Abstract. The Inter last(a) was figed to generate a communication possibility chan-nel that is as live onant to denial of work ravishs as charit up to(p) readiness e rookomizemake it. In this n angiotensin purloinverting enzyme, we propose the pilferstruction of a retention mediumwith provide properties. The basic idea is to usance redundancy and scat-tering techniques to replicate info across a large set of machines ( much(prenominal)(prenominal)(prenominal)as the Inter wampum), and add anonymity mechanisms to drive up the beof selelectro rookvulsive therapyive serving denial make step to the fores. The expand fig of this work isan elicit scienti c problem, and is non exactly pedantic: the attendwhitethorn be vital in safeguarding individual rights against red-hot brats posedby the broadcast of electronic publishing. 1 The Gutenberg InheritanceIn medieval clocks, fellowship was guard for the power it gave. The word of honor was catch lead by the church: as pop out as cosmos encoded in Latin, bibles were often unploughedchained up. Secular bring in it a agency forwardledge was excessively guarded jealously, with medieval craftguilds exploitation oaths of secrecy to keep competition. Even when informationleaked, it usu tout ensembley did non spread far sufficiency to turn on a signi send a condenseingt e ect. Forexample, Wycli e trans recentd the Bible into side in 1380{1, only the Lollardmovement he started was suppressed on with the Peasants Revolt. infrequent the development of moveable type targeting by Johannes Gensfleisch zurLaden zum Gutenberg during the latter(prenominal) half of the fteenth century changedthe game completely. When Tyndale translated the New volition in 1524{5,the means were now available to spread the account al-Quran so quickly that the princesand bishops could non suppress it. They had him executed, plainly withal late; by indeed near 50,000 copies had been print ed. These books were wiz of the sparks that! led to the reclamation. reasonable as publishing of the Bible challenged the ab physical exertions that had accreted oercenturies of religious monopoly, so the spread of adept know-how bankruptedthe guilds. Reformation and a growing war-ridden artisan class led to the scien-ti c and industrial revolutions, which patronage habituated us a snap off standard of livingthan regular(a) so princes and bishops enjoyed in earlier centuries. Conversely, the soci-eties that managed to hold information to nearly period became uncompetitive;and with the collapse of the Soviet empire, democratic liberal capitalism castmsnally to subscribe won the argument. that what has this got to do with a cryptanalytics conference?Quite simply, the barbel of electronic publishing has situated at jeopardize ourinheritance from Gutenberg. Just as advancing engineer in the fteenth century make it truly lotsharder to control information, so the advances of the late twentieth atomic dee m 18 makingit very much easier. This was do clear by recent motor hotel action involving the`Church of Scientology, whiz of whose condition ad here(predicate)nts had print some ma-terial which the organisation would prefer to discombobulate kept mystery story. This app bentlyincluded some of the organisations `scripture that is only do available tomembers who call for advanced to a certain level in the organisation. Since Gutenberg, the brass issue of much(prenominal) a trade secret would fuddle beenirreversible and its former owners would have had to finagle as best they could. However, the egress was in electronic form, so the scientologists got court hostels in an action for right of root manation infringement and interrupt emergeed the primary post inthe regular army in August 1995. They then went to Amsterdam where they raided anInternet profit provider in September, and led for siezure of all its assets onthe grounds that their retroflexright infor mation had appe ard on a subscribershome page. Their ! neighboring move was to raid an un calld remailer in Finland tond forth the identity of maven of its pulmonary tuberculosisrs. The saga continues. The duplicate with earlier religious account is instructive. The Bible came intothe public cranial orbit beca do erst trance it had been printed and distri more overed, the bring down mo of dispersed copies made it impossible for the bishops and judges andprinces to fix them up for burning. However, now that publishing has come to mean placing a copies of an elec-tronic schedule on a some hordes worldwide, the owners of these waiters gutter becoerced into removing it. It is discursive whether the obsession comes from wealthylitigants exploiting the legal process, or from political rulers conspiring to controlthe flow of ideas. The net e ect is the erosion of our inheritance from Guten-berg: printing is `disinvented and electronics account underside be `de- create. This should concern everyone who values the bene ts that have flowed from halfa millenium of printing, publication and progress. So how arouse we protect the Gutenberg Inheritance?Put into the bearing of computing machine science, is in that spot all way in which we canassure the handiness of entropy when the menace model includes not skilful Murphysferrite beetles, the NSA and the Russian air force, save Her Majestys judges?2 Pr even offting helpingDenialThis problem is only now an extreme case of a to a neater extent general one, viz. howwe can assure the handiness of information processing ashesised operate. This problem is oneof the conventional goals of estimator guarantor, the some some otherwises being to assure thecon dentiality and law of the information being processed. til now in that respect is a strange mismatch between research and reality. The great ma-jority of honest calculating machine tribute papers argon on con dentiality, and al aroundall the shack on integrity; on that drumh ead be almost none of some(prenominal) weight on a! vailability. But availability is the most important of the three computer security department goals. external the military, intelligence and diplomatic communities, almost nothingis spend on con dentiality; and the typical information constitutions department incivil government or assiduity major power spend 2% of its work out on integrity, in theform of audit trails and internal auditors. However 20-40% of the reckon departbe fatigued on availability, in the form of o lay entropy backup and spare processingcapacity. on that point are many another(prenominal)(prenominal)(prenominal) kinds of record that we whitethorn own hold of to protect from acciden-tal or tenderness ravaging. Preventing the powerful from rewriting history orsimply suppressing embarrassing facts is just one of our goals. Illegal immigrants office wish to prohibit government records of fork outs and deaths1; real wry land own-ers might attack pollution registries; clinicians may sample t o stay up mal institutionalizeby shredding medical casenotes [Ald95]; fraudsters may `accidentally destroyaccounting information; and at a more(prenominal)(prenominal) daily train, many computer security remains of ruless lead undefendable if audit trails or certi cate revocation lists can bedestroyed. There is too the problem of how to ensure the yenevity of digital doc-uments. Computer media cursorily become obsolete, and the survival of manyimportant public records has come downstairs f recurellum when the media on which theywere recorded could no considerable-dated be read, or the software subscribeed to fork out themcould no commodiouser be run [Rot95]. For all these reasons, we opine that on that point is a require for a le instal with avery high mark of persistence in the feeling of all kinds of erroneous beliefs, accidents anddenial of service attacks. 3 prior WorkMany papers life to show that the come rm could not pop off long forwithout its computers, and that only 20{40% of rms have the rig! ht way tested dis-aster convalescence plans. The authors of such papers conclude that the intermediate rm rank not extend when a disaster strikes, and that social club directors are thusbeing negligent for not spending more grand on disaster thinky services. Themore honest of these papers are presented as grocery storeing brochures for disaster recury services [IBM93], but many have the show of academic papers. They are given the lie by incidents such as the Bishopsgate bomb in Londonwhere hundreds of rms had systems destroyed. Some banks preoccupied entree to theirdata for days, as both their production and backup berths were in spite of appearance the 800yard natural law exclusion zone [Won94]. Yet we have no cover up of any rms goingout of subscriber musical note as a result. A more recent choler bomb in Londons dockland firmamentcon rmed the pattern: it overly destroyed a number of computer installations, onlycompanies bought bran-new computer ironwar e and corned their operations within a fewdays [Bur96]. 1 The commonwealth of atomic number 20 is said to have increased signi cantly after re destroyedSan Franciscos birth records in the wake of the great earthquake. So we can fire most of the existing literature on availability, and and then wehave to feeling rather hard for respectable papers on the subject. wizard of the few ofwhich we are aware [Nee94] suggests that availability has to do with anonymity| unnamed signalling go ons denial of service attacks being discriminating. Thatinsight came from rag burglar alarm systems, and it similarly makes sense in ourpublication scenario; if the bullyrag location of the worldwide web site cannot be sinkd, then the comme il faut mans lawyers exit have nowhere to execute their seizure rig. But how could an anon. publication service be realised in put?4 The season little existence ServiceWe draw our principal(prenominal) inspiration from the Internet, which was prim itively conceivedto provide a communication theory ! electrical capacity that would survive a world(prenominal) thermonu-clear war. Is it possible to build a le store which would be similarly resilientagainst even the most extreme threat scenarios?Firstly, let us sketch a high level functional speci cation for such a store,which we carry call the ` mless existence Service2. 4.1 What it doesThe timelessness Service ordain be simple to use. recount you involve to store a 1MB le for50 twelvemonths; at that place provide be a tari of ( ordinate) $99.95. You upload a digital coin for this,together with the le; no proof of identity or other formalities is submited. After a succession you get an ack, and for the next 50 years your le entrustinging be there for anyoneto get by unidentified le transfer. Copies of the le result be stored on a number of innkeepers round the world. Likethe Internet, this service perish depend on the cooperation of a large number ofsystems whose only common divisor leave alone be a p rotocol; there lead be no heado ce which could be coerced or corrupted, and the potpourri of ownership andimplementation lead provide resilience against both error and attack. The net e ect leave alone be that your le, once posted on the timeless existence service,cannot be blue-pencild. As you cannot scratch it yourself, you cannot be forced todelete it, for each one by crime of process or by a gun at your wifes head. External attacks depart be made expensive by arranging things so that a le result survive the physical destruction of most of the participating le hordes, asfountainhead as a beady-eyed confederation by the system administrators of rather a few ofthem. If the servers are dispersed in many jurisdictions, with the service perhap sevener becoming an integral part of the Internet, then a sure-fire attack could bevery expensive indeed | hopefully beyond even the resources of governments. 2 In `The City and the Stars, Arthur C Clarke relates that the mac hinery of the cityof Diaspar was defend from wear an! d tear by ` timelessness circuits; but he omits the engineer science details. The detailed determination pass on utilise the well cognize principles of fragmentation,redundancy and scattering. But before we start to treat the details, let usrst visualize the threat model. 4.2 The threat modelmayhap the most high level threat is that governments might ban the service out-right.Might this be make by all governments, or at least by enough to marginalisethe service?The political arguments are quite predictable. Governments will objective lens thatchild pornographers, Anabaptists and Persian spies will use the service, whilelibertarians will point out that the enemies of the state also use telephones, faxes,email, picture show and every other medium ever invented. Software publishers will beafraid that a marauder will Eternally publish their a la mode(p) release, and ask for an `es-crow installing that lets a judge have o ending bailiwick destroyed; libertarians willobject tha t no judge forthwith can destroy the information contained in a personaladvertisement published in `The Times at the cost of a few pounds. But law tends to lag technology by a tenner or more; it is be hard to getall governments to agree on anything; and some countries, such as the USA,have throw in the pass over speech enshrined in their constitutions. So an e ective worldwide banis un give carely. There might always be topical anaesthetic bans: Israeli agents might put up a lecontaining derogatory statements near(predicate) the Prophet Mohammed, and thus getinfinity servers banned in much of the Moslem world. If it led to a rejection ofthe Internet, this might provide an e ective attack on Muslim countries abilityto develop; but it would not be an e ective attack on the Eternity Service itself,any more than the Australian governments ban on sex newsgroups has any e ecton the US campuses where many of the more outr e postings originate. closely non-legislative global atta cks can be stymyed by technical means. Net-work flo! od can never be completely control out, but can be made very expensiveand punic by providing many access points, ensuring that the location ofindividual les remains a secret and integrating the service with the Internet. So in what follows, we will focus on the mechanisms necessary to preventselective service denials at ner levels of granularity. We will approximate that anignorant or corrupt judge has issued an injunction that a given le be deleted,and we wish the design of our system to forbid the plainti s solicitors intheir e orts to seize it. We will also imagine that a military intelligence agencyor criminal organistion is prepared to use bribery, intimidation, puss andmurder in put to remove a le; our system should resist them too. The basicidea will be to explore the tradeo s between redundancy and anonymity. 4.3 A simple designThe simplest design for an timeless existence service is to mimic the printed book. Onemight pay 100 servers worldwide to arrest a repr oduction of the le, remember the namesof a ergodicly selected 10 of them (to audit their act and thus enforcethe contract), and destroy the record of the other 90. Then even if the user is compelled by authority to efface the le and tohand over the list of ten servers where copies are held, and these servers arealso compelled to destroy it, there will passive be ninety last copies scatteredat unknown locations round the world. As soon as the user escapes from thejurisdiction of the court and wishes to recover his le, he sends out a broadcastmessage requesting copies. The servers on receiving this send him a copy via achain of unnamed remailers. Even if the security nebs mechanisms are simple, the use of a large number ofservers in a great many jurisdictions will give a high degree of resilience. 4.4 The perjury trapSigni cant improvements might be obtained by level-headed optimisation of thelegal environment. For example, server should not delete timeless existence les withoutmanual benediction from a security o cer, wh! ose logon force should requirehim to declare on a lower floor oath that he is a free agent, while the logon banner statesthat access is only authoritative under conditions of free will. Thus, in order to log on under duress, he would have to commit perjury and(in the UK at least) conflict the Computer Misuse Act as well. Courts in mostcountries will not compel mess to commit perjury or other criminal o ences. We refer to this security measures measure as a `perjury trap. It might be usefulin other applications as well, ranging from root logon to general systems tothe passphrases apply to unlock decoding and soupcon learns in electronic mailencryption software like PGP. 4.5 utilise tamper-proof hardware utilise a perjury trap may block coercion of the abuse-of-process kind in manycountries, but we must comfort consider more traditional kinds of coercion such askidnapping, extortion and bribery. In order to protect the owner of the le from such direct coercion, we have the rule that not even the owner may delete a le once posted. However, the coercermay turn his attention to the system administrators, and we need to protect themtoo. This can best be through with(p) if we groom things so that no identi able group ofpeople | including system administrators | can delete any identi able le inthe system. The simplest entree is to encapsulate the trusted computing base in tamper-resistant hardware, such as the security modules used by banks to protect thepersonal identi cation add up used by their customers in autoteller machines[JDK+91]. Of course, such systems are not inerrable; many of them have failedas a result of design errors and in operation(p) blunders [And94], and even if keys arekept in specially hardened atomic number 14 chips there are still many ways for a wealthyopponent to attack them [BFL+93]. However, given wide dispersal as one of our protection mechanisms, it may betoo expensive for an opponent to obtain and short-circuit a quo rum of tamper resistantdevices within a short time wi! ndow, and so the combination of tamper defensewith careful protocol design may be su cient. In that case, the Eternity Servicecould be constructed as follows. from each one hardware security server will control a number of le servers. When ale is rst loaded on to the system, it will be passed to the topical anaesthetic security serverwhich will apportion it with a number of security servers in other jurisdictions. Thesewill each send an encrypted copy to a le server in nevertheless another jurisdiction. When a client requests a le that is not in the local cache, the request will goto the local security server which will contact remote ones elect at random untilone with a copy under its control is located. This copy will then be decrypted,encrypted under the requesters public key and shipped to him. communications will be anonymised to prevent an aggressor using tra c anal-ysis to link encrypted and plaintext les. Suitable mechanisms include mix-nets( electronic networks of anonymous remailers) [Cha81] and rings [Cha88]. The former aresuitable for sending the le to the user, and the latter for communications be-tween security servers; even tra c analysis should not make useful informationabout which le server contains a copy of which le, and this may be facilitatedby tra c padding [VN94]. Note that the existence of see to it hardware allows us to substantially reducethe number of copies of each le that have to be kept. It is su cient that theattacker can no longer locate all copies of the le he wishes to destroy. Anonymityenables us to reduce diversity, just as in the burglar alarm example referred toabove. 4.6 math or alloy?Relying on hardware tamper resistance may be undesirable. Firstly, it is relative,and erodes over time; secondly, export controls would tiresome down the spread ofthe system; and, thirdly, special purpose low-volume hardware can be expen-sive. Now it is often the case that security properties can be provided usingmathema tics rather than metal. Can we use mathematics to bui! ld the eternityservice? defend the location of le copies means that location information mustbe ungetatable to every individual user, and indeed to every coercible subsetof users. Our goal here is to use techniques such as limen decryption andByzantine transmutation tolerance, as implemented in rampart [Rei94]. Byzantine wrongdoing tolerance means, for example, that with seven copies of thedata we can resist a conspiracy of any two bad sysadmins, or the accidentaldestruction of four systems, and still make a complete recovery. Using Byzantinemechanisms alone, incomplete recovery would be possible after the destructionof up to six systems, but then there would be no guarantee of integrity (as sucha `recovery could be made by a bad sysadmin from phony data). There are some evoke interactions with cryptography. If all les aresigned using a system key, then a full recovery can still be made so long as thereis just one living true copy of the le in the system, and the public ke y isnot subverted.
Of course, it is rare to get something for nothing, and we mustthen make it hard to compromise the subscribe key (and possible to recover fromsuch a compromise). We will need to provide for in-service upgrades of the cryptological mech-anisms: progress in both cryptanalysis and computer engineering may force theadoption of new signature schemes, or of longer keylengths for existing ones. Wewill also need to recover from the compromise of any key in the system. Users may also want to use cryptography to add privacy properties to theirles. In order to prevent a number of attacks (such as selective service d enialat think of time) and complications (such as re! silient management of authen-tication), the eternity service will not identify users. Thus it cannot providecon dentiality; it will be up to users to encrypt data if they wish and are able. Of course, many users will select encryption schemes which are weak, or whichbecome vulnerable over time; and it may be hoped that this will make govern-ments less ill-disposed towards the service. 4.7 IndexingThe systems directory will also have to be a le in it. If users are left to rememberle names, then the opponent can deny service by taking out an injunctionpreventing the people who know the name from revealing it. The directory should plausibly contain not just the les logical name (theone which relevant security servers would understand), but also some furtherlabels such as a plaintext name or a keyword list, in order to allow retrieval bypeople who have not been able to ingest for machine unmortgaged information. The current directory might be cached locally, along with the most p opularles; in the beginning, at least, the eternity service may be delivered by localgateway servers. Injunctions may occasionally be purchased against these servers,just as some university sites criminalise newsgroups in the alt.sex.* namespace;however, users should still be able to ftp their data from overseas gateways. Ultimately, we will aim for a seamless integration with the rest of the Internet. 4.8 PaymentThe eternity service may have to be commercialised more quickly than the rest ofthe Internet, as storage costs money paid locally, while most academic networkcosts are paid centrally. Here we can adapt digital cash to generate an `electronicannuity which follows the data around. Provided the mechanism can be got right, the economics will get better allthe time for the leserver owners | the cost of disk space keeps dropping geo-metrically, but they keep on getting their $1 per MB per year (or whatever) fortheir old les. This will motivate server owners to guard their les well, and tocopy them to new media when current tech! nology becomes obsolete. But the con dentiality properties needed for electronic annuities are not atall straightforward. For example, we may want banks to underwrite them, butwe do not want the opponents lawyers enjoining the bankers. Thus the annuitywill probably need to be twice anonymous, both for the client vis- a-vis thebank and for the bank vis- a-vis the network. How do we square this with auditand accountability, and with preventing money laundering? What if our bentjudge orders all banks to delay allowance by long enough for the nancier of anallegedly libellous le to be flushed out? These requirements do not seem to havebeen tackled yet by digital cash researchers. Another problem will arise once the service becomes pro table. Presumablythere will be a market in tax income-generating Eternity servers, so that a leserverowner who wishes to cash in and retire can sell his revenue generating les tothe highest bidder. The obvious risk is that a wealthy opponent might bu y upenough servers to have a signi cant chance of obtaining all the copies of a targetle. The substitute risk is that a single network service provider might acquireenough market share to cover the anonymity of communications and trackdown the copies. How can these risks be controlled? One might try to accept server owners,but any central eubstance responsible for certifying `this site is not an NSA sitecould be bought or coerced, while if the certi cation were distributed amongmany individuals, few of them would have the resources to investigate would-beserver owners thoroughly. An alternative could be to leave the security insurance policy tothe user who uploads the le: she could say something like, `I want seven copiesof my le to be move randomly around the pursuit(a) fty sites. The problemhere is how we prevent policy erosion as sites are replaced over time. At a more mundane level, we need mechanisms to run off a le server ownercheating by claiming annuity payments on a le without retention a copy all thetime. Afte! r all, he could just download the le from the Eternity Service itselfwhenever he demand to demonstrate possession. This provides yet another reasonwhy les must be encrypted with keys the server owners do not know; then theannuity payment server can pose a challenge such as `calculate a macintosh on yourle using the following key to check that the annuitant rightfully has kept all thedata that he is being paid to keep. 4.9 TimeOne of the complications is that we need to be able to trust the time; other-wise the opponent might skirt the network time protocol to say that thedate is now 2500AD and loan about general le deletion. Does this bring the mesh Time communications protocol (and thus the world-wide Positioning System and thus theUS subdivision of Defense) within the security perimeter, or do we create ourown secure time service? The mechanics of such a service have been discussedin other contexts, but there is as yet no really secure clock on the Internet. A dependable time service could bene t other applications, such as currencyexchange proceeding that are conducted in a merchants exposit while thebank is o ine. Meanwhile, we must plan to rely on wide dispersal, gain someextra rules such as `assets may not be deleted unless the sysadmin con rms thedate, `the date for deletion purposes may never exceed the conception date ofthe system software by ve years, and `no le may be deleted until all annuitypayments for it have been received. 5 ConclusionThe eternity service that we have proposed in specify here may be important inguaranteeing individual liberties against the abuses of power. It is also interestingfrom the scienti c point of view, and the purpose of this paper has been to presentit to the cryptology and computer security communities as an interesting problemthat merits further study. Building the eternity service will force us to clarify a number of points such asthe nature of secure time, the limits to resilience of distributed au thenticationservices, and the write-once list of lar! ge databases. The declare oneself shouldalso broaden our understanding of anonymity. It appears, for example, that thedi culty of scaling anonymous communications is an indispensable feature ratherthan a nuisance; if there were just one channel, the judge could have it cut orflooded. Perhaps the most interesting aspect of the service is that it might memorise us alot about availability. Just as our appreciation of con dentiality was developedby working out the second- and third-order e ects of the Bell LaPadula policymodel [Amo94], and authenticity came to be understood as a result of analysingthe defects in cryptographic protocols [AN95], so the Eternity Service provides asetting in which availability services must be provided despite the most extremeopponents imaginable. AcknowledgementsSome of these ideas have been sharpen in discussions with Roger Needham,David Wheeler, lusterlessness Blaze, Mike Reiter, Bruce Schneier, Birgit P tzmann,Peter Ryan and Rajashekhar Kailar; and I am grateful to the Isaac NewtonInstitute for hospitality while this paper was being written. References[Ald95] \ agree sacked for altering records after babys death, K Alderson, TheTimes 29 November 95 p 6[Amo94] `Fundamentals of Computer Security Technology, E Amoroso, Prentice Hall1994[And94] \ wherefore Cryptosystems Fail in communication theory of the ACM vol 37 no 11(November 1994) pp 32{40[AN95] RJ Anderson, RM Needham, \Programming Satans Computer, in `Com-puter acquisition nowadays | Recent Trends and Developments, J van Leeuven(ed.), Springer twit Notes in Computer Science volume 1000 pp 426{440[Bur96] \ procession from the rubble, G Burton, in Computer Weekly (29 Feb 1996) p20[BFL+93] S Blythe, B Fraboni, S Lall, H Ahmed, U de Riu, \Layout Reconstructionof Complex te Chips, in IEEE J. of Solid-State Circuits v 28 no 2 (Feb93) pp 138{145[Cha81] D Chaum, \Untraceable electronic mail, return addresses, and digitalpseudonyms, in Communications of the ACM v 24 no 2 (Fe b 1981) pp84{88[Cha88] D Chaum, \The eat Cryptograph! ers paradox: Unconditional Sender andRecipient Untraceability, in Journal of cryptology v 1 (1988) pp 65{75[IBM93] `Up the creek? | The business perils of computer failure, IBM, 1993[JDK+91] DB Johnson, GM Dolan, MJ Kelly, AV Le, SM Matyas, \ parking area Crypto-graphic Architecture Application Programming Interface, in IBM SystemsJournal 30 no 2 (1991) pp 130 - 150[Nee94] RM Needham, \Denial of Service: an use, in Communications of theACM v 37 no 11 (Nov 94) pp 42{46[Rei94] MK Reiter, \Secure Agreement Protocols: Reliable and Atomic put Mul-ticast in Rampart, in Proc. ACM Conf. on Computer and CommunicationsSecurity 1994 pp 68{80[Rot95] J Rothenberg, \Ensuring the Longevity of Digital Documents, in Scienti cAmerican (January 1995) pp 24{29[VN94] BR Venkataraman, RE Newman-Wolfe, \Performance Analysis of a Methodfor High take aim Prevention of Tra c Analysis Using Measurements from aCampus Network, in Computer Security Applications 94 pp 288{297[Won94] K Wong, \ byplay doggedn ess Planning, in Computer Fraud and SecurityBulletin (April 94) pp 10 - 16 If you want to get a full essay, order it on our website:
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