PKI by Gene Itkis

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PKI by Gene Itkis

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  • One way of adding change without re-issue: Say, certificates are short lived and a new date or new period/generation are required periodically – this is a change. The generation mechanism can be implemented as a hash chain (xi=hash(xi-1)). Some, xn is included in the certificate and every generation the previous x is released.
    A more sophisticated change: adding an private attribute. Generate a hash tree on the attributes (with some key bits). Certify root. To add a privilege/attribute send to the owner the attribute leaf value the siblings of the path from the attribute to the root. Can add any subset of attributes. Taking the attributes away is harder. Also, the attributes must be privileges (something the owner wants) – otherwise the owner pretends not to receive the above values.
  • PKI by Gene Itkis

    1. 1. Public Key Infrastructures Gene Itkis itkis@bu.edu Based on “Understanding PKI” by Adams & Lloyd
    2. 2. What and How?
    3. 3. Services ♦ Secure communication ♦ Notarization ♦ Time-Stamping ♦ Non-Repudiation ♦ Privilege Management – Authorization & Authentication – Authorization & Policy Authorities – Delegation • Blind vs. Auditable
    4. 4. PKI and the Services ♦ CLAIM: PKI can help in all ♦ Question (subjective – GI) – Where is the source of trust in all these? – Suggestion (subjective – GI) • Try to do the same without PKI, using only symmetric techniques (usually possible!); find the problem; see how this problem is manifested and addressed in the PKI solution. • Easier to “cheat” (including yourself!) with PKI. Symmetric techniques are more explicit. ♦ Make all the security & trust assumptions explicit!
    5. 5. Mechanisms ♦ Crypto – Signatures, hash, MAC, ciphers ♦ Infrastructure – Tickets – Certificates – Authorities (Trusted Third Parties) • Ticket Granting, Key Distribution • Certificate, Policy, Authorization,Time, Notary, etc. • Archives
    6. 6. Pitfalls ♦ Security breaches – Key compromises ♦ Inherent difficulties – Revocation ♦ Negligence – Certificates are routinely not checked or some of the attributes ignored – Alarms and warnings ignored (“certificate not valid. Press OK to proceed.”) ♦ Inconsistencies & human factors (“that’s not what I meant by this policy!”)
    7. 7. Certificates
    8. 8. Certificates ♦ Introduced in 1978 [Kohnfelder’s Bachelor’s thesis] ♦ X.509 – “the standard standard” today – v.1 (1988) – not extendable – v.2 – not much better – v.3 (1997) is much better – optional extensions Today, X.509=v.3 – Many other standards extend X.509 ♦ Others – PGP, SPKI, etc.
    9. 9. Certificates ♦ Certificates ≠ Signature – Certificates are implemented using Signatures ♦ Certificates ≠ Authentication – Authentication can be implemented using Certificates – Same for Authorization, etc. ♦ Certificates are static – Change => Re-Issue • *This could be challenged, but not in standard x509
    10. 10. X.509 Certificate Format ♦ See [AL] pg.76
    11. 11. Certificate Validation ♦ Integrity: signature is valid ♦ Signed by a trusted CA – or certification path is rooted in a trusted CA ♦ Certificate is valid now: – We are between Not Valid Before and Not Valid After time points in the certificate ♦ Not Revoked ♦ Use is consistent with the policy
    12. 12. Alternatives to X.509 Brief detour
    13. 13. SPKI – A Simple PKI ♦ Authorization certificates ♦ Delegation ♦ SDSI – a Simple Distributed Security Infrastructure ♦ Question #1: it may be very nice, but will it ever be used by anyone?
    14. 14. PGP – Pretty Good Privacy ♦ Tendencies – Email • Incompatibilities between PGP and S/MIME • OpenPGP v6.5 supports x509 certs, but still… – Personal (rather than corporate)
    15. 15. SET – Secure Electronic Transaction ♦ Credit card payment protocol ♦ Adopts and extends X.509 – See [AL] pg.84
    16. 16. Back to X.509 End detour
    17. 17. Infrastructure: Policies and Authorities
    18. 18. Certificate Policies ♦ Certificate Policy – “high level what is supported” document ♦ CPS – Certification Practice Statement – “detailed, comprehensive, technical how policy is supported” document ♦ No agreement on the roles and meanings of the above ♦ Might be not public; hard to enforce
    19. 19. Certificate Policies ♦ Distinguished by OIDs (Object ID) – “form letters” ♦ Equivalences – Policy Mapping ext. declare policies equivalent ♦ Established & registered by Policy [Management] Authorities – Internal – e.g. corporate – External – community
    20. 20. CA – Certification Authority ♦ Issuer/Signer of the certificate – Binds public key w/ identity+attributes ♦ Enterprise CA ♦ Individual as CA (PGP) – Web of trust ♦ “Global” or “Universal” CAs – VeriSign, Equifax, Entrust, CyberTrust, Identrus, … ♦ Trust is the key word
    21. 21. RA – Registration Authority ♦ Also called LRA – Local RA ♦ Goal: Off-load some work of CA to LRAs ♦ Support all or some of: – Identification – User key generation/distribution • passwords/shared secrets and/or public/private keys – Interface to CA – Key/certificate management • Revocation initiation • Key recovery
    22. 22. PKI management
    23. 23. Key & Certificate Management Key/Certificate Life Cycle Management – Identity ≠ Key. Focus on Key! Stages ♦ Initialization ♦ Issued (active) ♦ Cancellation • Generation • Issuance • [Usage] • Cancellation
    24. 24. Initialization ♦ Registration – Via RA – Identity verification • According to CP/CPS docs – If on-line, should be protected+authenticated (?) – Secret shared by user and CA • New or pre-existing relationship ♦ Key pair generation ♦ Certificate creation & delivery ♦ [Key backup]
    25. 25. Key pair generation ♦ Where? (by who?) – CA – RA – Owner (e.g. within browser) – Other Trusted 3rd Party ♦ What for? – Non-repudiation ⇒ owner generation ♦ Dual key pair model – Separate key pairs for authentication, confidentiality, etc.
    26. 26. Key pair generation ♦ Performance – Laptop, smart cards – used to be too slow • Today – many smart cards can generate own keys – Centralized generation • Scalability: bottleneck for performance & security ♦ Assurance – “Is the smart card’s random number generator good enough?” – Minimal security requirements guarantees ♦ Legal/Liabilities – Who to sue? Who backs up above assurances?
    27. 27. Certificate Creation+Distribution ♦ Creation – CA only ♦ Distribution (to the owner) – Certificate only – Certificate + private key • Deliver key securely! – X509 rfc2510 – Direct to owner – To depository – Both
    28. 28. Certificate dissemination ♦ Out-of-band ♦ Public repositories – LDAP-like directories – Used mostly for confidentiality ♦ In-band – E.g. signed e-mail usually carries certificate Issues: – Privacy, scalability, etc.
    29. 29. Key backup ♦ Backup ≠ Escrow – Backup= only owner can retrieve the (lost) key – Escrow= organization/government can retrieve the key even against owner’s wish ♦ Non-repudiation conflicts with Backup ♦ Where & how to backup securely???
    30. 30. Issued Phase ♦ Certificate retrieval – To encrypt msg or verify signature ♦ Certificate validation – Verify certificate integrity+validity ♦ Key recovery – Key backup – automate as much as possible ♦ Key update – When keys expire: new certificate [+new keys]
    31. 31. Certificate Cancellation ♦ Certificate Expiration – Natural “peaceful” end of life ♦ Certificate Revocation – Untimely death, possibly dangerous causes ♦ Key history – For owner: eg to read old encrypted msgs ♦ Key archive – “For public”: audit, old sigs, disputes, etc.
    32. 32. Certificate Expiration ♦ No action ♦ Certificate renewal – Same keys, same cert, but new dates – Preferably automatic – but watch for attributes change! ♦ Certificate update – New keys, new certificate
    33. 33. Certificate Revocation
    34. 34. Certificate Revocation ♦ Requested by – Owner, employer, arbiter, TTP, ???, … ♦ Request sent to – RA/CA ♦ Mechanisms for Revocation checks – Certificate Revocation Lists (CRLs) – On-line Certificate Status Protocol (OCSP) • Will it live? (SCVP) ♦ Revocation delay – According to Certificate Policy
    35. 35. Publication Mechanisms ♦ Complete CRLs ♦ Authority Revocation Lists (ARLs) ♦ CRL distribution points (partition CRLs) ♦ Delta CRLs ♦ Indirect CRLs ♦ Enhanced CRL distribution points & Redirect CRLs ♦ Certificate Revocation Trees (CRTs) White lists vs Black lists
    36. 36. CRL versions ♦ Version 1 (from x509 v1) – Flaws: • Scalability • Not extendable • Can replace one CRL with another ♦ Version 2 (similar to x509 v3) – Extensions • critical and non-critical • Per-CRL and per-entry – Format: see [AL] pg.112
    37. 37. Complete CRLs ♦ Advantage: – Self-contained, simple, complete ♦ Problems: – Scalability • CRL may grow too big – Timeliness • Also results from CRL size ♦ Conclusion: appropriate for some domains
    38. 38. Authority Revocation Lists ♦ ARL = CRL for Cas – Revokes certificates of Cas – Rarely needed/used • Decommissioned • Compromised
    39. 39. CRL Distribution Points ♦ Partition CRL into smaller chunks ♦ Static partitions: – Certificate points to its CRL distribution point ♦ Dynamic partitions – Enhanced/Redirect CRL DPs • Certificate points to a Redirect CRL • Redirect CRL directs to the proper CRL partition
    40. 40. Delta CRL ♦ Incremental change – From Complete or Partition CRL – CRLnew=BaseCompleteCRLold + DeltaCRL – Possibly many DeltaCRLs from same BaseCRL • E.g. complete CRL issued once a week, and a new DeltaCRL (containing the previous DeltaCRLs) issued every day
    41. 41. Indirect CRL ♦ Combines CRLs of many CAs – Potentially a “for fee” service by T3rdP
    42. 42. Certificate Revocation Trees – Valicert [Kocher] – Based on Merkle’s hash trees – Similar/Relevant work: [Micali; Naor&Nissim] ♦ Construct hash-tree; leaves – certificates ♦ Sign root ♦ To verify a certificate in the tree: path from the certificate to root + the siblings ♦ Certificate Owner can offer proof of not being revoked as of the current CRT date!
    43. 43. Trust models
    44. 44. Trust model issues ♦ Who to trust? – Which certificates can be trusted ♦ Source of Trust – How it is established? ♦ Limiting/controlling trust in a given environment
    45. 45. Common Trust Models ♦ CA Hierarchy ♦ Distributed ♦ Web ♦ User-centric Tool ♦ Cross-certification
    46. 46. Trust – definition(??) ♦ “A trusts B = A assumes B will behave exactly as A expects” – Problem 1: A expects B to try every way of cheating A that B can find, and A assumes B will do exactly that == A trusts B? – Problem 2: Is it a tautology? What’s the difference between “assumes” and “expects”? ♦ X trusts a CA = X assumes CA will establish and maintain accurate binding of attributes and PK’s – Maintain? Includes secure the binding, CA’s keys binding, security, etc…
    47. 47. Trusted Public Key ♦ PK is trusted by X when X is convinced the PK corresponds to SK which legitimately and validly belongs only to a specific named entity
    48. 48. CA Hierarchy ♦ Tree architecture ♦ Single Root CA – Number of subordinate CA’s • Etc… – Parent certifies children – Leaves are non-CA (end-) entities ♦ Typically CA either certifies other CA’s or end-entities, but not both ♦ Everyone has Root CA PK
    49. 49. Context is important ♦ Privacy Enhanced Mail (PEM) adopted strict hierarchy of CAs approach and failed ♦ DoD could use hierarchy fine
    50. 50. Distributed Trust Architecture ♦ A set of independent hierarchies – May evolve as independent historically ♦ Cross-certification or PKI networking – Connect the hierarchies ♦ Fully-meshed – all CAs are cross-certified ♦ Hub & spokes or bridge CA – Not= Hierarchy • No root CA: every end-entity holds its CA PK
    51. 51. Web Model ♦ A bunch of root CAs pre-installed in browsers ♦ The set of root CAs can be modified – But will it be? ♦ Root CAs are unrelated (no cross- certification) – Except by “CA powers” of browser manufacturer – Browser manufacturer = (implicit) Root CA
    52. 52. User-Centric ♦ PGP ♦ User = her own Root CA – Webs of trust ♦ Good – User fully responsible for trust ♦ Bad – User fully responsible for trust – Corporate/gov/etc. like to have central control • User-centric not friendly to centralized trust policies
    53. 53. Cross-Certification ♦ Mechanism: – Certificates for CAs (not end-entities) ♦ Intra- vs. Inter- domain ♦ One or two directions – CA1 certifies CA2 and/or CA2 certifies CA1 ♦ Control – Cross-certificate limits trust • Name, policy, path length, etc. constraints
    54. 54. Entity Naming ♦ What’s the identity? (the one bound by certificate to the PK [+sk]) – If a certificate is issued to “GeoTrust ”, rather than “Geotrust”, you may be talking to a different entity than what you think
    55. 55. Name Uniqueness ♦ X.500 Distinguished Name (DN) – Tree of naming authorities – X.509 Subject is a DN; – IP addresses, email, etc. are similar ♦ Problems – Not too user-friendly – Central naming authority not always there • => lots of cooperation required from participating entities
    56. 56. Names (continued) ♦ So, how useful are names? – SDSI, SPKI, etc – not very – X.509 allows alternative names • Extensions subjectAltName • If this extension is used Subject name (DN) is not required – Global uniqueness – not always crucial – Piggy-back on existing naming/identity infrastructures
    57. 57. Certificate Path ♦ Alice “trusts” CA1 – Alice has CA1’s PK in its browser • CA1’s PK = “trust anchor” – “trust anchor” depends on the model ♦ CA1 certifies CA2; CA2 certifies CA3 ♦ CA3 certifies Bob ♦ => Alice “trusts” Bob – Alice associates PK in Bob’s certificate with Bob
    58. 58. Certificate Path Processing ♦ Path construction – Aggregation of necessary certificates ♦ Path validation – Checking the certificates and the keys • Includes all steps of certificate validation
    59. 59. Path Construction ♦ “Just a [Shortest] Path graph algorithm” ♦ Not so simple – graph is not known – Edges (certificates) need to be queeried ♦ Once Path Construction is done Path Validation is straight-forward
    60. 60. Multiple Certificates per Entity

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