NSI: A Client-Server-Model for PKI Services

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NSI A Client-Server-Model for PKI Services
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Public Key Infrastructures

• PKIs setup by companies and organizations
• Allow certificates to be issued and retrieved
• May be interconnected through cross-certificates
• Allows for inter-organizational communication
• Authenticated, integrity protected, encrypted
• Problem PKIs not fully deployed nor easy to use

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Motivation Slow PKI Deployment

• Expensive
• Development of applications using PKI security
  services
• Administration cost of configuring and
  maintaining clients
• Complex
• Security enabled software is complex to write
• Non-user friendly, not transparent
• Encryption and digital signatures are not in
  widespread use

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Motivation II Complexities of PKI Trust Path
Construction

• Initial disjoint PKIs
• Communication between arbitrary users not
  possible
• Only useful within single PKI structure
• Cross-certificates
• Allows communication between separate PKIs
• However, makes path building more complicated
• PKIs too complicated for user
• Validation policies, policy mappings,
  configuration
• ? Client-Server model

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Complexity of Trust Path Construction
Fraunhofer
TA
CA
CA
Verifier
CA
CA
CA
CA
CA
CA
IBM
Johns certificate
CA
CA
CA
CA
CA - Certificate Authority
Possible certificate path
a
b
a cross certifies b
TA - Trust Anchor
a
b
a issues certificate b
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Problems for Security Applications

• Support of many protocols is necessary
• Certificate and CRL download (HTTP, FTP, LDAP,
  ...)
• Certificate Status (OCSP, LDAP)
• All applications must
• Support all protocols
• Know addresses of all needed repositories
• Have cryptographic functionality
• Be able to handle the complexities of PKI
• Complexity Bugs Less security

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Problems for Users

• Applications are expensive and large
• Small devices cannot support storage and
  computational requirements
• Must configure applications with addresses of
  repositories
• For path construction and encryption key
  retrieval
• Trust path construction is slow

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NSI Solution

• Develop a Client-Server based PKI
• Reduce complexity on client-side (Thin Client)
  by offering server based services such as
• Signature validation
• Trust path construction
• Management of CRLs and Revocation Status
• Central management of certificate policies
• Simple access to non-hierarchical interconnected
  PKIs

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Advantages for Clients

• Need not support multitude of PKI protocols
• Need support only one Client-Server-Protocol
• Need not be configured with repository addresses
• Application only needs to know 1 or 2 PKI-Servers
• Complex tasks delegated to the PKI Server
• Signature and certificate validation
• Encryption key retrieval
• Thus, applications become smaller and simpler
• Devices with limited resources can utilize PKI
  functionality
• Examples Cellular phones, PDAs (Personal Digital
  Assistants)

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PKI-Server Security Services Scenario
Trust path construction request
Signature validation request
Certificate retrieval request
Centrally managed policies
PKI Server
PKI Server
PKI Server
OCSP
LDAP
LDAP
LDAP
DNS
OCSP
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Who will benefit from the PKI Server?

• Companies
• Central management of Security Policies
• No longer need to reconfigure every client when
  PKI or policy changes
• Developers for small devices
• API on client side has low resource requirements
• More devices able to use PKI services
• Security application developers
• Decreased development time and costs
• More robust security code
• TrustCentre may provide PKI services

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NSI Goals

• Develop concrete protocols
• Develop client library such that clients with
  limited resources may use it
• Develop a working PKI Server that is deployable
• Run field tests

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Issues with NSI approach

• What is the architecture?
• Interconnection within existing PKIs
• What trust relationships are needed?

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PKI Architecture
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Comparison Internet Routing lt-gt PKI

• IP Routing
• Cooperation of many IP routers
• No computer knows every IP Address in the
  Internet
• Network changes are known only to routers, not
  clients
• Personal computer knows 1 to n DNS servers
• PKI
• Little cooperation between PKIs
• Application must know all repositories (incl. PKI
  meshes)
• Every client must be updated for every PKI change

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Interconnection

1. Client-Server-Protocol
2. Server-Server-Protocol
3. Standard-Protocols (LDAP,OCSP,etc.)

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NSIs role within PKI

• PKI Server is separate from CA
• Accesses available repositories to build paths
• Does not need to be certified by CA
• Trust in PKI Server is through PKI Servers
  certificate
• Must be configured on each client
• Revocation check of certificate not defined

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Trust Relationships

• Client trust in PKI Server
• Certificate validation complete trust
• Signature validation complete trust
• Path construction no trust
• Certificate retrieval no trust
• PKI Servers deployed within organizations
• Clients use organization validation policy and
  trust server

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Validity of PKI Server Responses

• All responses are authenticated
• Secure connection (eg. SSL, IPsec) or
• Digitally signed response
• Integrity of all requests and responses
  verifiable
• Hashes, signatures, encryption
• Replay attacks detectable
• nonces

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NSI comparison with XKMS

• Certificate retrieval and validation services
  supported
• NSI needs no connection with an RA or CA
• XKMS offers registration and revocation services
• Size of sent and stored responses
• XKMS uses XML tags
• NSI uses ASN.1 (support embedded within client
  library)
• Small storage requirements for audits

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NSI A Client-Server-Model for PKI Services

• Wolfgang Schneider
• wolfgang.schneider_at_sit.fhg.de
• Fraunhofer-Institute for Secure Telecooperation
• http//www.sit.fhg.de/NSI/