Digitial Signature Paradox White Paper

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ProoSpace White Paper

The Digital Signature Paradox

Je Stapleton, Paul Doyle, Steven Teppler, Esq.

Abstract Paradox is a term oten associated with Hollywoods anciul time travel;however in the real world a time paradox does, in act, exist. The system clock is theimmediate source o time or any computer, and is the sole source or a time stampdetermining when a document was created, modied and printed; or more interestinglywhen a digital signature was generated. Fraud has already been perpetrated by turningback system clocks, leading to the alsication o inormation or which individuals havebeen disbarred or incarcerated. The application o a digital signature would not haveresolved these issues; that is because digital signatures are time-insensitive. However,an independent clock source providing a trusted time stamp would and can circumventindividuals taking such illegal liberties. This paper posits that data integrity needs to beredened within the context o a time-sensitive mechanism.

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This document is an updated version o a paper titled The Digital Signature Paradox that was originallypublished in the Proceedings o the 2005 IEEE Workshop on Inormation Assurance and Security.It has been made available in this ormat with the permission o the authors.


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The Digital Signature Paradox Revised July 007

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Table o Contents

I. Introduction 3

A. Asymmetric Cryptography 3

B. Digital Signatures 3

C. Time Stamping 5

II. Paradox 7

A. Time-Insensitive Digital Signatures 7

B. Trusted Time Stamp 8

C. X9.95 Standard 9

III. Consequences o Time-Based Data Manipulation 10

A. Enron (CFO) 10

B. Rite-Aid (CEO, CFO) 10

C. NextCard (Auditors) 10

D. Autotote (Programmers) 10

E. Sirena Corp (CEO) 10

F. Parmalat (CEO, CFO, and Family) 10G. Adelphia Communications (executives) 11

IV. Conclusion 11

V. Reerences 1


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3The Digital Signature Paradox Revised July 007

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I. IntroductionA. Asymmetric CryptographyPublic key cryptography is the security discipline employing asymmetric cryptographythat enables key establishment o symmetric keys and digital signatures. The intrinsicmodularity o asymmetric cryptographic algorithms limits their applicability torelatively short data lengths such that they are typically used to establish symmetriccryptographic keys versus operating on the data itsel. The established symmetrickeys are then used in streaming or block ciphers with practically unlimited datalengths. Key establishment algorithms come in two favorskey agreement and keytransport [X9.44]:

Key agreement is a key establishment protocol whose secret key is a unctiono inormation contributed by two or more participants, so that no party can

predetermine the value o the key. Key transport is a key establishment protocol under which the secret key is

determined by the initiating party.

B. Digital SignaturesWith regard to digital signatures, a hashing unction is applied to very long data stringsto produce a hash value; and the asymmetric private is applied to the hash value togenerate a digital signature. The digital signature can be veried by a relying partyusing the corresponding asymmetric public key. In a classical public key inrastructurethe identity o the signer is provided to the relying party via a public key certicate[X.509] issued by a certication authority (CA). The CAs digital signature on the signerscerticate cryptographically binds the signers identity to the public key. Similarly, theCAs public key may likewise be encapsulated in another certicate issued by anotherCA such that a chain o certicates may exist; leading to a root CA. The underlyingassumption is that the relying party has and trusts one or more o the certicationauthorities in the certicate chain.


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Figure 1 Certifcate Chain

As shown in Figure 1 Certifcate Chain, the signer generates a digital signature on data.The relying party checks the signers white certicate and determines that it was issuedby the Red CA; the relying party checks the Red CAs certicate and determines that itwas issued by the green Root CA. Assuming that the relying party has a trusted copy othe green Root CAs public key, the relying party veries the green signature on the RedCAs certicate and thus can trust the Red CA public key; the relying party veries thered signature on the signers certicate and thus can trust the signers white public key;the relying party can nally veriy the signers signature on the data.

Root

CA

RedCA

Signer

????Data

Signature

Public key certificate

Asymmetric public key

Public key certificate

Signature

Signature


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C. Time StampingMost networks provide a system time such that data can be time stamped with theyear, month, day, hour, minute, and second. Figure 2 System Time Stamps shows therelationship between the system clock and the time stamp associated with each pieceo signed data.

T1

T2

T3

????

?

Data

????

?

Data

Signer

????

?

Data

Time

Figure System Time Stamps

Presumably the time stamp indicates a sequence to the relying party and implieswhen the digital signatures were generated. However, note that the system generatedtime stamp is not independent o the data generation, or the digital signature generation

processes.

The system clock, via the local or wide area network, may be synchronized to a nationalmeasurement institute whose clock is calibrated to the international time authorityBureau International des Poids et Mesures (BIPM) located in France. In the UnitedStates, the recognized national measurement institutes are the National Institute oStandards and Technology (NIST) and the United States Naval Observatory (USNO).


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Figure 3 US National Measurement Institutes

InternationalTimeAuthority

????USNO

United

StatesNavalObservatory

????NIST

NationalInstitute of

Standards &Technology

????ACTS

AutomatedComputerTimeService

???ITS

InternetTime

Service

???GPS

GlobalPositioningSystem

Device Under Test (DUT)

????FMAS

FrequencyMeasurement

& AnalysisService

Calibration Calibration

Calibration

Traceable

Monitored

???ITA

Figure 3 US National Measurement Institutes shows the relationship o the various NISTtiming services such as the:

Internet Time Service (ITS),

Automated Computer Time Service (ACTS),

Frequency Measurement Analysis Service (FMAS); and the

Global Positioning System (GPS) operated by the USNO [NIST].


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T1

T2

T3

Signer

Time

Data

Data

Data

II. ParadoxA. Time-Insensitive Digital SignaturesFigure 4 Digital Signature Paradoxbelow shows the same sequence o signed data;however the system clock has been reset such that the same time stamp is generatedat three dierent times (T1, T2, and T3) or dierent versions o the same data. The timeparadox is that a relying party now has three versions o the same data with the sametime stamp; and despite the presence o a legitimate digital signaturethe relying party can no longer have condence in the data. Thus digital signaturesare time-insensitive.

The relying party has no practical method to distinguish between the three dataversions, has no method to prioritize the data versions and has no option but to distrustall three versions. In order or the relying party to distinguish between the data versions

and continue to trust the digital signatures, the signer needs to implement a veriablemechanism such that the time stamp generation is independent o the signature

generation. This method is reerred to as a trusted time stamp.

Figure Digital Signature Paradox


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RTSA

Request

Response

DataHash

??MC

???NMI

???ITA

TimeSourceEntities

Requestor

National

Measurement

Institute

Master Clock

International

Time

Authority

Time

Stamp

Authority

Sign

?????

????

????

????

Token

Time

Sign

Sign

B. Trusted Time StampIn a trusted time stamp scheme, there are ve entities: time source entity, time stampauthority, requestor, verier, and relying party. The relying party can be the requestoror any other third party. The time stamp authority (TSA) calibrates its clock with anupstream time source entity such as Master Clock (MC) or directly with a nationalmeasurement institute. The TSA provides a trusted time stamp token to the requestor.The time stamp token can be veried by a third party verier. Figure 5 TrustedTime Stamp shows the relationship between the time source entities, the TSA,and the requestor.

The requestor generates a digital signature by hashing known data and applying itsasymmetric private key to the hash. The digital signature is presented to the TSAas a request or a time stamp token. Note that the TSA only knows the digitalsignature, not the original source data, and thereore has no liability as to the data

content. The TSA appends a time stamp to the requestors hash and binds themtogether with a cryptographic method, such as a digital signature [X9.95]. The hash,the time stamp and the crypto are the essential elements o the time stamp tokenthat the TSA returns to the requestor. The requestor can then provide the originaldata, the requestors digital signature, and the time stamp token to a relying party.

Figure Trusted Time Stamp


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Figure 6 Verifcation shows how the relying party can then make use o averication service who (i) veries that the hash in the time stamp token matchesthe data, (ii) veries the TSAs crypto binding, and (iii) veries the requestorsdigital signature this provides non-reputable evidence o who signed the data(authentication), when it was signed (timeliness) and what data was signed (integrity)provable to an independent third party.

V

Verifier

?????

????

????

????

Token

Time

Sign

Sign

????DATA

Figure Verifcation

The denition o data integrity a property whereby data has not been altered ordestroyed must thereore be expanded to embrace time-sensitive mechanisms

such as trusted time stamps. An better denition might then be the continuity o dataat a provable point in time. With this denition a relying party can veriy that dataintegrity is currently contiguous rom a previous point in time.

C. X. StandardThe American National Standard X9.95-2005 Trusted Time Stamps was developed basedon RFC 3161 [TSP] and the ISO/IEC 18014 standards [ISO], but goes much urther in itsanalysis and oerings. X9.95 denes time stamp schemes that provide a high assurancelevel o data integrity and non-repudiation not achievable by digital signatures alone;suitable or regulatory compliance. The standard denes roles, responsibilities, andthe management and security requirements or the time source entity, the time stampauthority, the requestor, and the verier. The standard species data objects; messageprotocol; and trusted time stamp methods, including the digital signature, MAC, linked

token, linked and signature, and transient key methods. The standard also providessample time stamp policy and practice statements along with evaluation compliancecriteria suitable or use by a proessional practitioner.


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III. Consequences o Time-Based Data ManipulationA. Enron (CFO)Mr. Fastow, the Chie Financial Ocer o Enron, and other members o his executiveteam made it a habit to engage in time-based data manipulation, i.e., altering orchanging nancial data to suit whatever it was they wanted the investing public, orgovernmental authorities, to know or not know. Mr. Fastow pleaded guilty and is now aguest o the ederal government.

B. Rite-Aid (CEO, CFO)The CEO and the CFO o this publicly-traded company backdated compensationgrant letters to enrich themselves by millions o dollars. They then attempted to removeany evidence o their wrongdoing by dumping the computer they used to backdate the

documents into the Atlantic Ocean. These gentlemen are now guests o the ederalgovernment.

C. NextCard (Auditors)Now deunct NextCard was the largest issuer o Internet MasterCard and Visa creditcards. Executives o this ormer high-fying public company raudulently and illegallyre-characterized loan losses, thereby reducing the amount o cash reserves required.Assisting in no small way in this billion dollar fameout, auditors rom Ernst & Youngperpetuated the companys raud by backdating their work papers and their nalreports to conorm to the raudulent representations by company executives. Theseauditors are also currently guests o the ederal government. The SEC attorneyinvestigating this matter lamented that the real crime here was that there was noway to ascertain or recover the real, or the true data, because o the time-based data

manipulation o these insiders.

D. Autotote (Programmers)A senior trusted-insider programmer or the largest electronic wagering organizationin the United States back-dated data to create a 6 million dollar winning ticket in theMaryland Breeders Cup race. This gentleman is now a guest o the state.

E. Sirena Corp (CEO)The Securities and Exchange Commission ned publicly traded Sirena Corp. or holdingthe quarter open or days ater the end o that quarter in order to squeeze additionalrevenues to meet analysts projections. Sirena eventually declared bankruptcy.

F. Parmalat (CEO, CFO, and Family)In this 18 billion dollar 2003 bankruptcy, the entire CxO level o this multi-nationalconglomerate engaged in time-based data manipulation by creating an authentic-appearing conrmation by Bank o America, on Bank o America letterhead, andsigned by a Bank o America Vice President, to the eect that there existed an oshorebank account holding 5 billion euro on account. In reality both the unds and the accountwere non-existent, and the alleged Bank o America letter used by the Company to


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raise billions in the public credit market was pieced together by the company executivesusing a scanner and Adobe Photoshop, rom three totally unrelated sources. Thesignature o the Bank o America VP was rom the inormation technology department.There are currently at least three lawsuits, including two class actions, pending invarious courts around the world.

G. Adelphia Communications (executives)Once one o the largest cable providers in the United States, Adelphias top executivesengaged in time-based data manipulation to hide the thet o more than 400 milliondollars rom the company. Adelphia subsequently entered bankruptcy, its top executiveswere tried (and two convicted), and the company is now being acquired by one oits competitors.

IV. ConclusionSo is time-based data manipulation really that much o a problem? It most certainlyis: a multi-billion dollar problem, as the case studies above make clear. When given theopportunity, a certain percentage o trusted insiders will always abuse their authorityto manipulate records to their advantage. Digital signature-based schemes cannotadequately address this problem, but trusted timestamp technologies might be justwhat the doctor ordered: a practical, inexpensive and technically robust solution.


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2007 ProoSpace. All Rights Reserved. ProoSpace, Transient Key, the ProoSpace logo, ProoMark and the ProoMarkSystem are trademarks o ProoSpace Inc. All other trademarks are owned by their respective companies.

V. Reerences[X9.44] X9.44 (drat) Public Key Cryptography or the Financial Services Industry Key Establishment Using Integer Factorization Cryptography, Accredited StandardsCommittee X9, www.x9.org, October 2003

[X.509] ISO/IEC International Standard 9594-8 | ITUT Recommendation X.509 (1997),Inormation Technology Open Systems Interconnections The Directory:Authentication Framework

[NIST] National Institute o Standards and Technology, Physics Laboratory Time andFrequency Division, http://www.boulder.nist.gov/timereq/index.html

[X9.95] American National Standard X9.95-2005 Trusted Time Stamps, AccreditedStandards Committee X9, www.x9.org, March 2005

[TPS] Request or Comments (RFC) 3161 Internet X.509 Public Key InrastructureTime-Stamp Protocol (TSP), Internet Engineering Task Force (IETF), C. Adams, P. Cain,D. Pinkas, R. Zuccherato, August 2001

[ISO] ISO/IEC 18014 Inormation Technology Security Techniques Time StampingServices, ISO/IEC Joint Technical Committee One (JTC1), 2003

Digital Signature Paradox, J. Stapleton, P. Doyle, S. Teppler Esq., Proceedings o the2005 6th IEEE, Workshop on Inormation Assurance and Security, United States MilitaryAcademy, West Point, NY

Digital Signatures are Not Enough, J. Stapleton, S. Teppler Esq, THE ISSA JOURNAL ,January 2006

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