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Enforcing honesty in fair exchange protocols

Enforcing honesty in fair exchange protocols. Asandei George Ilascu Marian. Master Securitatea Informa tiei , An I, Facultatea de Informatica. Universitatea Alexandru Ioan Cuza, Iasi , 2014. Structura. Introducere Tipuri de protocoale „Fair exchange” Optimistic Fair Exchange Protocol

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Enforcing honesty in fair exchange protocols

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  1. Enforcing honesty in fair exchange protocols Asandei George Ilascu Marian Master SecuritateaInformatiei, An I, Facultatea de Informatica Universitatea Alexandru Ioan Cuza, Iasi, 2014

  2. Structura • Introducere • Tipuri de protocoale „Fair exchange” • Optimistic Fair Exchange Protocol • Comparatii intre protocoale • Q&A • Referinte

  3. Certified Email – Nenadic et al.[2] • S -> R : {h(Message), SigS(h(Message))} • R -> S : {Enc.kR(Receipt)} • S -> R : {Message} • R -> S : {kR}

  4. Certified Delivery - Nenadic et al.[3] DP = Digital product • M -> C : {Enc.kM(DP), Cert(DP) SigM(DP))} • C -> M : {Enc.kC(SigC(ReceiveDP))} • M -> C: {kM} • C -> M : {kC}

  5. Certified Delivery Asokan et al. [4] Contract N is previously agreed on by both parties A and B. a = random generated by A b = random generated by B • A -> B : {SigA(h(a), N)} • B -> A : {SigB(h(b), {SigA(h(a), N)}} • A -> B : {a} • B -> A : {b}

  6. Types of fairness • Strong fairness • Weak fairness

  7. TTP involvement • No TTP • TTP • Inline • Online • Offline

  8. Protocoale ce nu implica TTP-uri • Protocoale „gradual exchange” • Parti mici din produs is trimise in mai multe runde • Jakobson prpoune impartirea datelor in doua bucati ce pot fi interpretate doar cand sunt • Protocoale probabilistice • Probabilitatea corectitudinii creste cu fiecare runda

  9. Protocolul Zhang et al. [5] • C -> M : Request product • M -> C : Invoice • C-> M : Enc.kC(Payment) • C -> B : Enc.kC(Payment) • B -> M : Enc.kC(Payment) • M -> C : Product • C -> M : kC

  10. Protocolul Ray et al.[6] • Comerciantul M se inregistreaza la un TTP. • TTP-ul genereaza kM1 si dkM1 • Similar pentru clientul C (inregistrare + generare chei) • TTP-ul va stoca produsul, descrierea si pretul acestuia

  11. Protocolul Ray et al.[6] • C -> M : {Purchase order, Enc.C1xC2(Payment)} • M -> C : {Enc.M1xM2(Product)} • C -> M : C2 • M -> C : M2

  12. Protocolul Asokan et al.[7] • C -> M : Request product • M -> C : Invoice • C-> M : Payment • C -> B : Product TTP will cancel the payment if the product is not as described in the description.

  13. Protocolul Devane et al.[8] • C -> M : SigC(Purchase request) • M -> C : SigM(Invoice), Enc.kM(DP) • C-> M : SigC(Payment) • M -> B : kM, SigB(SigC(Payment)) • B -> M : kM, SigB(SigC(Payment)) • M -> C : kM, SigB(SigC(Payment)) • C -> B: Aknowledgement

  14. Protocolul EMH (Enforcing Merchant Honesty) • Utilizat pentru schimbul de plati si produse intre un cumparator si un vanzator • Forteaza comportamentul corect al vanzatorului • Contine un mecanism de rezolvare al disputelor, in urma caruia ambele parti sunt multumite

  15. Agentisiroluri • C -> cumparator • Efectueazacererea de achizitionare a unuiprodus • M -> vanzator • Detine un anumitproduspe care ilcomercializeazaclientului in schimbuluneiplati

  16. Agentisiroluri • TTP -> terta parte in care C si M au incredere • Genereaza o cheiepublicapkctsi o cheieprivataskctpe care le partajeaza cu C • Genereaza un certificatC.ctpentruskctcecontinesemnaturalui TTP skct ->cripteazacheiasimetricautilizata pt. decriptareaplatii • Rezolvaanumiteconflicte initiate de M

  17. Agentisiroluri • CB -> bancaundeclientuldetine un cont • Genereazaplataprin care care clientulvaachizitionaprodusul • Genereaza un certificat P-Cert al platiicecontine: • Suma de plata • Identitateavanzatorului M • hP -> un hash al platii • heP -> un hash al platiicriptate cu cheiasimetricakc • heKc -> un hash al platiicriptate cu cheiapublicapkct • Semnaturabancii CB

  18. Agentisiroluri • CA -> autoritatea care certificaprodusul • Genereaza un certificat DG-Cert pentruprodusuldetinut de M cecontine: • Pret • d -> Descriereasau ID-ulprodusului D • hDG -> Hash-ulprodusului D • Sig.CA -> Semnaturaautoritatiipecertificat • M vafolosiacestcertificatpentru a comercializaprodusul

  19. Fazeleprotocolului 1. Faza de pre-exchange

  20. Fazeleprotocolului 2. Faza de exchange

  21. Fazeleprotocolului 2. Faza de exchange • E-M1: C-> M: • desc: specificaprodusuldorit de client, contine: • Descriere + ID • hDG: hash-ulprodusului D • enc.kc(P): plata. criptata cu kc • P-Cert • C.ct • enc.pkct(kc): cheiasimetricagenerata de C • enc.pkm(km): cheiesimetrica km ceva fi utilizata de vanzatorpentru a criptaprodusul • Sig.c(P): semnatura RSA a clientuluiceasigura non-repudierea

  22. Fazeleprotocolului 2. Faza de exchange • M verifica: • P-Cert prinverificareasemnaturiilui CB • C.ctprinverificareasemnaturiilui TTP • enc.kc(P) prin: • HP = hP? (verificareaegalitatiidintre hash-ullui P din sig.C(P) hP din P-Cert) • HeP = heP? (calculeaza h(enc.kc(P) sicompara cu heP din P-Cert) • HeKc = heKc? (calculeaza h(enc.pkct(kc)) sicompara cu heKc din P-Cert)

  23. Fazeleprotocolului 2. Faza de exchange • E-M2: M -> C: • enc.km(D) – M obtine km din pkm(km) sicripteazaprodusul • Sig.m(D) – asigura non-repudierea • E-M3: C -> M: • Enc.pkm(skct) – C trimitecheiaprivataskct pt. ca M sadecriptezeplata

  24. Rezolvareadisputelor • Modalitati: • Arbitration: existentaunui mediator cecolecteazainformatii din ambelepartisiia o decizieasuprarezolvariidisputei • Evaluation: mediatorulcontineinformatii din ambeleparti, insadoarrecomanda o solutiepentrurezolvareadisputei • Mediation: ajutapartilesacomuniceintreelepentrurezolvareadisputei • Automated negotiation: se refera la rezolvareaconflictelor cu privire la suma de baninegociataintreparti, fiecare parte ofera o anumitacantitate din bani/produs, faracacealaltasastie. Mediatorulrezolvadisputacandceidoiofera in mod egal.

  25. Rezolvareadisputelor • Modalitati: • Mock trial: Existentaunuijuriuceofera o sugestieasuprarezolvariidisputei • Complaint Assistance: Instrument ceajuta la creareacererii de rezolvare a disputei, precumsi la rezolvareaacestuia • Credit card charge back: Mediatorulconstaintr-un furnizor de card-uri de credit cevainmanasuma de banicuvenitapartiivictime

  26. Rezolvareadisputelor • In EMH: • Rezolvareaeste in modul Arbitration, insainformatiilesuntpreluatedintr-o singura parte: M • Disputapoate fi datorata de contestareacomportamentului ne-onest al cumparatorului C cu privire la (ne)trimitereacheii de decriptare a platii, precumsi a comportamenului ne-onest al lui M in vedereaobtineriiuneiplati in plus • Cererea de rezolvare a disputeipoate fi emisadoar de vanzatorul M

  27. Rezolvareadisputelor

  28. Rezolvareadisputelor • DR-M1: M -> TTP: • desc • P-Cert • C.ct • Sig.c(P) • enc.pkt(km) • enc.km(D) • TTP verifica P-Cert, C.ct, semnaturalui C pedescsidecripteaza km

  29. Rezolvareadisputelor • DR-M2: TTP -> C: enc.km(D) + enc.pkc(km) • Motive: • M a trimisprodusulincorect D in E-M2 • M nu a trimisprodusuldeloc (M a contactat TTP inaintea E-M2) • M nu a criptatprodusul D cu km trimis in E-M1 de client • DR-M3: TTP -> M: enc.pkm(skct) sau abort

  30. Rezolvareadisputelor

  31. Scenarii de executie • C si M se comportaonest • C se comporta ne-onestiar M onest • C onest, M ne-onest • C si M nu se comportaonest

  32. Scenarii de executie

  33. Scenarii de executie • (1): C si M onesti executienormala • (2): Dupareceptialui E-M1, M opresteexecutiaprotocolului • (3): M receptioneaza E-M1 sicontacteaza TTP inainteatrimiterii E-M2. TTP retureaza abort => M triseaza • (4): Similar (3) insa DR-M1 estecorect, TTP rezolvadisputa • (5): M receptioneaza E-M1, verificasitrimite E-M2, insa nu maiprimeste E-M3. M contacteaza TTP, dar DR-M1 esteincorect. Motive: E-M2 nu estecorectsau C ne-onest(M treb. satrimita DR- M1 corectpentrurezolvare) • (6): Asemanator (5) insa DR-M1 estecorectsi TTP rezolvadisputa in cazul in care E-M2 estecorect

  34. Scenarii de executie • (7): Protocolul se executapana la capat, insa M contacteaza TTP, dar DR-M1 esteincorect -> TTP returneaza abort • Motive: • E-M3 incorect • M vreasaobtina un avantaj fata de C • (8): Asemanator (7), insa DR-M1 estecorectsi TTP rezolvadisputa

  35. Comparatie cu alteprotocoale

  36. Referinte • [1] Enforcing Honesty in Fair Exchange Protocols - Abdullah M. Alaraj and Malcolm Munro • [2] Nenadic, A., Zhang, N., Barton, S.: Fair certified email delivery. • [3] Nenadic, A., Zhang, N., Cheetham, B., Goble, C.: RSA-based certified delivery of e-goods using verifiable and recoverable signature encryption. • [4] Asokan, N., Shoup, V.,Waidner, M.: Asynchronous protocols for optimistic fair exchange. • [5] Zhang, Q., Markantonakis, K., Mayes, K.: A practical fair exchange e-payment protocol for anonymous purchase and physical delivery.

  37. Referinte • [6] Ray, I., Ray, I., Narasimhamurthy, N.: An anonymous failure resilient fair-exchange ecommerce protocol. • [7] Asokan, N., Schunter, M., Waidner, M.: Optimistic protocols for fair exchange. • [8] Devane, S., Chatterjee, M., Phatak, D.: Secure e-commerce protocol for purchase of e-goods using smart card.

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