hal berghel watermarking cyberspace t - university of...

COMMUNICATIONS OF THE ACM November 1997/Vol. 40, No. 11 19

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Hal Berghel

The use of watermarks isalmost as old as paper man-ufacturing. Ancients poured

their half-stuff slurry of fiber andwater onto mesh molds to collectthe fiber, then dispersed theslurry within deckle frames toadd shape and uniformity, andfinally applied great pressure toexpel the water and cohere thefiber. This process hasn’t changedtoo much in 2,000 years, evenwith the benefit of automation.One by-product of this process isthe watermark—the technique ofimpressing into the paper a form,image, or text derived from thenegative in the mold, as thepaper fibers are squeezed anddried.

Paper watermarks have beenin wide use since the late middleages. Their earliest use seems tohave been to record the manufac-turer’s trademark on the productso that the authenticity could beclearly established withoutdegrading the aesthetics and util-ity of the stock. In more recenttimes, watermarks have beenused to certify the composition ofthe paper, including the natureof the fibers used. Today, mostdeveloped countries also water-mark their paper currencies andpostage stamps to make forgerymore difficult.

The digitization of our worldhas expanded our concept of

watermarking to include im-material, digital impressions foruse in authenticating ownershipclaims and protecting proprietaryinterests. However, in principle,digital watermarks are not unliketheir paper ancestors. They sig-nify something about the tokenof a document or file in whichthey inherit. Whether the prod-uct of a Fourdrinier paper press ora discrete cosine transformation,watermarks of varying degrees ofvisibility are added to presenta-tion media as a guarantee ofauthenticity, quality,ownership, andsource.

Watermarks in ContextA digital watermark is adigital signal or pattern insertedinto a digital document (text,graphics, multimedia presenta-tions). As such, it is a form ofelectronic watermark much like

the corporate logos used by thecable television industry to iden-tify the source of the program,

typically

along the lower periph-ery of the televisionscreen. Such cable com-panies, we may assume,believe that the adver-tising advantage of theever-present, on-screen

logo, together with thelegal benefit of having a

source signature persistunder video recording,

more than offset the aggregateuser annoyance and distraction.

Digital watermarks extendthese advantages to digital docu-ments. A signal or pattern maybe digitally imposed on a docu-

Watermarking Cyberspace

20 November 1997/Vol. 40, No. 11 COMMUNICATIONS OF THE ACM

ment prior to sale or distribu-tion. The persistence of thewatermark under transmission,and some common forms oftransformation, contribute toour ability to authenticatecopies. This, in turn, shouldenable us to protect our owner-ship rights of digital informa-tion, even in the undisciplined,anarchistic world of the Internet(see Figure 1).

Before going into detail aboutwhat digital watermarking is,we’ll first explain what it is not.Digital watermarking is notencryption, which also involvesfile transformation. It is a com-mon practice nowadays toencrypt digital documents sothat they become unviewablewithout a decryption key. Unlikeencryption, however, digitalwatermarking leaves the originalimage or file basically intact andrecognizable.

Further, decrypted documentsare free of any residual effects ofencryption, whereas visible digi-tal watermarks are designed to bepersistent in viewing, printing,or subsequent retransmission ordissemination.

Digital watermarking differsfrom digital fingerprinting,which produces a “meta” filethat describes the contents of thesource file. Cyclic redundancychecking and checksum algo-

rithms are both simple uses offile fingerprinting for errordetection applications. A moreadvanced use of fingerprinting isfound in RSA Data Security’s useof message digests for authenti-cation purposes. Digests are theresult of applying a hashingalgorithm (for example, MD5,SHA) to a document or file to

produce an identifying bit string(fingerprint). If the receiver’shash algorithm produces thesame message digest for the fileas the sender’s, the file is authen-tic. Of course, this assumes thatsender and receiver use the samesoftware, hence the same hash algorithm.

Fingerprints may also serve

VISIBLE WATERMARKS ARE A MORE OVERT

means of discouraging theft and 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.

Figure 1: Digitized copy of artwork from a 16th century Aztec manuscript.Note the circular digital watermark is most visible against a light background.

Faint watermarks tend to hide in the intense, foreground imagery.

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as digital signatures. If themessage digest just discussedwere further encrypted, con-verted to plain text, andattached to the original file ormessage in transit, the plain-text version of the messagedigest (fingerprint) would alsoserve as a digital signature forthe original file. While bothfingerprints and signaturesaccompany unaltered sourcedocuments, signatures, liketheir penned counterparts, areembedded in the documentitself even if in encrypted form.

Watermarks in Use

Authentication is just one useof digital watermarking.Both symmetric and asym-

metric hashing algorithms can beused to embed a unique digitalimprint on a document or file. Ifthe removal of an imprint yieldsthe original document (which is tosay that the stripped watermark isidentical to the embedded water-mark), then the copy is authentic.Once again, this assumes thestripping algorithm is available tothe end user. Such authenticationtechniques are usually associatedwith some sort of encryption forthe distribution of keys, programs,and so forth, which are related tothe watermarked documents.

In addition, watermarks arealso used as a check for non-repudiable duplication and transmission. In this case, theowner, creator, or senderimprints a watermark unique foreach receiver. The watermarkholds under subsequent re-transmission, so the authorizedsource of unauthorized copiesmay be easily identified afterextraction. A collateral benefit is

that the intended recipient of adocument token could always beidentified.

However, these applicationsreally only apply to the class ofinvisible watermarks. Visiblewatermarks (as in Figure 1) con-tribute to document and trans-mission security in differentways. To illustrate, visible water-marks are a more overt means ofdiscouraging theft and unautho-rized use both by reducing the

commercial value of a documentand making it obvious to thecriminally inclined that the docu-ment’s ownership has been defini-tively established. Invisiblewatermarks only have this effectif the digital thief is aware of thetechnology and the possibilitythat watermarks may be presenton a document of interest.

There are several characteris-tics of effective watermarks. Forone, they must be difficult orimpossible to remove. Foranother, they must survive com-mon document modifications andtransformations such as croppingand compressing image files.They must also, in principle at

least, be easily detectable andremovable by authorized userswith such privileges (law enforce-ment agencies). Invisible water-marks should also beimperceptible, while visiblewatermarks should be perceptibleenough to discourage theft butnot perceptible enough todecrease the utility or apprecia-tion of the document.

Watermarking PracticeWatermarking techniques tend todivide into two categories—textand image—according to thetype of document to be water-marked. In the case of imagery,several different methods enablewatermarking in the spatialdomain from simply flippinglow-order bits of selected pixelsto superimposing watermarksymbols over an area of a graphic.Spatial domain watermarking isillustrated in Figure 2. Thisdemonstrates the degree of visi-bility of the watermark. Consid-erable latitude is available, interms of placement, size, andintensity, to blend the watermarkinto a graphic.

Another spatial watermarkingtechnique uses color separation.In this way, the watermarkappears in only one of the colorbands. This renders the water-mark visually subtle so it is diffi-cult to detect during regularviewing. However, the water-mark appears immediately whenthe colors are separated for print-ing. This renders the documentuseless to the printer unless thewatermark can be removed fromthe color band. This approach isused commercially for journaliststo inspect digital pictures from aphoto-stockhouse before buying

Figure 2. Two watermarked images identical but for the intensity

of the image


unwatermarked versions.An alternative to spatial

watermarking is frequencydomain watermarking. In thiscase, transforms like the FastFourier Transform (FFT) alterthe pixel values of the image forchosen frequencies. Since highfrequencies will be lost by com-pression or scaling, the water-mark signal is applied to lowerfrequencies, or applied adap-

tively to frequencies containingimportant information of theoriginal picture (feature-basedschemes). Since watermarksapplied to the frequency domainwill be dispersed over theentirety of the spatial imageupon inverse transformation, thismethod is not as susceptible todefeat by cropping as the spatial

technique. However, there ismore of a trade-off here betweeninvisibility and decodability,since the watermark is in effectapplied indiscriminately acrossthe spatial image.

Watermarking can be appliedto text images as well. Three pro-posed methods are: text line cod-ing, word-space coding, andcharacter encoding. For text linecoding, the text lines of a docu-

ment page are shifted impercep-tibly up or down. For a 40-linetext page, for instance, this yields240 possible code words.

Figure 3a illustrates text linecoding as it would appear to thecasual reader. According to theline code box, the first, second,fourth and sixth lines are ele-vated by one pixel, although the

alteration is practically impercep-tible. The effectiveness of suchwatermarking is confirmed inFigure 3b. Even with the affectedlines set apart in red, it is stilldifficult to determine the linesare elevated.

For word-shift coding, thespacing between words in a lineof justified text is altered. Theplain text in Figure 4a has threewords shifted right one pixel.

Figure 4b high-lights theaffected words.

The remain-ing text water-markingtechniqueinvolves charac-ter coding. Thisinvolves minoralterations to theshapes of charac-ters such as clip-ping a serifimperceptibly, orextending adescender. Anadvantage ofthese methodsover thoseapplied to pic-ture images isthat, by combin-ing two or threeof these to onedocument, two

documents with different water-marks cannot be spatially regis-tered to extract the watermark.Of course, the watermark can bedefeated by retyping the text.

Limitations of Digital WatermarkingAs of this writing, a counterfeit-ing scheme has been demon-

Figure 3a. Text with lines 1, 2, 4 and 6 elevatedfrom normal position by one pixel

Figure 3b. Elevated lines highlighted

strated for a class of invertible,feature-based, frequency domain,invisible watermarking algo-rithms. This counterfeitingscheme could be used to subvertownership claims because therecovery of the digital signaturefrom a watermarked image

requires a comparison with anoriginal. We may illustrate thepoint simply with graphics.

Standard watermarking (seeFigure 5a) involves the creationof a watermarked image byencoding a signature into anoriginal image. Authentication

proceeds in two stages. First, thewatermarked signature is“removed” from the watermarkedcopy. The watermark signature isthe “difference” between theoriginal (white) and the water-marked copy of the original(blue). Next, the extracted signa-ture (blue) is compared againstthe original signature (gold).Identity signifies authenticity ofthe copy.

The counterfeiting scheme(see Figure 5b) works by firstcreating a counterfeit water-marked copy (violet) from thegenuine watermarked copy (blue)by effectively inverting the gen-uine watermark. This inversionproduces a counterfeit signature(violet) as well.

The trick is the original imageand bona fide signature stand inthe same relationship to thewatermarked image as the coun-terfeit image and counterfeit sig-nature (see Figure 5c). Thus, thetechnique of establishing legiti-mate ownership by recoveringthe signature watermark by com-paring a watermarked imagewith the original image breaksdown. While it may be demon-strated that at least one recipienthas a counterfeit watermarkedcopy, it cannot be determinedwho it is.

This research suggests not allwatermarking techniques will beuseful in resolving ownership dis-putes in courts of law. There willlikely be noncommercial applica-tions, or those with limited vul-nerability to theft, where “goodenough watermarking” will suf-fice. More sensitive applicationsmay require noninvertable ornonextracting watermarkingtechniques.


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Figure 4b: Text with offset words highlighted

Figure 4a. Text with three words offset by one pixel

The Future of WatermarkingThe enormous popularity of theWorld Wide Web in the early1990s demonstrated the commer-

cial potential of offering multime-dia resources through the digitalnetworks. Since commercial inter-ests seek to use the digital net-

works to offer digitalmedia for profit, theyhave a strong interestin protecting theirownership rights.Digital watermarkinghas been proposed asone way to protectsuch interests.Though muchresearch remainsbefore watermarkingsystems becomerobust and widelyavailable, there ismuch promise theywill contribute signif-icantly to the protec-tion of proprietaryinterests of electronicmedia. Collateraltechnology also willbe necessary to auto-mate the process ofauthentication, nonre-pudiable transmission,and validation.

Pointers:Some of this materialwas adapted from H.Berghel and L.O’Gorman, “Protect-ing OwnershipRights through Digi-

tal Watermarking,” IEEE Com-puter 29, 7 (1996), 101–103.

A good overview of how coun-terfeiters could attack water-marked images based on thecorrelation of the differencesbetween samples is reported inH. Stone, “Analysis of Attacks onImage Watermarks with Ran-domized Coefficients,” NECResearch Institute TechnicalReport, May 17, 1996. Thecounterfeiting scheme describedhere will appear in S. Craver, N.Memon, B. Yeo, and M. Yeung,“Resolving Rightful Ownershipswith Invisible WatermarkingTechniques: Limitations, Attacksand Implications,” IEEE J. onSelected Areas of Communications,Dec. 1997.

The latter two articles also con-tain useful references to the water-marking literature. Information onIBM’s digital library projects areto be found at www.ibm.com/IBM/ibmgives/ diglib.htm.

The images in Figures 2 and 3were taken from our digitalwatermarking demonstration pro-gram available as freeware fornoncommercial use through theauthor’s ftp site (see bio).

Hal Berghel (www.acm.org/~hlb) is aprofessor of computer science at the Univer-sity of Arkansas and a frequent contributorto the literature on cyberspace.

ACM 0002-0782/97/1100 $3.50

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Figure 5c. Counterfeit logic

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Figure 5a. Basic watermarking technique

Figure 5b. Watermark “inversion” for counterfeiting

IN THE CASE OF IMAGERY, SEVERAL DIFFERENT METHODS

enable watermarking in the spatial domain from simply

flipping low-order bits of selected pixels to superimposing

watermark symbols over an area of a graphic.

hal berghel watermarking cyberspace t - university of...

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