Transcript Public Key Infrastructure (X509 PKI) - HP Labs

Public Key Infrastructure
(X509 PKI)
Marco Casassa Mont
Trusted E-Services Laboratory - HP Labs - Bristol
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Outline
• Basic Problem of Confidence and Trust
• Background: Cryptography, Digital Signature,
Digital Certificates
• (X509) Public Key Infrastructure (PKI)
• (X509) PKI: Trust and Legal Issues
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Confidence and Trust
Issues in the Digital
World
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Basic Problem
Bob
Intranet
Extranet
Internet
Alice
Bob and Alice want to exchange data in a digital world.
There are Confidence and Trust Issues …
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Confidence and Trust Issues
• In the Identity of an Individual or Application
AUTHENTICATION
• That the information will be kept Private
CONFIDENTIALITY
• That information cannot be Manipulated
Intranet
Extranet
Internet
Bob
Alice
INTEGRITY
• That information cannot be Disowned
NON-REPUDIATION
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Starting Point:
Cryptography
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Starting Point: Cryptography
Cryptography
It is the science of making the cost of acquiring or altering
data greater than the potential value gained
Cryptosystem
It is a system that provides techniques for mangling a
message into an apparently intelligible form and than
recovering it from the mangled form
Plaintext
Hello World
Encryption
Key
Ciphertext
&$*£(“!273
Public Key Infrastructure (PKI)
Decryption
Key
Plaintext
Hello World
HP Laboratories, Bristol, UK
Cryptographic Algorithms
All cryptosystems are based only on three Cryptographic
Algorithms:
• MESSAGE DIGEST (MD2-4-5, SHA, SHA-1, …)
Maps variable length plaintext into fixed length ciphertext
No key usage, computationally infeasible to recover the plaintext
• SECRET KEY (Blowfish, DES, IDEA, RC2-4-5, Triple-DES, …)
Encrypt and decrypt messages by using the same Secret Key
• PUBLIC KEY (DSA, RSA, …)
Encrypt and decrypt messages by using two different Keys: Public Key,
Private Key (coupled together)
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Cryptographic Algorithms based
on Private Key
Plaintext
Pros
Encryption
Ciphertext
Private Key
Decryption
Plaintext
Private Key
• Efficient and fast Algorithm
• Simple model
 Provides Integrity, Confidentiality
Cons
• The same secret key must be shared by all the entities involved in the data exchange
• High risk
• It doesn’t scale (proliferation of secrets)
 No Authentication, Non-Repudiation
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Pros
Cryptographic Algorithms based
on Public Key
• Private key is only known by the owner: less risk
• The algorithm ensures Integrity and Confidentiality by encrypting with
the Receiver’s Public key
Intranet
Extranet
Internet
Bob
Plaintext
Encryption
Ciphertext
Alice’s Public Key
Public Key Infrastructure (PKI)
Alice
Decryption
Plaintext
Alice’s Private Key
HP Laboratories, Bristol, UK
Pros
Cryptographic Algorithms based
on Public Key
• The algorithm ensures Non-Repudiation by encrypting with
the Sender’s Private key
Intranet
Extranet
Internet
Bob
Plaintext
Encryption
Ciphertext
Bob’s Private Key
Public Key Infrastructure (PKI)
Alice
Decryption
Plaintext
Bob’s Public Key
HP Laboratories, Bristol, UK
Cons
Cryptographic Algorithms based
on Public Key
• Algorithms are 100 – 1000 times slower than secret key ones
They are initially used in an initial phase of communication and then
secrets keys are generated to deal with encryptions
• How are Public keys made available to the other people?
• There is still a problem of Authentication!!!
Who ensures that the owner of a key pair is really the person whose
real life name is “Alice”?
Intranet
Extranet
Internet
Bob
Public Key Infrastructure (PKI)
Alice
Moving towards PKI …
HP Laboratories, Bristol, UK
Digital Signature
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Digital Signature
A Digital Signature is a data item that vouches the origin
and the integrity of a Message
• The originator of a message uses a signing key (Private Key) to sign the
message and send the message and its digital signature to a recipient
• The recipient uses a verification key (Public Key) to verify the origin of
the message and that it has not been tampered with while in transit
Intranet
Extranet
Internet
Bob
Public Key Infrastructure (PKI)
Alice
HP Laboratories, Bristol, UK
Digital Signature
Message
Digest
Algorithm
Message
Digest
Hash Function
Algorithm
Digest
Private Key
Public Key
Encryption
Decryption
Signature
Expected
Digest
Signer
Hash Function
Channel
Public Key Infrastructure (PKI)
Actual
Digest
Receiver
HP Laboratories, Bristol, UK
Digital Signature
There is still a problem linked to the
“Real Identity” of the Signer.
Why should I trust what the Sender claims to be?
Moving towards PKI …
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Digital Certificate
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Digital Certificate
A Digital Certificate is a binding between an entity’s
Public Key and one or more Attributes relating its Identity.
• The entity can be a Person, an Hardware Component, a Service, etc.
• A Digital Certificate is issued (and signed) by someone
- Usually the issuer is a Trusted Third Party
• A self-signed certificate usually is not very trustworthy
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Digital Certificate
CERTIFICATE
Issuer
Subject
Subject Public Key
Issuer
Digital
Signature
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Digital Certificate
Problems
• How are Digital Certificates Issued?
• Who is issuing them?
• Why should I Trust the Certificate Issuer?
• How can I check if a Certificate is valid?
• How can I revoke a Certificate?
• Who is revoking Certificates?
Moving towards PKI …
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Public Key Infrastructure
(PKI)
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Public Key Infrastructure
(PKI)
A Public Key Infrastructure is an Infrastructure
to support and manage Public Key-based
Digital Certificates
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Public Key Infrastructure
(PKI)
“A PKI is a set of agreed-upon standards, Certification
Authorities (CA), structure between multiple CAs,
methods to discover and validate Certification Paths,
Operational Protocols, Management Protocols,
Interoperable Tools and supporting Legislation”
“Digital Certificates” book – Jalal Feghhi, Jalil Feghhi, Peter Williams
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Public Key Infrastructure
(PKI)
Focus on:
• X509 PKI
• X509 Digital Certificates
 Standards defined by IETF, PKIX WG:
http://www.ietf.org/
… even if X509 is not the only approach (e.g. SPKI)
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI – Technical View
Basic Components:
• Certificate Authority (CA)
• Registration Authority (RA)
“Provider” Side
• Certificate Distribution System
• PKI enabled applications
Public Key Infrastructure (PKI)
“Consumer” Side
HP Laboratories, Bristol, UK
X509 PKI – Simple Model
CA
Certification
Entity
Cert. Request
Application
Service
Signed
Certificate
Internet
Remote
Person
RA
Certs,
CRLs
Directory
Local
Person
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Certificate Authority (CA)
Basic Tasks:
• Key Generation
• Digital Certificate Generation
• Certificate Issuance and Distribution
• Revocation
• Key Backup and Recovery System
• Cross-Certification
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Registration Authority (RA)
Basic Tasks:
• Registration of Certificate Information
• Face-to-Face Registration
• Remote Registration
• Automatic Registration
• Revocation
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Certificate Distribution System
Provide Repository for:
• Digital Certificates
• Certificate Revocation Lists (CRLs)
Typically:
• Special Purposes Databases
• LDAP directories
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Revocation List
Certificate Revocation List
Revoked Certificates
remain in CRL
until they expire
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Revocation List (CRL)
• CRLs are published by CAs at well defined
interval of time
• It is a responsibility of “Users” of certificates to
“download” a CRL and verify if a certificate has
been revoked
• User application must deal with the revocation
processes
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Online Certificate Status Protocol
(OCSP)
• An alternative to CRLs
• IETF/PKIX standard for a real-time check if a
certificate has been revoked/suspended
• Requires a high availability OCSP Server
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
CRL vs OCSP Server
Download CRL
CRL
User
CA
CRL
Directory
Certificate IDs
to be checked
User
Answer about
Certificate States
OCSP
Server
Public Key Infrastructure (PKI)
Download
CRL
CRL
CA
Directory
OCSP
HP Laboratories, Bristol, UK
X509 PKI
PKI-enabled Applications
Functionality Required:
• Cryptographic functionality
• Secure storage of Personal Information
• Digital Certificate Handling
• Directory Access
• Communication Facilities
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Trust and Legal Issues
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Trust and Legal Issues
• Why should I Trust a CA?
• How can I determine the liability of a CA?
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Approaches to Trust and
Legal Aspects
• Why should I Trust a CA?
Certificate Hierarchies, Cross-Certification
• How can I determine the liability of a CA?
Certificate Policies (CP) and Certificate Policy
Statement (CPS)
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Approach to Trust
Certificate Hierarchies
and
Cross-Certification
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
CA Technology Evolution
CA
CA
CA
CA
RA
CA
CA
CA
Internet
RA
RA
RA
RA
Try to reflect
Real world Trust Models
Public Key Infrastructure (PKI)
Directory
Services
RA
CA
Internet
LRA
LRA
HP Laboratories, Bristol, UK
Simple Certificate Hierarchy
Root CA
Sub-CAs
Each entity has its own certificate (and may
have more than one). The root CA’s certificate
is self signed and each sub-CA is signed by its
parent CA.
Each CA may also issue CRLs. In particular
the lowest level CAs issue CRLs frequently.
End Entities
End entities need to “find” a certificate path to
a CA that they trust.
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Simple Certificate Path
Trusted
Root
Alice trusts the root CA
Bob sends a message to Alice
Alice needs Bob’s certificate, the certificate of
the CA that signed Bob’s certificate, and so on
up to the root CA’s self signed certificate.
Alice also needs each CRL for each CA.
*
Alice
Bob
Only then can Alice verify that Bob’s certificate
is valid and trusted and so verify the Bob’s
signature.
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Cross-Certification and
Multiple Hierarchies
1
2
3
1. Multiple Roots
2. Simple cross-certificate
3. Complex cross-certificate
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Approach to Trust : Problems
Things are getting more and more
complex if Hierarchies and
Cross-Certifications are used
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Trusted
Root
Cross-Certification and
Trusted
Path
Discovery
Root
3
*
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
X509 PKI
Approach to Legal Aspects
Certificate Policy
And
Certificate Practice Statement
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Policy (CP)
• A document that sets out the rights, duties and
obligations of each party in a Public Key
Infrastructure
• The Certificate Policy (CP) is a document which
usually has legal effect
• A CP is usually publicly exposed by CAs, for
example on a Web Site (VeriSign, etc.)
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Policy (CP)
COMMUNITY &
APPLICABILITY
POLICY OUTLINE
RIGHTS, LIABILITIES
& OBLIGATIONS
CP
CERTIFICATE &
CRL PROFILES
IDENTIFICATION &
AUTHENTICATION
TECHNICAL
SECURITY CONTROL
Public Key Infrastructure (PKI)
OPERATIONAL
REQUIREMENTS
HP Laboratories, Bristol, UK
Policy Issues (CP)
• Liability Issues
• Repository Access Controls
• Confidentiality Requirements
• Registration Procedures
- Uniqueness of Names
- Authentication of Users/Organisations
• Certificate Acceptance
• Suspension and Revocation (Online/CRL)
• Physical Security Controls
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Policy Statement
(CPS)
• A document that sets out what happens in practice
to support the policy statements made in the CP
in a PKI
• The Certificate Practice Statement (CPS) is a
document which may have legal effect in limited
circumstances
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Certificate Policy Statement
(CPS)
GENERAL
PROVISIONS
INTRODUCTION
IDENTIFICATION &
AUTHENTICATION
SPECIFICATION
ADMINISTRATION
CPS
OPERATIONAL
REQUIREMENTS
CERTIFICATE &
CRL PROFILES
TECHNICAL
SECURITY
CONTROLS
Public Key Infrastructure (PKI)
PHYSICAL,
PROCEDURAL &
PERSONNEL
HP Laboratories, Bristol, UK
IETF (PKIX) Standards
• X.509 Certificate and CRL Profiles
• PKI Management Protocols
• Certificate Request Formats
• CP/CPS Framework
• LDAP, OCSP, etc.
http://www.ietf.org/
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK
Identity is Not Enough:
Attribute Certificates
IETF (PKIX WG) is also defining standards for Attribute
Certificates (ACs):
• Visa Card (Attribute) vs. Passport (Identity)
• Attribute Certificates specify Attributes associated
to an Identity
• Attribute Certificates don’t contain a Public key
but a link to an Identity Certificate
Public Key Infrastructure (PKI)
HP Laboratories, Bristol, UK