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Answer Machine Hacking (Posted on 2004-05-24) Difficulty: 4 of 5
Consider an answering machine with remote inquiry facility, where you can call the answering machine and enter a 4 digit passcode into your telephone keypad, so you can listen to the messages from anywhere you like. Many of these machines will let you in if you enter the correct consecutive sequence of digits, regardless of what preceded that sequence.

Example: Passcode is 1234.

if you feed the machine 1234, you're in.
if you feed the machine 01234, you're in.
if you feed the machine 0121234, you're in.
if you feed the machine 94129838701234, you're in.

To brute-force hack the machine, you could try all numbers from 0000 to 9999, sending 40000 sounds across the wire. But since you are a smart hacker, you see that there's room for optimization. What is the shortest series of digits you have to send to the answering machine in order to break the code in any case?

No Solution Yet Submitted by SilverKnight    
Rating: 4.4286 (7 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution re: de Bruijn sequence | Comment 9 of 11 |
(In reply to de Bruijn sequence by Kenny)

Apparently the forum software didn't like the long string:

0000100020003000400050006000700080009001
1001200130014001500160017001800190021002
2002300240025002600270028002900310032003
3003400350036003700380039004100420043004
4004500460047004800490051005200530054005
5005600570058005900610062006300640065006
6006700680069007100720073007400750076007
7007800790081008200830084008500860087008
8008900910092009300940095009600970098009
9010102010301040105010601070108010901110
1120113011401150116011701180119012101220
1230124012501260127012801290131013201330
1340135013601370138013901410142014301440
1450146014701480149015101520153015401550
1560157015801590161016201630164016501660
1670168016901710172017301740175017601770
1780179018101820183018401850186018701880
1890191019201930194019501960197019801990
2020302040205020602070208020902110212021
3021402150216021702180219022102220223022
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5023602370238023902410242024302440245024
6024702480249025102520253025402550256025
7025802590261026202630264026502660267026
8026902710272027302740275027602770278027
9028102820283028402850286028702880289029
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3050306030703080309031103120313031403150
3160317031803190321032203230324032503260
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3720373037403750376037703780379038103820
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3045404550456045704580459046104620463046
4046504660467046804690471047204730474047
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7049804990505060507050805090511051205130
5140515051605170518051905210522052305240
5250526052705280529053105320533053405350
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5470548054905510552055305540555055605570
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5920593059405950596059705980599060607060
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4066506660667066806690671067206730674067
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7720773077407750776077707780779078107820
7830784078507860787078807890791079207930
7940795079607970798079908080908110812081
3081408150816081708180819082108220823082
4082508260827082808290831083208330834083
5083608370838083908410842084308440845084
6084708480849085108520853085408550856085
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8086908710872087308740875087608770878087
9088108820883088408850886088708880889089
1089208930894089508960897089808990909110
9120913091409150916091709180919092109220
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9450946094709480949095109520953095409550
9560957095809590961096209630964096509660
9670968096909710972097309740975097609770
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9890991099209930994099509960997099809991
1112111311141115111611171118111911221123
1124112511261127112811291132113311341135
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1148114911521153115411551156115711581159
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1198119912121312141215121612171218121912
2212231224122512261227122812291232123312
3412351236123712381239124212431244124512
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5812591262126312641265126612671268126912
7212731274127512761277127812791282128312
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9612971298129913131413151316131713181319
1322132313241325132613271328132913321333
1334133513361337133813391342134313441345
1346134713481349135213531354135513561357
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2214231424142514261427142814291432143314
3414351436143714381439144214431444144514
4614471448144914521453145414551456145714
5814591462146314641465146614671468146914
7214731474147514761477147814791482148314
8414851486148714881489149214931494149514
9614971498149915151615171518151915221523
1524152515261527152815291532153315341535
1536153715381539154215431544154515461547
1548154915521553155415551556155715581559
1562156315641565156615671568156915721573
1574157515761577157815791582158315841585
1586158715881589159215931594159515961597
1598159916161716181619162216231624162516
2616271628162916321633163416351636163716
3816391642164316441645164616471648164916
5216531654165516561657165816591662166316
6416651666166716681669167216731674167516
7616771678167916821683168416851686168716
8816891692169316941695169616971698169917
1718171917221723172417251726172717281729
1732173317341735173617371738173917421743
1744174517461747174817491752175317541755
1756175717581759176217631764176517661767
1768176917721773177417751776177717781779
1782178317841785178617871788178917921793
1794179517961797179817991818191822182318
2418251826182718281829183218331834183518
3618371838183918421843184418451846184718
4818491852185318541855185618571858185918
6218631864186518661867186818691872187318
7418751876187718781879188218831884188518
8618871888188918921893189418951896189718
9818991919221923192419251926192719281929
1932193319341935193619371938193919421943
1944194519461947194819491952195319541955
1956195719581959196219631964196519661967
1968196919721973197419751976197719781979
1982198319841985198619871988198919921993
1994199519961997199819992222322242225222
6222722282229223322342235223622372238223
9224322442245224622472248224922532254225
5225622572258225922632264226522662267226
8226922732274227522762277227822792283228
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7229822992323242325232623272328232923332
3342335233623372338233923432344234523462
3472348234923532354235523562357235823592
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3762377237823792383238423852386238723882
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6242724282429243324342435243624372438243
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4248524862487248824892493249424952496249
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5362537253825392543254425452546254725482
5492553255425552556255725582559256325642
5652566256725682569257325742575257625772
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5942595259625972598259926262726282629263
3263426352636263726382639264326442645264
6264726482649265326542655265626572658265
9266326642665266626672668266926732674267
5267626772678267926832684268526862687268
8268926932694269526962697269826992727282
7292733273427352736273727382739274327442
7452746274727482749275327542755275627572
7582759276327642765276627672768276927732
7742775277627772778277927832784278527862
7872788278927932794279527962797279827992
8282928332834283528362837283828392843284
4284528462847284828492853285428552856285
7285828592863286428652866286728682869287
3287428752876287728782879288328842885288
6288728882889289328942895289628972898289
9292933293429352936293729382939294329442
9452946294729482949295329542955295629572
9582959296329642965296629672968296929732
9742975297629772978297929832984298529862
9872988298929932994299529962997299829993
3334333533363337333833393344334533463347
3348334933543355335633573358335933643365
3366336733683369337433753376337733783379
3384338533863387338833893394339533963397
3398339934343534363437343834393444344534
4634473448344934543455345634573458345934
6434653466346734683469347434753476347734
7834793484348534863487348834893494349534
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3546354735483549355435553556355735583559
3564356535663567356835693574357535763577
3578357935843585358635873588358935943595
3596359735983599363637363836393644364536
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3748374937543755375637573758375937643765
3766376737683769377437753776377737783779
3784378537863787378837893794379537963797
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5438553856385738583859386438653866386738
6838693874387538763877387838793884388538
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3944394539463947394839493954395539563957
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5568556955765577557855795586558755885589
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6856695676567756785679568656875688568956
9656975698569957575857595766576757685769
5776577757785779578657875788578957965797
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7858795886588758885889589658975898589959
5966596759685969597659775978597959865987
5988598959965997599859996666766686669667
7667866796687668866896697669866996767686
7696777677867796787678867896797679867996
8686968776878687968876888688968976898689
9696977697869796987698869896997699869997
7778777977887789779877997878797888788978
9878997979887989799879998888988998989999


  Posted by Kenny on 2006-02-25 00:06:49
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