Wolfgang Schneider NSI: A Client-Server-Model for PKI Services.

Wolfgang SchneiderWolfgang Schneider

NSI: A Client-Server-Model for PKI NSI: A Client-Server-Model for PKI ServicesServices

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Public Key InfrastructuresPublic 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 DeploymentMotivation: 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 Motivation II: Complexities of PKI – Trust Path Construction– 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 ConstructionComplexity of Trust Path Construction

Possible certificate patha cross certifies ba ba issues certificate ba b

CA - Certificate AuthorityTA - Trust Anchor

IBM

Fraunhofer

Verifier

CA

CA CA

CA CA

CA

CA

CA

CA

TA

CACA CA

John‘scertificate

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Problems for Security ApplicationsProblems 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 UsersProblems 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 SolutionNSI 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 ClientsAdvantages 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 ScenarioPKI-Server Security Services Scenario

PKI Server

PKIServer

PKIServer

OCSPLDAP LDAPLDAP DNS OCSP

Centrally managed policies

Trust path construction

request

Certificate retrieval request

Signature validation

request

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Who will benefit from the PKI Server?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 GoalsNSI 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 approachIssues with NSI approach

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

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PKI ArchitecturePKI Architecture

PKIClient

ApplicationLocal Database

DirectRoutes

Server ’sKeys

TrustAnchors

Ind irectRoutes

CertificatePolic ies

CoreFunctionality

Adm inistratorInterface

PK IS LDAP O CSP DNS ...

Protocol M odules

Acc

ess

Pro

toco

l Ma

nag

er

- PKI -Server

PK

I Ro

uti

ng

ValidationPolic ies

CacheStorage

...PK I C lient A PI

M aintenance APICom m unication API

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Comparison: Internet Routing <-> PKIComparison: Internet Routing <-> 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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InterconnectionInterconnection

(A) Client-Server-Protocol(B) Server-Server-Protocol(C) Standard-Protocols

(LDAP,OCSP,etc.)

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NSI‘s role within PKINSI‘s 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 Server‘s certificate– Must be configured on each client– Revocation check of certificate not defined

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Trust RelationshipsTrust 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 ResponsesValidity 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 XKMSNSI 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 NSI: A Client-Server-Model for PKI ServicesServices

Wolfgang [email protected]

Fraunhofer-Institute for Secure Telecooperation

http://www.sit.fhg.de/NSI/