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1 SteganographyinaVideoConferencingSystem? AndreasWestfeld1andGrittaWolf2 2InstituteforOperatingSystems,DatabasesandComputerNetworks 1InstituteforTheoreticalComputerScience DresdenUniversityofTechnology D-01062Dresden,Germany Abstract.Wedescribeasteganographicsystemwhichembedssecret includesdiscretecosinetransformation(dct)based,lossycompression. securityisestablishedbyindeterminismwithinthesignalpath. Resultisthetechnicalrealisationofasteganographicalgorithmwhose messagesintoavideostream.weexaminethesignalpathwhichtypically 1Introduction Theescalationofcommunicationviacomputernetworkhasbeenlinkedtothe informationhidingexistfordigitalimageles,audioles,andinbackground soundsofphonecalls[2].therearemorethan20programsontheinternet(for increasinguseofcomputeraidedsteganography[5],[6].steganographicmethods examplesseethelistbelow). thatathirdpersoncannotdetectorevenprovethisprocess.examplesfor usuallyhidecipheredmessagesinother,harmless-lookingdatainsuchaway {S-ToolsbyAndyBrownembedsdataasleastsignicantbitsinaudioles {JstegbyDerekUphamembedsdatainJFIFimages.Itoverwritestheleast (.wav)orasleastsignicantbitsofthergbcolorvaluesingraphicles (.bmp).athirdmethodhidesdatainfreesectorsofdiskettes.severalsymmetricencryptionmethods(des,idea,...)areoeredforadditionalencryptionofthesecretdata.[9] signicantbitsofthecoecients.[8],[10] {HideandSeekbyColinMaroneyhidesdata(encryptedwithIDEA)inGIF {MandelstegbyHenryHasturcalculatesaGIFfractalfromale.Theresultingimagesareverysimilar.Dierencescanonlybeseenwhencomparing {PGE(PrettyGoodEnvelope)byRoche-CryptpacksdatainGIForJPEG les.theuseofanadditionalsecureencryptionmethodisrecommended.[12] les.[11]?thisworkissponsoredbythegermanfederalministryofeducation,science,researchandtechnology(bmbf). {StegobyJohnWalkertransformsanyletoanonsensicaltextbymeansof theircolorvalues.[13] afreechoosabledictionary.

2 2{TextobyKevinMahertransferslesintopoeticEnglishsentences(comparablewithstego,whichproducesnonsensicaltexts).[14] stereophony,colortv,videotext,traccontrolsystem(tcs),andradiodata system(rds)atfmradio. Datacamouageisalsousedforcompatibleenlargementofnorms,suchas 2VideoConferencingSystems Videoconferencesusecompressionalgorithmstoensureanacceptablequality Thispaperdoesnotdealwithwatermarkingsystemsatall[1]. evenonlowdataratesystemslikeisdn.usually,compressionmethodsarelossy whichmeansthatthereconstructedimageisnotidenticalwiththeoriginal. tempresentedinthispaperworksontheh.261standard.thisisthemost commonstandardforcompressioninvideoconferencesandisrecommendedby [3].InFig.4,wecanseethepointsforembeddingandextractingwithinthe thecomiteconsultatifinternationaltelegraphiqueettelephonique1(ccitt) Thevideoconferenceusedfortheimplementationofthesteganographicsys- H.261informationow. weneedacarrierthatallowsthepossibilityofunnoticeablemodications.signal possibilitiesfordataembeddingwehave.inchapter5weinvestigateatypical noiseandirrelevancearecommonexamplesforit.compressionmethodstryto removesignalnoiseandirrelevance.thebetterasignaliscompressed,theless Compressionanddataembeddinghavecontrarygoals.Fordataembedding signalpathfordataembedding. ThesteganographicalgorithmdescribedinChapter6embedsdataintransformedblocks.Thereforewedescribethetransformationprocessusedinthis videoconferencingsystem. image.thesubsequentquantizationremovestheinsignicantpartsoftheimage. forthehumaneye)frommarginalparts(invisibleforthehumaneye)ofthe Thetransformationemployedhastobeinvertibleinordertoregaintheessential Asuitabletransformationisameanstoseparateessentialinformation(visible 3DiscreteCosineTransformation (DCT).Ittransformesanimageof88pixelswith88=64brightnessvalues H.261,M-JPEG,MPEG,usethetwo-dimensionaldiscretecosinetransformation F(0;0):::F(7;7)into64values(so-calledDCTcoecients)f(0;0):::f(7;7) (seeequation1).thetransformationcausesnosignicantloss(roundingerrors Manydigitalvideoconferencingsystems,forinstancebasedonthestandards only).theretransformedimageresultsfrombacktransformationofthedct coecients(seeequation2).itcanalsobeunderstoodaslinearcombination Bk;n(seeFig.1). ofthedctcoecients(seeequation3andfig.2)withthedctbaseimages 1theformerCCITTisnowtheInternationalTelecommunicationUnion(ITU)

3 increasingverticalfrequency kn ????88 image increasinghorizontalfrequency Fig.1.DCTbaseimagesBk;n - =f(0;0) +f(0;1) +:::+f(7;7) Fig.2.Presentationofan88imageby64baseimageparts?

4 4 F(x;y)=7Xk=07Xn=0C(k) f(k;n)=c(k) 2C(n) 22C(n) 7Xx=07Xy=0F(x;y)cos(2x+1)k 2f(k;n)cos(2x+1)k (1) 16 cos(2y+1)n 16 (2) with F(x;y)=7Xk=07Xn=0f(k;n)Bk;n(x;y) C(z)=12p2forz=0 (3) 4AnExample 1else InordertoillustratetheDCT,wetransformadotoverthei.Fig.3showsit stronglyenlarged.aspresentedinfig.3b)thedotovertheihasagratingof 64brightnessvalues.Let'slookatitstransformedmatrix: 264?840? ? ?110?1?40 50?4?84?2?31?2?12? ? ?2?20?10 00?10?20 00?10?10 0?1 3 Thequantizationcausesanaccumulationofzerosbyapplyingastepfunction 75 (dividingandrounding)tothedctcoecients: 264?30?80?20 20?80?30? ? Theexampledemonstratesthatonly15coecientsdierentfromzeroareleftof 75 coded.[3]and[4]encloseadescriptionofrunandlevelandhumancoding.the theinitial64brightnessvalues.thecoecientsarearrangedinlinearordering andthenarerunandlevelcoded.thenewcreatedsequencewillbehuman- signicantfactisthattheyareloss-freecodings.fig.3c)showstheresultof thebacktransformation.

5 5 a) b) c) Fig.3.\Dotoverthei":a)original,b)rasteredc)afterdecompression Input Motion Estimationand Motion Compensation 6Output Compensation Discrete Cosine Transform Cosine Transform InverseDiscrete Quantization Quantization Inverse Coding Model EmbedExtract? Model Decoder Entropy Coder Entropy SS??SSS??HHHHH Decoder Fig.4.InformationowintheH.261Codec[4] Transmission 666 -

6 65SignalPath Apreciseknowledgeofthesignalpathisimportantinordertobeabletoestimate thesafetyofasteganographictechnique.fromthecameratothecodedsequence ofpictures,thesignalpathissubjecttolossesbytransformationsaswellasto inuencesandtodisturbances.inthefollowing,sometransformationpointson thepatharedesignated,andinparentheses,thealteredquantity. imageoftheoriginalispreprocessedopticallybythelensofthecamera.additionally,attitudeaperturesetting(depthoffocus),thefocuses(partofhigh videofrequencies),thefocallength(detail,videodepth)andthequalityofthe Theappearanceoftheoriginalisinuencedbythelightingconditions.The lens(distortion)contributeessentially.throughdispersion,thefocusesaredependentonthecoloroflight.thelightisusuallytransformedintoanelectrical signalinthecamerabyachargecoupleddevice(ccd).thetinyccdsare characterizedthroughtheirhighsensitivitytolight.thelightinfrontofthe about380000photosensitivepointsislteredbymanycolored,narrow,vertical stripes.eachthreeadjacentsensors,receivingrespectivelyred,green,andblue lteredlight,makeonepixel.thehorizontaldistanceofthethreesensorsisonly apartialpixeldistanceandthus,isneglected.accdhasatemporalinertness (thereaderpossiblyobservedthe\tracing"inthecaseofacamerapanshot) andisoperatedwithaspecicsamplingfrequency.afterwards,thealteredand runsthroughacircuitwhichcontainssemiconductors(temperaturedependence, rasteredimageisavailableintheformofanelectricalsignal.inthisform,it whichcontainstheimagewhichisnowevenmorecoarselyrasteredthaninthe transmittedtothedevicedriver.2adatastructureinthergbformatresults, camera.onthevideocardinthecomputerthepictureislocked,digitizedand noise),anditischangedintoantscorpalsignal.thesignalpathnowleads tothecomputeroveracoaxialcable(spectralphaseshift,attenuation)fromthe boundarytodeterminism(seefig.5).allfurtherprocessingstepsaredigitaland NTSCorPALsignal.Withthelaststeponthesignalpath,weexceededthe deterministic.withthetransformationandquantization,desirableroundingerrorsoccur.aloss-freeentropycodingcompressesthedatabetweenquantization andtransfer. -non-deterministic Processing - lossy Processing deterministic,embedding?-deterministic, Fig.5.Sectionsofthesignalpath loss-free Processing 2Oftenonlytheinterfaceofthedevicedriverisdocumented.Theprogrammeris unabletoseparatetheactivitiesofthevideocardandthoseofthedevicedriver.

7 meansthatresultingoutputsignalsdierfromeachotherwithhighprobability Thesignalpathcanbedividedintothreeparts.Non-deterministicprocessing7 inthecaseofidenticalinputsignals. noiselevelisdependentoftheconsideredbandwidth.theimageincorporated linespersecond(for25fullframes).thevoltagewithinalinevariesduringhorizontalcolorchange,withverticalfromlinetoline.therefore,uprightbrightness Anexampleisthenoiseofthesemiconductordevicesmentionedalready.The bythecameraisledinthepalsignallinebylinefromtoptobottom,15625 modicationsareputinthesignalatabandwidthamaximumof8khz(oneline iswhite,onelineisblack,alternating).upto800imageelementsperlinecan beplacedincomparisontothis,whichcorrespondstoabandwidthof6mhz. Thebandwidthforhorizontalvideofrequencies{andthereforethenoiselevel{ isupto800timesaslargeasthebandwidthforverticalvideofrequencies. whichisincludedintheccvssignal.lineinterlacingdividesafullframeinto twohalfframes.ifonenumbersthelinesofafullframefromtoptobottom,the rstframecontainsalloddlinesandthesecondframeallevenlines.therefore, twostraightadjacentlinesofthefullframeare1 Twoscanlinesarealignedbymeansofthehorizontalsynchronizationpulse other.sincethehalfframesarealsoregisteredattheframerate,thescreen contentcanalreadyhavechanged. mightbeplacedinformat720by540.threeadjacentsensorswillbesummarized Herefollowsanexamplewithnumbers.Theabout380000sensorsofaCCD 50frameseparatedfromeach asoneimagepoint(rgb),althoughthesensorshaveadistanceof1 spectrum. theimagecontentof110pixelsallowsconsiderablemodicationsofthefrequency length.thishorizontaldistanceis,referringtothesmallesth.261format(176by 144),14pixeldistance.Table1shows,thatalreadyahorizontaldisplacementof 720line generatedwithhorizontaldephasingandtransformed.thedephasingresults ifterm2y+1isreplacedby2y+1:2inequation1.throughconsideration offig.1,itisobviousthatahorizontal\dephasing"ofthebaseimagesbk;0 bringsnochange.underexclusionofthecoecientsf(0;0)(basebrightness) WedevelopedalittleprogramwhichcreatesTable1whenbaseimagesare andf(k;n)(appropriateforthebaseimage),thecoecientf(k0;n0)withthe truncatedvaluesduringcomputation.inthelineforb0;1,thecoecientwiththe inbracketsarenottobetracedbacktodisplacementandcanbeexplainedby strongestdieringamountprobablyisf(0;0).sincethiscoecientisexcluded largestabsolutevaluewasalwayssearchedinthetransformedmatrix.thevalues fromconsideration,thesmallernextappears.otherwise,forn>0,thefollowing patternisvalid: thatcoecientf(k0;n0)currentlychangesby1.sincethecoecientsareinteger, Columnfmincontainstheminimumamountforthecoecientf(k;n)from n0=n?1andf(k0;n0) k0=k f(k;n)n3% amodicationlessthan1isnotpossible.

8 8 Table1.RelativechangeofDCTcoecientsf(k0;n0)whilehorizontaldephasingof baseimagesbk;nby1 knk0n0f(k0;n0) 00{{(0.00)%(1) 10pixel f(k;n)fmin % 6.75% 9.19% knk0n0f(k0;n0) % 15.16% 18.90% (0.00)%(1) (0.27)%(369) % 3.61% % f(k;n)fmin % % % % (0.13)%(780) 12.28% 15.13% 18.98% 24.55% (0.14)%(736) 12.29% % % % % % % % % % (0.14)%(737) 12.18% 15.19% 18.82% 24.62% (0.26)%(390) 12.30% % % % % % % % % % 9.29% % 15.16% 18.98% 24.55% (0.14)%(736) 15.21% 18.82% % % 6.80% % 15.16% 18.98% 24.60% 9.28%

9 6Algorithm 9 time,wewanttoshowanunveriablemodication,unveriableinthesenseof Inthischapterwewilldiscusstheimage(dotoverthei)ofChapter4again.This changescauseanimperceptiblehorizontaldislocationoftheimage,thealgorithmdoesnotinuenceso-calledmotionvectors,atleastnotinadirectway.a motioncompensationstep(seefig.4)isnotnecessaryfortheimplementation ofthissteganographicvideoconferencingsystem.anattackercouldgetmore changethecarriersignal.theheartofasteganographicalgorithmisaprocess thecalculatedexampleinchapter5.whenembeddingsomething,wehaveto thatchangesthesignal.inourcaseitchangesdctcoecients.althoughthe onlyonceasakeyframe.hence,itistransmittedandusedforsteganography easierforsteganographers.anunchangingpictureinfrontofthecameracomes forembedding.however,deltaframecodingincaseofstillimagesmakeslife whichhecouldmatchagainsttheactualframe.thiswouldreducethespace preciseimagedatabyinterpolatingconsecutiveframesofanunchangingpicture steganographicprocessing. agecontainabigcoecient(seefminintable1)makingthemsuitablefor onlyonce.itisveryunlikelythatthedierence(ordelta)framesofastillim- a) b) c) d) e) f) (unchanged),c)afterdecompression(changedbyalgorithm),d)by115pixelshifted Fig.6.\Dotoverthei"andhorizontalshifting:a)original,b)afterdecompression original,e)movedimageaftergrating,f)movedimageafterdecompression result,theabsolutevalueofoneofthedctcoecientsaccordingtotable1is highercontrast. largeenoughtoallowamodication.(refertocoecientf(0;2)intheexample.) Thefollowingmatrixincludesthe64brightnessvaluesoftheoriginalimagewith Thecontrastoftheoriginalimagehasbeenincreased(seeFig.6a)).Asa

10 ThefollowingleftmatrixincludestheDCTcoecientsafterquantization. 75 Theboldhighlightedcoecientf(0;2)allowsamodicationof6.75%,which means160:0675=1:08.therightmatrixshowsthismodicationforf(0;1). 264?160?503020?60 40?160?60? ? ?1?160?60?20 64?160?503020?20? Afterrecovery,thefollowingmatrixesofbrightnessvaluesresult,presented 75 infig.6b)andc),too.themodicationleadstoaslightshiftingtotheright The\natural"shiftingasacomparison:Fig.6d)showstheoriginal,shifted 75 tionofthecoecient. Theshiftingoftheoriginalimagewouldhavecausedamoreintensivemodica- transformationandquantizationoftheleftfollowingmatrix(seealsofig.6e). by115pixel.thecoecientspresentedintherightfollowingmatrixresultfrom ?2?160?60?20 64?160?513020?20?

11 Finally,Fig.6f)showsthefollowing,recoveredmatrix Astheexampleshows,early,non-deterministiceectsatthebeginningof 75 7Implementation thesignalpathcanbereproducedinalaterpart(seefig.5). coecients.blocksare\suitable"iftheyincludeacoecientwhichislarger exploitstheeectdescribedinchapter6:thefrequencyspectrumchangesconsiderablyalreadyatminorchangingsofthephasingoftheimage. Theimplementedsteganographicfunction\Embedding"(seethemodelin[7]) thanitsminimumamountfmin(seetable1).inthesourcecode,allminimum amountsarerepresentedbydelta[].allotherblocksare\unsuitable"andwill betransmittedwithoutsteganographicmodication. Atrst,wedistinguishbetween\suitable"and\unsuitable"blocksofDCT */ /*Tobeclassifiedas"suitable",ablockmustcontainone unsignedintdelta[64]={ coefficientgreaterorequaltoitscorrespondentvalue inthefollowingmatrix. -1,-1,-1,-1,-1,-1,-1,-1, -1,28,28,28,29,28,28,28, 15,15,15,15,15,15,15,15, 11,11,11,11,11,11,11,11, 9,9,9,9,9,9,9,9, /*-1meansinfinity*/ }; 5,5,5,5,5,5,5,5 6,6,6,6,6,6,6,6, 7,7,7,7,7,7,7,7, /*stego_in(p)isthesteganographicfunction"embedding". */ Theparameterppointstoamatrixof64coefficients.

12 12 voidstego_in(int*p) { inti,most_suitable,sum_of_block,is_stego,*steg_ptr; is_stego=0; for(i=1;i<64;i++){/*skipdccoefficientp[0]*/ sum_of_block=0;/*forsum(mod2)*/ if(p[i]){ sum_of_block+=abs(p[i]); /*1means"suitable"block*/ if(abs(p[i])>=delta[i]){ /*coefficientlargeenough?*/ is_stego=1; /*considernon-zerocoefficients*/ /*moresuitable?thenkeepthepointer*/ /*"suitable"block*/ /*sumup*/ if(abs(p[i])-delta[i]>=most_suitable){ steg_ptr=&p[i-8];/*thisisf(k',n')*/ } } } } most_suitable=abs(p[i])-delta[i]; embedding,theblockwillbetransmittedunchanged.iftheparityisnotequal, 2sumofitscoecients(akindofparity).Iftheparityisequaltothenextbitfor...Forablockclassiedassuitable,itsfurthertreatmentdependsonthemodulo- ithastobechanged....if(is_stego) /*comparethemodulo-2sumwiththenextbittoembed*/ if((sum_of_block&1)!=get_bit_to_embed()){ if(*steg_ptr>0) /*decrementabs(*steg_ptr),thecoefficient*/ /*suitableblock?*/ elseif(*steg_ptr<0) (*steg_ptr)--; }Letf(k;n)bethecoecientofasuitableblockwhich,correspondingto } else/*0==>1*/ (*steg_ptr)++; *steg_ptr=1; Table1,allowsthemaximummodication.Inthiscase,theabsolutevalueof thecoecientf(k;n?1)willbedecreasedby1orifitiszero,setto1.this way,acoecientofablockischangedby1anditsparityips.

13 necessary.thewholescenarioisshowninfig.7.therecipientreceivessuitable Allchangedblocksaretransmittedaswellasthose,wherenochangewas 13 andunsuitableblockswhichareseparatedbythesamecriteriaasatthesender. Ithastoberemarkedthatchangedsuitableblocks(suitableblockswithipped parity)willalwaysstaysuitableblocksbecausethecoecientf(k;n)hasnot beenchanged,butfulllsthecriterion\suitable". imagereconstruction.thesteganographicalgorithmpresentedhereactslikea bitsofthesuitableblockssequentially.therecipientsystemusesallblocksfor quantizationwithahigherdivisor.itincreasesthecompressionratesothatthe introducederrorlooksnatural.alowerquantizershouldequalizetheeectof Therecipientcanextracttheembeddeddatathroughreadingouttheparity thealgorithm. voidstego_out(int*p)/*steganographicfunction"extraction"*/ { inti,sum_of_block,is_stego; sum_of_block=0; is_stego=0; for(i=1;i<64;i++){ if(p[i]){ sum_of_block+=abs(p[i]);/*sumup*/ /*1means"suitable"block*/ /*considernon-zerocoefficients*/ /*formod-2sum*/ /*coefficientlargeenough?*/ if(abs(p[i])>=delta[i]) }InFig.8weshowthesurfaceoftheapplication.ivsdisthedaemonwhich }if(is_stego)put_embedded_bit(sum_of_block&1); } is_stego=1; /*YES!suitableblock!*/ otherdaemons(single-ormulticast-addresses).afterestablishingthevideoconferenceconnection,theuseroftheconferencewithsteganographicenhancement receivesconferencecalls.permouseclickitispossibletoopenawindowtocall andonefordisplayingtheembeddedmessagesofthecommunicatingpartner. Theapplicationiscomparabletoacombinationofavideoconferenceandthe Unixstandardcommandtalk3. 8Conclusion Throughcompression,asusedwithvideoconferencing,theleastsignicantbits gainimportance,soeverybitofthecompressedsignalcontributesasignicant parttothepicture.thedetectionofarandomreplacementofthesebitsis hastwoadditionalwindows:onefortheinputoftext(thesecretmessagetohide) 3talkisacommunicationprogramforterminals.

14 14 Embedding digital brightness values Extracting compressed video signal (embedded data) Transformation Entropy Decoding unsuitable measure the distance to quantized value detect whether block is suitable secret message secret message calculate modulo-2 sum Quantization lossy unsuitable Inverse detect whether block is suitable Quantization calculate modulo-2 sum sum=bit compare sum with the bit to embed change one coefficient Fig.7.EmbeddingandExtracting by 1, if necessary Entropy Coding loss-free Inverse Transformation compressed video signal (embedded data) digital brightness values totheunchangedcarrier,whichshouldneverleavethesecuritydomain.we carrier,makingitimpossibletodetectthesechangeswithoutdirectcomparison possibleasshownforjstegin[8].however,itispossibletochangepartsofthe Transmission raiseanysuspicionforapossibleattacker.forthisreason,wescrutinisedthe picturereceptionclosely. analysisoftheinputdevicesshowsfreespacespermittingembeddeddata.if steganographictechniquessimulatepeculiaritiesofacamera,thechangesdonot usespecialfeaturesoftheinputdevices,suchasacameraorscanner.the ceptibly.adirectcomparisonwiththeoriginalallowsdierentiation,butthis stilldoesnotenabletheobservertodiscernbetweentheoriginalandthealtered signals.furthermore,thesendermerelytransmitsthechangedframes.inthis manner,asecretmessagecanbeembedded.theslighthorizontaldephasingis Ouralgorithmreproducestheseeectsarticially;thesignalchangesimper- unnoticeable. thisitisnecessarytoseparatethealgorithmfromthesecrets.thesimplest receiverneedthesamekeyandproceduretogeneratethesebitsandusethem possibilityisthegenerationofpseudorandombits.boththesenderandthe asapseudoone-timepad.becausethedistributionofthesebitshasthesame Algorithmsareonlytrustworthy,whentheyareopentopublicscrutiny.For randomuniformityasbitsextractedfromanyvideoconference,theattackercan notdiscernbetweenanormalvideoconferenceandoneinwhichsecretdatahas beenembeddedafterencryption. picture,becauseitisimpossibletoembeddatainblackframes. phoneconversation(upto8kbit/s).thisdependsuponthetextureofthe InanISDNvideoconferencingsystemitispossibletoembedaGSMtele-

15 15 References 1.IngemarJ.Cox,JoeKilian,TomLeighton,TatalShamoon,ASecure,RobustWa- 2.ElkeFranz,AnjaJerichow,SteenMoller,AndreasPtzmann,IngoStierand: termarkformultimedia,in:proceedings:informationhiding.workshop,cam- Fig.8.Userinterfaceoftheimplementedapplication 3.CCITTRecommendationH.261,VideoCodecForAudiovisualServicesAtp64 Cambridge,U.K.,May/June,1996,LNCS1174. ComputerBasedSteganography.In:Proceedings:InformationHiding.Workshop, 7.BirgitPtzmann:InformationHidingTerminology.In:Proceedings:Information 6.MaritKohntopp,SteganographiealsVerschlusselungstechnik,iX4/ AndyC.Hung,PVRG-P64Codec1.1,StanfordUniversity, NeilF.Johnson,Steganography,GeorgeMasonUniversity,1996 kbit/s,genf, RobertTinsley,SteganographyandJPEGCompression,FinalYearProjectReport,UniversityofWarwick,1996 Hiding.Workshop,Cambridge,U.K.,May/June,1996,LNCS.

16 169.ftp://idea.sec.dsi.unimi.it/pub/security/crypt/code/s-tools4.zip 10.ftp://ftp.funet.fi/pub/crypt/steganography/ ftp://ftp.funet.fi/pub/crypt/mirrors/idea.sec.dsi.unimi.it/ cypherpunks/steganography/mandelsteg1.0.tar.gz