Download - PKI by Gene Itkis
What and How?
Services
Secure communication Notarization Time-Stamping Non-Repudiation Privilege Management
– Authorization & Authentication– Authorization & Policy Authorities– Delegation
• Blind vs. Auditable
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!
Mechanisms
Crypto– Signatures, hash, MAC, ciphers
Infrastructure– Tickets– Certificates– Authorities (Trusted Third Parties)
• Ticket Granting, Key Distribution• Certificate, Policy, Authorization,Time, Notary, etc.• Archives
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!”)
Certificates
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.
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
X.509 Certificate Format
See [AL] pg.76
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
Alternatives to X.509
Brief detour
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?
PGP – Pretty Good Privacy
Tendencies– Email
• Incompatibilities between PGP and S/MIME
• OpenPGP v6.5 supports x509 certs, but still…
– Personal (rather than corporate)
SET – Secure Electronic Transaction
Credit card payment protocol Adopts and extends X.509
– See [AL] pg.84
Back to X.509
End detour
Infrastructure:Policies and Authorities
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
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
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
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
PKI management
Key & Certificate Management
Key/Certificate Life Cycle Management– Identity Key. Focus on Key!
Stages Initialization Issued (active) Cancellation
• Generation
• Issuance
• [Usage]
• Cancellation
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]
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.
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?
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
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.
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???
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]
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.
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
Certificate Revocation
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
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
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
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
Authority Revocation Lists
ARL = CRL for Cas– Revokes certificates of Cas– Rarely needed/used
• Decommissioned
• Compromised
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
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
Indirect CRL
Combines CRLs of many CAs– Potentially a “for fee” service by T3rdP
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!
Trust modelsTrust models
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
Common Trust Models
CA Hierarchy Distributed Web User-centric
Tool Cross-certification
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…
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
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
Context is important
Privacy Enhanced Mail (PEM) adopted strict hierarchy of CAs approach and failed
DoD could use hierarchy fine
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
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
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
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
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
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
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
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
Certificate Path Processing
Path construction– Aggregation of necessary certificates
Path validation– Checking the certificates and the keys
• Includes all steps of certificate validation
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
Multiple Certificates per Entity