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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 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.

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.

ENCRPTION ALGORITHEM DECRYPTION

ALGORITHEM

PLAIN TEXT

CIPHER TEXT

DECRYPTION KEY

CIPHER TEXT

PLAIN TEXT

ENCRYPTIONKEY


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.


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

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.

6.1 CLASSIFICATION OF WATERMARKING

AUTHENTICATION 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.

WATERMARKING

FRAGILEWATREMARKING

VISIBLEWATERMARKING

INVISIBLEWATERMARKIN

G

ROBUST WATERMARKING

DUAL WATERMARKIN

G


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 DATA

AUTHENTICATION

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.

6.2.3 OTHER REQUIREMENTS

The authentication checking procedure should be easily performed by

authorized person and it should be difficult for non authorized person to forge an

authenticated image.

6.2.4 VS – SPECIFIC REQUIREMENTS

Since water marking is performed inside the video camera, water mark

embedding should not have high computational demands and should be

compressing resistant.

To ensure data integrity, the video sequence is tied to the time and date it

has been produced. The easiest way to detect the removal of one or more frames

although alternation of the original frame order is to embed on each image a

serial number before authenticating tools are applied. In fact such a number can

neither be removed nor modified without affecting the authentication check, this

making it impossible to remove or change the position of any frame of the

sequence.

By embedding in each frame the time and date of its creation in dissoluble

link is created between the sequence content and the time instant, so that legal

value of the sequence is completely preserved.


The embedded water mark can be made to depend on the frame number

and to bear time information. Frame exchange or substitution would thus be

easily detected and acquisition time can be reliably extracted.

In figure below a sketch of a simple VS system in which water marking is

used to authenticate VS data in its raw form is given. Time, date and frame serial

number are over written to every single frame before authentication. The

authenticated sequence is possessed by a central unit for detecting pre-alarm

situations and then is compressed for storage purposes. In this case authentication

tools should be transparent to the image possessing algorithms applied by the

central unit and resistant to compression.


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.


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.

Water mark detection ratio Quality loss (%)

(a) (b)


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).

(a) (b) (c)

a) Initial tampered frame b) False detection image c) Detected tampered regions


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.

2. 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.


12. REFERENCES

1. C. Ragazoni, G.Fabri, “Image Authentication Techniques for VS”,

Proceedings of IEEE, October 2001.

2. M.M.Yeung and F.Mintzer “A watermark for digital image” IEEE

Spectrum, April 2002.

3. “Digital Watermarking for protecting piracy”, Electronics for you,

January 2003.

4. “Encryption wars”, IEEE Spectrum, April 2000.

5. www.ctr.columbia.edu

6. citeseer.nj.nec.com/wolfgang96watermark.html


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.


ACKNOWLEDGEMENT

I extend my sincere gratitude towards Prof. P.Sukumaran Head of

Department for giving us his invaluable knowledge and wonderful technical

guidance

I express my thanks to Mr. Muhammed Kutty our group tutor and also

to our staff advisors Ms. Biji Paul, Mr. Noushad V Moosa, Mr. Baiju Karan

for their kind co-operation and guidance for preparing and presenting this

seminar.

I also thank all the other faculty members of AEI department and my

friends for their help and support.


CONTENTS

1. INTRODUCTION

2. AUTOMATIC VISUAL SURVEILLANCE SYSTEM

3. AUTHENTICATION TECHNIQUES

4. CRYPTOGRAPHY

5. CRYPTOGRAPHIC DATA AUTHENTICATION

5.1 DRAWBACKS OF CRYPTOGRAPHIC AUTHENTICATION

6. WATERMARKING

6.1 CLASSIFICATION OF WATERMARKING AUTHENTICATION

SCHEMS

6.2 REQIREMENTS OF WATERMARKING BASED VS DATA

AUTHENTICATION

7. WATERMARKING ALGORITHM

7.1 WATERMARK GENERATION AND EMBEDDING

7.2 WATERMARK DETECTION

7.3 AUTHENTICATION CHECK

8. OTHER APPLICATIONS

9. ADVANTATGES

10. DISADVANTAGES

11. CONCLUSION

12. REFERENCES

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Image Authentication Techniques

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