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Dept. Of Comp. Engg.

Digital Signature And Wat








Digital Signature And Watermarking

























Digital Signature And Watermarking





In today's commercial env
ironment, establishing a framework for the authentication
of computer
based information requires a familiarity with concepts and professional skills
from both the legal and computer security fields. Combining these two disciplines is not an
easy task. Con
cepts from the information security field often correspond only loosely to
concepts from the legal field, even in situations where the terminology is similar. For
example, from the information security point of view, "digital signature" means the result

applying to specific information certain specific technical processes described below.
The historical legal concept of "signature" is broader. It recognizes any mark made with the
intention of authenticating the marked document.


It is
probably not surprising that the inventors of writing, the Sumerians, were also
the inventors of an authentication mechanism. The Sumerians used intricate seals, applied
into their clay cuneiform tablets using rollers, to authenticate their writings. Seals

continued to be used as the primary authentication mechanism until recent times.

Use of signatures is recorded in the Talmud (fourth century), complete with security
procedures to prevent the alteration of documents after they are signed. The
Talmud even
describes use of a form of "signature card" by witnesses to deeds. The practice of
authenticating documents by affixing handwritten signatures began to be used within the
Roman Empire in the year AD 439, during the rule of Valentinian III. The


short handwritten sentence at the end of a document stating that the signer "subscribed" to
the document

was first used for authenticating wills. The practice of affixing signatures to
documents spread rapidly from this initial usage, and
the form of signatures (a hand

Digital Signature And Watermarking



representation of one’s own name) remained essentially unchanged for over 1,400 years. It
is from this Roman usage of signatures that the practice obtained its significance in
Western legal tradition.

When do you ne
ed to verify identity?

New ways of verification are being developed daily.

Biometrics and other methods
keep getting formulated and incorporated into the information technology industry.

interesting biometric authentication mechanism developed by a
leading Japanese biometric
company has found a way to get your DNA into a pen.

You sign a document and it is
digitally scanned.

This document then can be scanned in the future to verify its
authenticity. Identity should be verified when ever there is dou
bt of the 3rd party being
whom they say they are or when there is personal information at risk.

Personal information
like credit card details and banking information should be kept safe using digital
certification as one of the security layers.

Some bank
ing institutions require that a user

verifies his/her identity by validating identification credentials using a digital
certificate. Important e
mail can also use Digital signatures that verify that the e
mail is
from the originating sender and that it ha
s not been tampered with.

On many occasions
users are unsure if they are dealing with reputable suppliers of institutions.

certification gives the user a sense of legitimacy and formalizes the process.

It ensure that
the company that the user is

dealing with has a registration with a trusted authority and that
the transaction is guaranteed to be done with the intended parties.


Digital signatures

are a way to ensure the integrity of a message or other
data using public key crypt
ography. Like traditional signatures written with ink on
paper, they can be used to authenticate the identity of the signer of the data.
However, digital signatures go beyond traditional signatures in that they can also
ensure that the data itself has not
been altered. This is like signing a check in such a
way that if someone changes the amount of the sum written on the check, an
“Invalid” stamp becomes visible on the face of the check.Digital signatures take the
Digital Signature And Watermarking



concept of traditional paper
based signing
and turn it into a digital "fingerprint".
This "fingerprint", or coded message, is unique to both the document and the signer.
The digital signature ensures that the signatory is indeed the originator of the

Any changes made to the document after

it was signed invalidate the
signature, thereby protecting against forgery. Digital signatures help organizations
sustain signer authenticity, accountability, data integrity and non
repudiation of
documents and transactions.

Reasons for using digital se

It insures by means of verification and validation that the user is whom he/she
claims to be.

This is done by combine the users credential to the digital
certificate and in turn this method uses one point of authentication.

Digital certificates i
nsure data Integrity giving the user piece of mind that the
message or transaction has not been accidentally or maliciously altered.

is done cryptographically.

Digital certificates ensure confidentiality and ensure that messages can only be
read by

authorized intended recipients.

Digital certificates also verify date and time so that senders or recipients can
not dispute if the message was actually sent or received.

The components that a digital signature comprise of.


Your public key:

This is the
part that any one can get a copy of and is part of
the verification system.


Your name and e
mail address:

This is necessary for contact information
purposes and to enable the viewer to identify the details.


Expiration date of the public key:

This part of

the signature is used to set a
shelf life and to ensure that in the event of prolonged abuse of a signature
eventually the signature is reset.

Digital Signature And Watermarking




Name of the company:

This section identifies the company that the signature
belongs too.


Serial number of the
Digital ID:

This part is a unique number that is bundled
to the signature for tracking ad extra identification reasons.


Digital signature of the CA (certification Authority):

This is a signature that is
issued by the authority that issues the certificates

Signatures and the Law

A signature is not part of the substance of a transaction, but rather of its representation or
form. Signing writings serve the following general purposes:


A signature authenticates a writing by identifying the signer
with the
signed document. When the signer makes a mark in a distinctive manner, the
writing becomes attributable to the signer.

The act of signing a document calls to the signer's attention the legal
significance of the signer's act, and thereby
helps prevent "inconsiderate


In certain contexts defined by law or custom, a signature expresses the
signer's approval or authorization of the writing, or the signer's intention that it have
legal effect.

Efficiency and logistics:


signature on a written document often imparts a sense of
clarity and finality to the transaction and may lessen the subsequent need to inquire
beyond the face of a document. Negotiable instruments, for example, rely upon
formal requirements, including a s
ignature, for their ability to change hands with
ease, rapidity, and minimal interruption.

The formal requirements for legal transactions, including the need for signatures, vary in
different legal systems, and also vary with the passage of time. There is

also variance in the
legal consequences of failure to cast the transaction in a required form. The statute of
frauds of the common law tradition, for example, does not render a transaction invalid for
lack of a "writing signed by the party to be charged,"

but rather makes it unenforceable in
Digital Signature And Watermarking



court, a distinction which has caused the practical application of the statute to be greatly
limited in case law.



A Singing algorithm


A key generation algorithm


A verif
ication algorithm



Digital signatures are created and verified by cryptography, the branch of applied
mathematics that concerns itself with transforming messages into seemingly unintelligible
forms and back aga
in. Digital signatures use what is known as "public key cryptography,"
which employs an algorithm using two different but mathematically related "keys;" one for
creating a digital signature or transforming data into a seemingly unintelligible form, and
ther key for verifying a digital signature or returning the message to its original form.
Computer equipment and software utilizing two such keys are often collectively termed an
"asymmetric cryptosystem."

The complementary keys of an asymmetric cryptosys
tem for digital signatures are
arbitrarily termed the private key, which is known only to the signer and used to create the
digital signature, and the public key, which is ordinarily more widely known and is used by
a relying party to verify the digital si
gnature. If many people need to verify the signer's
digital signatures, the public key must be available or distributed to all of them, perhaps by
publication in an on
line repository or directory where it is easily accessible. Although the
keys of the pa
ir are mathematically related, if the asymmetric cryptosystem has been
designed and implemented securely it is "computationally infeasible to derive the private
key from knowledge of the public key. Thus, although many people may know the public
key of a
given signer and use it to verify that signer's signatures, they cannot discover that
signer's private key and use it to forge digital signatures. This is sometimes referred to as
the principle of "irreversibility."

Digital Signature And Watermarking




Another fundamental process, t
ermed a "hash function," is used in both creating and
verifying a digital signature. A hash function is an algorithm which creates a digital
representation or "fingerprint" in the form of a "hash value" or "hash result" of a standard
length which is usuall
y much smaller than the message but nevertheless substantially
unique to it. Any change to the message invariably produces a different hash result when
the same hash function is used. In the case of a secure hash function, sometimes termed a
way hash

function," it is computationally infeasible to derive the original message
from knowledge of its hash value. Hash functions therefore enable the software for creating
digital signatures to operate on smaller and predictable amounts of data, while still
roviding robust evidentiary correlation to the original message content, thereby efficiently
providing assurance that there has been no modification of the message since it was
digitally signed.

Thus, use of digital signatures usually involves two processe
s, one performed by the signer
and the other by the receiver of the digital signature:

Digital signature creation

uses a hash result derived from and unique to both the
signed message and a given private key. For the hash result to be secure, there must

only a negligible possibility that the same digital signature could be created by
the combination of any other message or private key.

Digital signature verification

is the process of checking the digital signature by
reference to the original message an
d a given public key, thereby determining
whether the digital signature was created for that same message using the private
key that corresponds to the referenced public key.

Digital Signature And Watermarking



The creation of a digital signature

In the simplest terms a digital signatur
e is a stream of bits appended to a document. The
purpose of a digital signature is to provide assurance about the origin of the message and
the integrity of the message contents. When a message with a digital signature is
transmitted and received, the fol
lowing parties are involved:

the signer who signs the document;

the verifier who receives the signed document and verifies the signature ;

the arbitrator who arbitrates any disputes between the signer and the verifier if there
is a disagreement on the v
alidity of the digital signature.

Digitally signing a document begins with producing a summary of the document using
mathematical functions known as hash functions. Some examples are Message Digest
(MD5), Secure Hash Algorithm
1 (SHA
1) and Réseaux IP E
uropéens (RIPE) Message
160). The output of a hash function, a document summary called the
hash, always has the same number of bits e.g. 128 for MD5 and 160 for SHA
1, regardless
of the length of the input document. It is obvious that di
fferent documents will produce
different hashes. It is considered virtually impossible to have an identical hash even from
two similar documents.

The hash function is encrypted by the signer using his/her private key and forms the digital
signature of the
encrypted document.

The verifier receives both the document and the signature, calculates the summary of the
document using the same hash function used by the signer. The signature is decrypted
using the signer’s public key. The last step is to compare the

decrypted summary with the
one previously computed by the verifier from the document. If the two summaries are
identical then the signature has been verified. The verifier is now sure of the identity of the
signer and that the data was not been modified.

Digital Signature And Watermarking



The figure below shows the signing process again in steps.

Let us suppose that Alice is the signer and Bob the verifier:

Let us suppos
e that Alice is the signer and Bob the verifier:

Alice calculates the summary of the document, the hash;

Alice encrypts the summary with her own private key to create the digital signature;

Alice sends the digital signature and the document to Bob, the

Bob calculates the summary of the document, the hash;

Bob decrypts the digital signature with Alice’s public key and obtains a summary;

Bob compares the two summaries he has made;

if they are equal Bob is sure that the document was not modifi
ed and that Alice
really did sign the document herself.

Digital Signature And Watermarking




Explanation: This Standard specifies a Digital Signature Algorithm (DSA) appropriate for
applications requiring a digital rather than written signature. The DSA digit
al signature is a
pair of large numbers represented in a computer as strings of binary digits. The digital
signature is computed using a set of rules (i.e., the DSA) and a set of parameters such that
the identity of the signatory and integrity of the data
can be verified. The DSA provides the
capability to generate and verify signatures. Signature generation makes use of a private
key to generate a digital signature. Signature verification makes use of a public key which
corresponds to, but is not the same
as, the private key. Each user possesses a private and
public key pair. Public keys are assumed to be known to the public in general. Private keys
are never shared. Anyone can verify the signature of a user by employing that user's public
key. Signature ge
neration can be performed only by the possessor of the user's private key.

A hash function is used in the signature generation process to obtain a condensed version of
data, called a message digest . The message digest is then input to the DSA to generat
e the
digital signature. The digital signature is sent to the intended verifier along with the signed
data (often called the message). The verifier of the message and signature verifies the
signature by using the sender's public key. The same hash function

must also be used in the
verification process. The hash function is specified in a separate standard, the Secure Hash
Standard (SHS), FIPS 180. Similar procedures may be used to generate and verify
signatures for stored as well as transmitted data.

Digital Signature And Watermarking



Using the SHA with the DSA

What is a digital certificate?

An electronic credential tha
t vouches for the holder's identity, a digital certificate has
characteristics similar to those of a passport

it has identifying information, is forgery
proof, and is issued by a trusted third party. Digital certificates are published in on
ies. Typically, a digital certificate contains:

The user's distinguished name (a unique identifier)

The issuing Certification Authority's distinguished name

The user's public key

The validity period

The certificate's serial number

The issuing Certific
ation Authority's digital signature, verifying the information in
the digital certificate.

How Strong are Signatures?

No security mechanism, whether manual or automated, provides absolute assurance. There
is evidence that forgery was practiced shortly afte
r the invention of writing, and that it has
remained a problem ever since.

Digital Signature And Watermarking



Modern forensic document examiners commonly compare a suspect signature with several
examples of known valid signatures, and look for signs of forgery, which include:

• Signatures
written at a speed which is significantly slower than the genuine signatures;

• Frequent change of the grasp of the writing implement;

• Blunt line endings and beginnings;

• Poor line quality with wavering and tremor of the line;

• Retracing and patching;

• Stops in places where the writing should be free.

These techniques are supplemented with ink and paper analysis, electrostatic detection of
writing imprints, and so on.

If one were to argue in court that "I didn’t sign this document, my pen did," the res
would probably be tittering in the courtroom, a lost case, and a possible court
psychiatric evaluation. However, if one were to argue in court that "I didn’t sign the data,
my computer did," the response from the court might be more sympathetic
, as anyone who
has used a computer has had the experience of the computer doing something the operator
didn’t want it to do. Ultimately, people do not sign electronically

they command their
computers to sign electronically on behalf of the signer. Somed
ay an attacker will seize
control of a victim’s signing application to fraudulently sign data, and when this attack
becomes public, confidence in digital signatures may be forever shaken.

Digital signatures cryptographic authentication systems bind signatu
res to individuals
through technical and procedural mechanisms. There are strong, mathematical links
between a private signature key, its associated public key, and the message signature, but
the association between the signer and her private key depends o
n the protection afforded
the private key. The association between the signer and her public key depends on the
honesty and diligence of the Certification Authority (CA) issuing the signer’s public key
Digital Signature And Watermarking



certificate (a public key certificate is a digitally s
igned statement by a CA that binds a
public key to a signer’s identity). Hence, the strength of the security services provided by a
digital signature is a function of the methods used to safeguard the private signature key,
methods used by the CA to identi
fy and authenticate those applying for digital certificates,
the protections provided against corrupt CAs, safeguards against the computers used by the
CA being subverted, and so on. The standards, practices and procedures used to ensure the
validity of th
e binding between a signer and the signer’s public key represent a "certificate
policy." The Internet Engineering Task Force (IETF) Public Key Infrastructure/X.509
(PKIX) working group has developed a guide for developing certificate policies that
s certificate policies more precisely as:

"A named set of rules that indicate the applicability of a certificate to a particular
community and/or class of application with common security requirements."

The IETF goes on to list about 250 "policy elements"
which can be factored into the
establishment of a certificate policy. These policy elements include methods used to
identify an individual, how the public/private key pairs are generated, how the private keys
are protected, liability limits, and so on. Sin
ce different CAs establish and follow different
policies, the strength of digital signatures varies according the policy of the CA who issued
the signers’ certificates. Furthermore, digital signature certificates normally state a "validity
interval," deter
mined by the CA, during which the certificate may be used to verify
signatures. The matter of what to do about signatures applied using a private key for which
the associated public key has expired is one of many associated with the long
term validity
of d
igital signatures.

Digital Signatures

Will They Last?

When considering digital data archival, it is important to remember digital signature
verification requires each and every bit in the signed document be preserved and read
correctly, just as it was wh
en the signer applied the signature. For example, the flipping of
a bit that changes an "s" character to an "S," while undesirable in any electronic document,
would render a digitally signed document completely unverifiable, just as if every word in
the do
cument had been changed.

Digital Signature And Watermarking



There are at least four problems associated with the long
term archival of signed electronic
records. Briefly, they are:

• Deterioration of the source media;

• Obsolescence of the record data format;

• Evolution of cryptographic al
gorithms and related standards; and,

• Certificate life

Source media (tapes, optical disks, floppy disks, etc.) are subject to deterioration over time.
Magnetic media are prone to hydrolysis of the binder in which the magnetic particles are
. Hydrolysis causes the binder to become soft and sticky, and transfer from the
media substrate to read/write heads and other surfaces. Another problem with magnetic
media is the magnetic domains within the media "top coat" can reverse, thus changing
ded 1’s to 0’s and vice versa. The length of time a tape may be used to archive data
varies from a minimum of about one year under tropical conditions, to about 64 years
under ideal (cool, dry) conditions.

The "weak link" in terms of optical disk archival

is the metal reflecting layer, used to
reflect the optical disk reader’s laser. This reflecting layer is typically made of aluminum,
and subject to oxidation, because the reflecting surface is enclosed in materials that can be
permeable. As with ma
gnetic tape, quality of the media and storage conditions play
the dominant role in determining the useful archive lifetime, but manufacturers estimates
and independent studies indicate that read
only optical disks should last for 100 years under
ideal cond
itions. Lifetimes for writable optical disks are usually less

between 10

years (Dual alloy disks being an exception, with an estimated life of 100 years.)

Digital signatures exacerbate the problem of technological obsolescence. They make the
most c
ommon coping technique

conversion to new formats during transition periods

impossible unless the original signer can resign under the new format

a solution which is
always burdensome and often impossible. From a digital signature perspective, a chang
e to
a document format is indistinguishable from a change to the document content, and will
result in an unverifiable signature.

Digital Signature And Watermarking



A similar problem is associated with the mercurial nature of cryptographic algorithms and
standards. Aside from the signer’s pr
ivate signature key, a digital signature is a function of:

The message being signed (including any encoding of the data);

The hashing algorithm used; and,

The signature algorithm used.

We have already seen that the formatting of data is changing continu
ously. It appears that
digital signature standards are also likely to undergo continuous evolution. Hashing
algorithms that have been used in the short history of digital signatures include MD2,
MD4, MD5, and the Secure Hashing Algorithm

1 (SHA
1). There

are frequent proposals
for improving upon these algorithms as new cryptanalytic attacks are found, more efficient
hashing mechanisms are devised, and computer hardware (for example the move from 16
bit to 32 bit machines) changes algorithm requirements.

Earlier we explored the role of the Certification Authorities in binding identities to public
keys. It must be stressed that digital signatures cannot be verified without certificates.
Certificates expire. VeriSign Corporation, for example, issues certific
ates to end
entities for
one year periods. Certificate validity dates vary from one Certification Authority to
another, and a single CA can support several certificate policies with different certificate
validity periods. Certificates can be renewed, but
if they are not renewed, they expire, and
are not supposed to be used to verify signatures thereafter.

Challenges and Opportunities

The prospect of fully implementing digital signatures in general commerce presents both
benefits and costs. The costs consis
t mainly of:

Institutional overhead:

The cost of establishing and utilizing certification
authorities, repositories, and other important services, as well as assuring quality in
the performance of their functions.

Subscriber and Relying Party Costs:

A dig
ital signer will require software, and will
probably have to pay a certification authority some price to issue a certificate.
Digital Signature And Watermarking



Hardware to secure the subscriber's private key may also be advisable. Persons
relying on digital signatures will incur expenses f
or verification software and
perhaps for access to certificates and certificate revocation lists (CRL) in a

On the plus side, the principal advantage to be gained is more reliable authentication of
messages. Digital signatures, if properly imp
lemented and utilized offer promising
solutions to the problems of:

, by minimizing the risk of dealing with imposters or persons who attempt
to escape responsibility by claiming to have been impersonated;

Message integrity
, by minimizing the ris
k of undetected message tampering and
forgery, and of false claims that a message was altered after it was sent;

Formal legal requirements
, by strengthening the view that legal requirements of
form, such as writing, signature, and an original document, ar
e satisfied, since
digital signatures are functionally on a par with, or superior to paper forms; and


Digital watermarking

is a technique which allows an individual to add hidden

notices or other verification messages to digital audio, video, or image signals and
documents. Such hidden message is a group of bits describing information pertaining to the
signal or to the
author of the signal (name, place, etc.). The technique takes its name from

of paper or money as a security measure. Digital watermarking is not a
form of
, in which data is hidden in the message without the end user's
knowledge, although some watermarking techniques have the steganographic feature of not
being perceivable by the human

The enormous popularity of the World Wide Web in the early 1990's demonstrated the
commercial potential of offering multimedia resources through the digital networks. Since
commercial interests seek to use the digital networks to offer digital media f
or profit, they
Digital Signature And Watermarking



have a strong interest in protecting their ownership rights. Digital watermarking has been
proposed as one way to accomplish this.

A digital watermark is a digital signal or pattern inserted into a digital image. Since this
signal or patter
n is present in each unaltered copy of the original image, the digital
watermark may also serve as a digital signature for the copies. A given watermark may be
unique to each copy (e.g., to identify the intended recipient), or be common to multiple
(e.g., to identify the document source). In either case, the watermarking of the
document involves the transformation of the original into another form. This distinguishes
digital watermarking from digital fingerprinting where the original file remains int
act, but
another file is created that "describes" the original file's content. As a simple example, the
checksum field for a disk sector would be a fingerprint of the preceding block of data.
Similarly, hash algorithms produce fingerprint files.


Two types of digital watermarks may be distinguished, depending upon whether the
watermark appears visible or invisible to the casual viewer. Visible watermarks are used in
much the same way as their bond paper ancestors, where the

opacity of paper is altered by
physically stamping it with an identifying pattern.This is done to mark the paper
manufacturer or paper type. One might view digitally watermarked documents and images
as digitally "stamped".

The visible watermarks which app
ear in Figures 1 and 2 illustrate the technique. The
watermark in Figure 1 appears is quite obtrusive because of the high contrast between the
background and foreground drawing. There is no place for the watermark to "hide" as it
were. The colored image in

Figure 2 renders the visible watermark less obvious.

Figure 1.
Digital Copy of fifteenth century drawing with digital watermark

Digital Signature And Watermarking



Figure 2.
Digitized copy of artwork from a sixteenth century Aztec

Invisible watermarks, on the other hand, are potentially useful as a means of identifying the
source, author, creator, owner, dis
tributor or authorized consumer of a document or image.
For this purpose, the objective is to permanently and unalterably mark the image so that the
credit or assignment is beyond dispute. In the event of illicit usage, the watermark would
facilitate the c
laim of ownership, the receipt of copyright revenues, or the success of

Watermarking has also been proposed to trace images in the event of their illicit
redistribution. Whereas past infringement with copyrighted documents was often limited by

the unfeasibility of large
scale photocopying and distribution, modern digital networks
make large
scale dissemination simple and inexpensive. Digital watermarking makes it
possible to uniquely mark each image for every buyer. If that buyer then makes an
copy, the illicit duplication may be convincingly demonstrated.


Visible and invisible watermarks both serve to deter theft but they do so in very different
ways. Visible watermarks are especially useful for conveyin
g an immediate claim of
ownership. The main advantage of visible watermarks, in principle at least, is that they
virtually eliminate the commercial value of the document to a would
be thief without
lessening the document's utility for legitimate, authorize
d purposes. A familiar example of
a visible watermark is in the video domain where CNN and other television networks place
their translucent logo at the bottom right of the screen image.

Digital Signature And Watermarking



Invisible watermarks, on the other hand, are more of an aid in catchi
ng the thief than
discouraging the theft in the first place.

Though neither exhaustive nor definitive, Table 1 shows some anticipated primary (p) and
secondary (s) benefits to digital watermarking.

Table 1.




validation of inte
nded recipient



repudiable transmission



deterrence against theft



diminish commercial value without utility



discourage unauthorized duplication



digital notarization and authentication



identify source




To be effective in the protection of the ownership of intellectual property, the invisibly
watermarked document should satisfy several criteria:


the watermark must be difficult or impossible to remove, at least without visibly
degrading the origi
nal image,


the watermark must survive image modifications that are common to typical image
processing applications (e.g., scaling, color requantization, dithering, cropping, and
image compression),


an invisible watermark should be imperceptible so as not

to affect the experience of
viewing the image, and


for some invisible watermarking applications, watermarks should be readily
detectable by the proper authorities, even if imperceptible to the average observer.
Such decodability without requiring the ori
ginal, un
watermarked image would be
necessary for efficient recovery of property and subsequent prosecution.

Digital Signature And Watermarking



One can understand the challenge of researchers in this field since the above requirements
compete, each with the others. The litmus test of a wa
termarking method would be that it is
accepted and used on a large, commercial scale, and that it stands up in a court of law.
None of the digital techniques have yet to meet these tests.


Watermarking techniques tend to divide i
nto two categories, text and image, according to
the type of document to be watermarked. Techniques for images: Several different methods
enable watermarking in the spatial domain. The simplest (too simple for many applications)
is to just flip the lowest
order bit of chosen pixels in a gray scale or color image. This will
work well only if the image will not be subject to any human or noisy modification. A
more robust watermark can be embedded in an image in the same way that a watermark is
added to paper.

Such techniques may superimpose a watermark symbol over an area of the
picture and then add some fixed intensity value for the watermark to the varied pixel values
of the image. The resulting watermark may be visible or invisible depending upon the value
(large or small, respectively) of the watermark intensity. One disadvantage of spatial
domain watermarks is that picture cropping (a common operation of image editors) can be
used to eliminate the watermark.

Spatial watermarking can also be applied using c
olor separation. in this way, the watermark
appears in only one of the color bands. This renders the watermark visibly subtle such that
it is difficult to detect under regular viewing. However, the watermark appears immediately
when the colors are separate
d for printing or xerography. This renders the document
useless to the printer unless the watermark can be removed from the color band. This
approach is used commercially for journalists to inspect digital pictures from a photo
stockhouse before buying un
watermarked versions. Watermarking can be applied in the
frequency domain (and other transform domains) by first applying a transform like the Fast
Fourier Transform (FFT). In a similar manner to spatial domain watermarking, the values
of chosen frequencie
s can be altered from the original. Since high frequencies will be lost
by compression or scaling, the watermark signal is applied to lower frequencies, or better
yet, applied adaptively to frequencies that contain important information of the original
Digital Signature And Watermarking



ture (feature
based schemes). Since watermarks applied to the frequency domain will be
dispersed over the entirety of the spatial image upon inverse transformation, this method is
not as susceptible to defeat by cropping as the spatial technique. However,
there is more of
a tradeoff here between invisibility and decodability, since the watermark is in effect
applied indiscriminately across the spatial image.

Watermarking can be applied to text images as well. Three proposed methods are: text line
coding, wo
rd space coding, and character encoding. For text line coding, the text lines of a
document page are shifted imperceptibly up or down. For a 40
line text page, for instance,
this yields 2**40 possible codewords. For word
shift coding, the spacing between w
ords in
a line of justified text is altered (see Figure 3). For character coding, a feature such as the
endline at the top of a letter, "t" is imperceptibly extended. An advantage of these methods
over those applied to picture images is that, by combining
two or three of these to one
document, two documents with different watermarks cannot be spatially registered to
extract the watermark. Of course, the watermark can be defeated by retyping the text.

Types of Watermark

Visible watermarks

Visible waterma
rks are an extension of the concept of logos.
Such watermarks are applicable to images only. These logos are inlaid into the image but
they are transparent. Such watermarks cannot be removed by cropping the center part of the
image. Further, such watermark
s are protected against attacks such as statistical analysis.

The drawbacks of visible watermarks are degrading the quality of image and detection by
visual means only. Thus, it is not possible to detect them by dedicated programs or devices.
Such watermar
ks have applications in maps, graphics and software user interface.

Invisible watermark

Invisible watermark is hidden in the content. It can be detected
by an authorized agency only. Such watermarks are used for content and/or author
authentication and fo
r detecting unauthorized copier.

Digital Signature And Watermarking



Public watermark:

Such a watermark can be read or retrieved by anyone using the
specialized algorithm. In this sense, public watermarks are not secure. However, public
watermarks are useful for carrying IPR information. The
y are good alternatives to labels.

Fragile watermark

Fragile watermarks are also known as tamper
proof watermarks.
Such watermarks are destroyed by data manipulation.

Private Watermark:

Private watermarks are also known as secure watermarks. To
read or re
trieve such a watermark, it is necessary to have the secret key.

Perceptual watermarks

A perceptual watermark exploits the aspects of human
sensory system to provide invisible yet robust watermark. Such watermarks are also known
as transparent watermarks
that provide extremely high quality contents.

stream watermarking

The term is sometimes used for watermarking of
compressed data such as video.

Text document watermark

Text document is a discrete information source. In discrete sources, cont
ents cannot be
modified. Thus, generic watermarking schemes are not applicable. The approaches for text
watermarking are hiding watermark information in semantics and hiding watermark in text

In semantic
based watermarking, the text is designed aro
und the message to be hidden.
Thus, misleading information covers watermark information. Such techniques defy
scientific approach.

By text format, we mean layout and appearance. Commonly used techniques to hide
watermark information are line shift coding,
word shift coding and feature coding.

Digital Signature And Watermarking



How It Works

Digital Watermarking software looks for noise in digital media and replaces it with useful
information. A digital media file is nothing more than a large list of 0’s and 1’s. The
watermarking software de
termines which of these 0’s and 1’s correspond to redundant or
irrelevant details. For example, the software might identify details in an image that are too
fine for the human eye to see and flag the corresponding 0’s and 1’s as irrelevant noise.
Later the

flagged 0’s and 1’s can be replaced by a digital watermark.

A real
world example

The following two sequences of images demonstrate a typical watermark embedding and
ion process applied to a static image. It is notable that a slight degradation of the
original image occurs when the watermark is embedded. However, the retrieved watermark
is very close to the original watermark, which can help resolve ownership issues.

Digital Signature And Watermarking




Video watermarking

Video watermarking can be considered as a superset of normal image watermarking. As
such, all the techniques applicable to static images can be applied to video images.
However, due to the high frame rate of video,
the embedding process must occur almost in
real time for live transmissions (it takes a finite time to embed the watermark, which might
influence the transmission rate). If the content is generated off
line, this limitation does not
exist. A very popular f
orm of on
line (live) video watermarking is the usage of a visible
watermark (normally a logo or other distinguishing sign placed in an unobtrusive place on
each frame of video footage).

Audio watermarking

Audio watermarking is currently at the forefront o
f technology development in an attempt
to prevent illegal reproduction and redistribution. One implementation receiving
widespread attention is the MP3 approach to audio compression and watermarking.

Audio watermarking can be successfully implemented at fr
equencies outside the normal
human audible range. (This is also the approach followed by compression schemes, in
which frequencies outside the human audible range are removed from the original audio

Text watermarking

Text can be subdivided int
o two categories: raw unformatted ASCII text and formatted text
(typically Postscript, PDF or RTF formats).

Watermark information can be embedded into a formatted document using an approach
based on the slight adjustment of inter
line and inter
word spacin
gs. Another approach to
watermark embedding is to consider the typeset text as one large image and thus to use the
typical approaches used for images.

Digital Signature And Watermarking



Raw text presents a big problem to the watermark process. At this stage no successful
approach is known.
One possible approach is based on adding white space characters after
each sentence (and is thus hidden to the casual observer). However, this approach is easily
bypassed using a normal text editor.


Authentication is but one use of digit
al watermarking. Both symmetric and asymmetric
hashing algorithms can be used to embed a unique digital imprint on a document or file. If
the removal of an imprint yields the original document (which is to say that the "stripped"
watermark is identical to
the embedded watermark), then the copy is authentic. Once again,
this assumes that the "stripping" algorithm is available to the end
user. Such authentication
techniques are usually associated with some sort of encryption for the distribution of keys,
rams, etc. which are related to the watermarked documents.

In addition, watermarks are also used as a check for non
repudiable duplication and
transmission. In this case, the owner, creator or sender imprints a watermark which is
unique for each receiver.
The watermark holds under subsequent re
transmission, so the
"authorized" source of unauthorized copies may be easily identified after extraction. A
collateral benefit is that the intended recipient of a document token could always be


these applications really only apply to the class of invisible watermarks. Visible
watermarks (as in Figure 1) contribute to document and transmission security in different
ways. To illustrate, visible watermarks are more overt means of discouraging theft

unauthorized use both by reducing the commercial value of a document and making it
obvious to the criminally inclined that the document's ownership has been definitively
established . We observe that invisible watermarks only have this effect if the d
igital thief
is aware of the technology and the possibility that watermarks may be present on a
document of interest.

Digital Signature And Watermarking



There are several characteristics of effective watermarks. For one, they must be difficult or
impossible to remove. For another, they must

survive common document modifications and
transformations (e.g., cropping and compressing image files). Third, they must, in principle
at least, be easily detectable and removable by authorized users with such privileges (e.g.,
law enforcement agencies).
Invisible watermarks should also be imperceptible, while
visible watermarks should be perceptible enough to discourage theft but not perceptible
enough to decrease the utility or appreciation of the document.


Watermarking techniques t
end to divide into two categories, text and image, according to
the type of document to be watermarked. In the case of imagery, several different methods
enable watermarking in the spatial domain from simply flipping low
order bits of selected
pixels to su
perimposing watermark symbols over an area of a graphic. Spatial domain
watermarking is illustrated in Figures 2a and 2b that demonstrate how the degree of
visibility of the watermark depends upon its intensity and the nature of the background.

Figures Figures 2a and 2b.
Two (of many) Two watermarked images identical but for the
intensity of the image. Considerable latitu
de is available, in terms of placement, size and
intensity to blend the watermark into a graphic.

Another spatial watermarking technique uses color separation. In this way, the watermark
appears in only one of the color bands. This renders the watermark v
isibly subtle such that
Digital Signature And Watermarking



it is difficult to detect under regular viewing. However, the watermark appears immediately
when the colors are separated for printing. This renders the document useless to the printer
unless the watermark can be removed from the co
lor band. This approach is used
commercially for journalists to inspect digital pictures from a photo
stockhouse before
buying un
watermarked versions.


As of this writing, a counterfeiting scheme has been demonstrated fo
r a class of invertible,
based, frequency domain, invisible watermarking algorithms. This counterfeiting
scheme could be used to subvert ownership claims because the recovery of the digital
signature from a watermarked image requires a comparison w
ith an original. The
counterfeiting scheme works by first creating a counterfeit watermarked copy from the
genuine watermarked copy by effectively inverting the genuine watermark. This inversion
creates a counterfeit of the original image which satisfies t
wo properties: (a) a comparison
of the decoded versions of both the original and counterfeit original yields the owner's
(authorized) signature, and (b) a comparison of decoded versions of both the original and
counterfeit original yield the forged (invert
ed) signature. This, the technique of establishing
legitimate ownership recovering the signature watermark by comparing a watermarked
image with the original image breaks down. It can be shown that both the legitimate
signature and counterfeiter's signatur
e inhere in both the watermarked and counterfeit
watermarked copies. Thus, while it may be demonstrated that at least one recipient has a
counterfeit watermarked copy, it can not be determined which it is.

This research suggests that not all watermarking t
echniques will be useful in resolving
ownership disputes in courts of law. There will likely be non
commercial applications, or
those with limited vulnerability to theft, where "good enough watermarking" will suffice.
More sensitive applications may requir
e non
invertable or non
extracting watermarking
techniques. These issues are under consideration at this writing. Standard watermarking
involves the creation of a watermarked image by encoding a signature into an original
image. Authentication proceeds in
two stages. First, the watermarks signature is "removed"
from the watermarked copy. The watermark signature is the "difference" between the
Digital Signature And Watermarking



original (white) and the watermarked copy of the original (blue). Next, the extracted
signature (blue) is compared a
gainst the original signature (gold). Identity signifies
authenticity of the copy.


Though publishers have been clamoring for some means to protect the
ir material on
electronic networks, there has been no rush yet to embrace any of the current schemes. This
could be just due to a period of inspection and appraisal, but our opinion is that publishers
and scientists have yet to fully understand the practic
al specifications associated with the
problem. Should the watermarks be visible or invisible? What constitutes invisibility? How
difficult should it be to remove watermarks from images? How might one characterize
Enough Watermarking" for different co
mmercial and non
commercial applications?
What constitutes a "reasonable" level of photo
editing? Or of degradation? Can the original
image be required for decoding? Is transferal of the watermark from the electronic medium
to the printed medium important?

How are the watermarks to be policed? Etc.

As scientists propose solutions and publishers experiment with them and debate their
merits, some methods of watermarking will emerge as useful and widely used. When that
happens, there will also be the emergenc
e of external agencies for monitoring electronic
copyright infringement (much the same as there are agencies for music and print copyright
management). In the meantime, the challenge is for the scientists to develop ever more
invisible, decodable, and perm
anent watermarking methods, and perhaps to meet even
more specifications as they are demanded.

Digital Signature And Watermarking



Here are some problems that you might encounter when deploying a watermark

Ease of Destruction:

Although watermarks are designed to survive manipulation of
source media, it is nonetheless possible to perform manipulations that irrecoverably break
the watermark. Furthermore, the small number of watermark software vendors results in
easily detectable watermark signature patterns. There are numerous effectiv
e techniques for
identifying and disabling commercial watermarks in media.

Efficient Detection of Watermarks:

Imagine you’re working for a stock photography
company. Browsing the web one day, you come across an image that looks very familiar.
you scan the image with your company’s watermarking software. Sure enough,
it’s one of your images, and the site never purchased the right to use it! The watermark
gives your legal team the ammunition it needs to force payment from the freeloaders.

This s
cenario makes watermarking sound incredibly useful. Unfortunately, the method of
detection (accidental) is not very reproducible or reliable. Automated watermark search
engines exist, but they have some significant limitations. For starters, the amount of
media on the Internet is staggering. It could take hundreds of millions of dollars in
equipment to effectively scan a significant amount of Internet data for watermarks. Then
there’s traditional media

scanning newspapers, magazines, TV broadcast
s and films for
watermarks requires a lot of manual work and therefore is rarely cost effective.

Stock photography, clip art, and other variants of digital artwork are ideal candidates for
watermarking. Without watermarks, a visual artist can’t display th
eir commercial images
online without worrying that someone will just download and use their imagery without
paying. By using a watermark search engine, our example scenario becomes a business
saving strategy for these companies.

Digital Signature And Watermarking



The following figure pre
sents a very abstract watermark
embedding process.

The extraction of the embedded watermark is depicted in the next figure. Upon successful
extraction of t
he watermark, ownership information (and other information initially
embedded in the original image) is available for inspection.

Digital Signature And Watermarking




In this paper we

introduced the concept of digital watermarking used to protect intellectual
property rights, copyrights and rightful ownership. We presented required criteria for a
watermarking scheme to be successful. We also identified areas (types of digital media)
ere watermarking can be applied as well as other areas (such as raw text) where no
current watermarking scheme exists. We finally addressed the issue of whether the
presence of a watermark can prove ownership and concluded that this is only possible
h the use of a higher, controlling governing body where all original media can be

A final conclusion: Digital watermarking can successfully be employed if the value of the
digital media warrants the added expense. If not, it is an exercise in f

Digital signature relies on the protection afforded a private signature key by the signer, and
the procedures implemented by a Certification Authority.

Digital signatures must be applied by a computer commanded by the signer.

Digital Signature And Watermarking



Forgery of digital si
gnatures, in the absence of compromise of the private signature key, or
hijacking of the signature mechanism, is virtually impossible.

Due to the cryptographic nature of digital signatures, attempted forgeries are immediately
obvious to any verifier, exc
ept in the case where a private signature key has been
compromised, or control of the signing mechanism has been seized. In these cases,
distinguishing between a valid and invalid digital signature may be impossible, even for a
computer forensics specialis

. Digital signatures are fiendishly complex, involving arcane number theory, the workings
of computer operating systems, communications protocols, certificate chain processing,
certificate policies, and so on. There are very few people on this planet (
if any) who
completely understand every process involved in generating and verifying a digital
signature. The potential for confused lawyers, judges and juries is extreme.

Digital signatures have the potential to have the greatest impact on commerce since
invention of money. Digital signatures allow us to identify ourselves and make
commitments in cyberspace in much the same way as we do in actual space. Nonetheless,
digital signature have important limitations, the most significant being their temporar