Non-repudiation

1. Non-repudiation Robin Burke ECT 582

2. Midterm scores • Ave: 69 • Std. dev: 23 • Median: 75 • Max: 100 • Min: 35

3. Approximate grade • Mid 80s and up: As • High 60s and to mid80s: Bs • 50s to 60s: Cs • 40s: Ds

4. Midterm • Answers

5. Law and Business • Legal systems make business possible • (sorry libertarians) • Law establishes • conditions for contract validity • venues for disinterested mediation and dispute resolution • remedies for breach of contract • mechanisms of enforcement

6. Law and E-Commerce • E-Commerce also needs legal systems • Complexities • global scope / jurisdiction • evolving technology landscape • automation / liability

7. Evidence • Legal systems require evidence • evidentiary statutes predate digital era • slowly catching up • Non-repudiation • maintaining digital evidence for e-commerce transactions

8. Legal structures • Common law • long-established precedents in US and UK • Concepts • writing • signing • notary • competence • presence • negotiability

9. Problems for e-commerce • Is a digital contract "written"? • digital media impermanent • Is a digital signature a "signature"? • must be qualified with respect to key purpose, policy, etc. • Who bears liability? • private key compromise • service disruption • Who will archive and how? • digital media volatile • archives must be secure

10. Example • Financial services law • banks must retain canceled checks • or facsimiles thereof (microfilm) • pre-dates digital era • If we define "digital representation" • as equivalent to physical facsimile • then banks can store electronic scans of canceled checks

11. Example • Jurisdiction • location where suit can be brought • party must have "minimum contacts" with a jurisdiction to be summoned there • US Constitutional law • Does the availability of web site constitute "minimum contacts"?

12. Legal frameworkUS Federal • Federal law • Federal E-Sign act • provisions • Technology-neutral • Electronic signatures have same status as written ones • limits • applies mostly to sale and lease contracts, will, trusts and other transactions explicitly excluded)

13. Legal FrameworkUS State Law • Uniform Electronic Transactions Act • More specific than Federal law • Enacted by 43 states • Still technology-neutral • Doesn't mention certificates, PKI, etc. • Uniform Computer Information Transactions Act • Extremely controversial • Enacted by 3 states: Maryland, Virginia, Iowa • Major concern • imposition of onerous license terms: self-help, reverse engineering, prevention of archiving, fair-use, etc.

14. UETA Provisions • Electronic Signature • "an electronic sound, symbol. or process attached to or logically associated with a record and executed or adopted by a person with the intent to sign the record." • Effect of Electronic Signature: A • "signature may not be denied legal effect or enforceability solely because it is in electronic form.""If a law requires a signature, an electronic signature satisfies the law." • Electronic Record • "Means a record created, generated, sent, communicated, received, or stored by electronic means." • Effect of Electronic Record • A record "may not be denied legal effect or enforceability solely because it is in electronic form." • If a law requires a record to be in writing, an electronic record satisfies the law." • A contract may not be denied legal effect or enforceability solely because an electronic record was used in its formation." • Effect of Electronic Agents • "The actions of machines ("electronic agents") programmed and used by people will bind the user of the machine, regardless of whether human review of a particular transaction has occurred."

15. Digital Signature Law • Utah Digital Signature Act (1995) • Very specific • Mentions public key cryptography, certificates, CRLs, etc. • Licensing and regulation of CAs • Liabilities of users and CAs • Not widely emulated • "Digital Signature Guidelines" (1999) • American Bar Association • Guidelines for the deployment of PKI • Expectations and liability associated with CAs, RAs, and users

16. International Laws • UN Model Law on Electronic Commerce • similar to UETA • EU Directive on Digital Signatures • similar to Utah law • specific requirements for PKI

17. State of law • Complex and unsettled • Different laws in different states / countries • Catch-22 • Slow adoption of PKI is tied to legal uncertainties • Lack of legal precedents / guidelines due to slow adoption

18. Break

19. Non-repudiation • System property • Protocol • provides for the retention of evidence • that can be used to resolve disputes • regarding transactions

20. Non-repudiation • Strong and substantial evidence of the identity of the signer of a message and of message integrity, sufficient to prevent a party from successfully denying the origin, submission or delivery of the message and the integrityof its contents. • ABA Digital Signature Guidelines

21. Disputes • "I never said that." • origin • "I never got your message." • reception • "Check's in the mail." • submission

22. Types needed • Non-repudiation of origin • NRO • Non-repudiation of delivery • NRD • Non-repudiation of submission • NRS

23. Non-repudiation of Origin • Evidence needed • Identity of originator • Contents of message • Time of generation • this may matter for establishing a negotiation sequence • Techniques • two party • three party

24. Originator Digital Signature • Alice • creates message M • dates it T • and signs it S • Alice sends M + T + S to Bob • Bob uses Alice's public key certificate to verify signature • Bob archives • M + T + S • Alice's public key certificate and CRL used to verify it

25. Features • Identity and contents are protected • Timestamping depends on the accuracy of Alice's clock • Alice needs digital signature capability

26. TTP Signature • Trusted third-party (Vicky) • Receives Alice's transaction M • message • Generates time stamp T • Signs M + T • creating S' • Returns to Alice • Bob gets M + T + S' • can verify that whole transaction matches S' • archives the message for dispute resolution • also Vicky's certificate and CRL used to verify it

27. Features • Alice doesn't need to sign • she can review message before sending • Alice doesn't need a key pair • lower PKI overhead • Timestamp • Vicky's timestamp will be more reliable than Alice's • Identity less secure • no digital signature from Alice • Vicky has access to message contents

28. TTP Digest Signature • Alice doesn't want to disclose M • Same operation with hash of M using key k • creates hash H • Sends H to Vicky • gets back H + T + S' • Attaches M • encrypts M + k + H + T + S' • Bob receives message • verifies that H is a true hash of M • verifies Vicky's signature • archives the transaction

29. Features • Alice needs encryption / hashing capability • Confidentiality is preserved • Identity still a problem

30. In-line TTP • Receives Alice's transaction M • message • Generates time stamp T • Signs M + T • creating S' • Archives M + T + S' • Forwards M to Bob • perhaps with transaction id • Bob can contact Vicky to get evidence

31. Features • Vicky does archiving • Alice and Bob don't need encryption capability • Content and identity guarantees

32. TTP Token • Receives Alice's transaction M • Generates time stamp T • Creates a secure hash H of M + T using a cryptographic key k • Returns to Alice M + T + H • Bob gets M + T + H • Bob can contact Vicky with H • Vicky verifies that H matches message

33. Features • Content secure • No PKI • Ordinary symmetric encryption sufficient • Identity less secure

34. Combination of methods • Originator Signature + TTP Digest Signature • if we care about disclosure • and recipient can archive • Originator Signature + In-line TTP • if we don't care about disclosure • and we want 3rd party archiving • In-line TTP could • archive encrypted message • Bob would need private key to access evidence

35. Non-repudiation of delivery • Same information needed • Identity of recipient • Content of message • Timestamp • Think of NRO • but the origin message is the acknowledgement of receipt

36. Signed receipt • Alice sends Bob M • Bob • generates a timestamp T • computes a hash of M = H • signs H + T = S' • sends Alice a receipt message H + T + S' • Alice • checks H against her original message • validates Bob's signature • archives the receipt message

37. Features • Like digital signature NRO, but in reverse • message = acknowledgement • Standardized part of S/MIME • secure receipt of email • available in MS Outlook • Other variants • TTP Signature, In-Line etc. • all the same options available

38. Problem • Requires that the recipient generate the receipt • What about the "reluctant recipient"? • reason for NRD in the first place

39. Trusted Delivery Agent • Alice sends message of Vicky • Bob must contact Vicky to access message • Vicky generates receipt

40. Non-repudiation of submission • Useful when what matters is submitting something • a bid • acceptance • Like NDD • but with the mail system • or the bidding engine • doing the verification

41. Basic idea • Parties agree to non-repudiation mechanism • Evidence is generated during transaction • Evidence is transmitted • Evidence is verified • Evidence is archived • If necessary • Evidence is retrieved • Evidence is presented for dispute resolution

42. Digital evidence • Evidence will be strong if • secure chain of custody from creation to presentation • properties of authenticity and integrity • policies of the CA and TTP

43. Secure bidding • Suppose Alice doesn't want Bob to know the contents of her message • a bid to be unsealed later • Additional safeguards • Alice shouldn't be able to change her mind • Bob shouldn't be able to read her bid • "Commitment protocol" • Alice commits to an answer but doesn't reveal it

44. Commitment protocol • Alice encrypts M with symmetric key k • produces ciphertext C • generates the transaction based on C • Bob gets Alice's bid C • he can verify identity and timestamp • gets copy of C • When bids are revealed • Alice transmits k • Bid can be read

45. Homework #4 • Use secure email • digital signature • encryption • Get certificate from www.thawte.com • cannot use web mail • if necessary, open a new hotmail account • Use Outlook Express or Netscape Communicator