Overview
This lecture presents an overview and technical profile of X.509 v3 certificates and X.509 v2 certificate revocation lists (CRLs) as used in the Internet Public Key Infrastructure (PKI), covering key data structures, extensions, validation procedures, and interoperability requirements.
Introduction to X.509 Internet PKI
- X.509 certificates bind public keys to subjects using signatures from trusted certificate authorities (CAs).
- Certificates and CRLs are structured using ASN.1 with DER encoding.
- The standard addresses both basic infrastructure and extensions for Internet interoperability.
- Multiple versions of certificates (v1, v2, v3) and CRLs (v2) exist; v3 and v2 are currently profiled.
Core PKI Entities and Concepts
- PKI entities: End entities (certificate users), CAs, Registration Authorities (RAs), CRL issuers, and repositories.
- CAs issue and may revoke certificates; CRLs list revoked certificates.
- Certificate chains (paths) link trust from a user to a trusted root CA.
X.509 v3 Certificate Structure
- Main fields: tbsCertificate (includes version, serial number, issuer, subject, public key, validity, extensions), signatureAlgorithm, signatureValue.
- Extensions add functionality, e.g., key usage, policies, alternative names.
- Subject and issuer fields use distinguished names (DNs) following X.501.
Certificate Extensions
- Common extensions: Authority/Subject Key Identifier, Key Usage, Certificate Policies, Policy Mappings, Subject/Issuer Alternative Name, Basic Constraints, Name Constraints, Extended Key Usage, CRL Distribution Points.
- Each extension has a defined criticality; unknown critical extensions cause validation failure.
- Private extensions exist for Internet-specific needs: Authority Info Access, Subject Info Access.
CRL and Extensions Profile
- CRLs list the serial numbers of revoked certificates and may include extensions (e.g., CRL Number, Delta CRL Indicator, Authority Key Identifier).
- Delta CRLs provide only updates since the previous base CRL.
- CRL issuers must include key identifier and CRL number extensions.
Certification Path Validation Algorithm
- Validation ensures the binding between a subject and their public key, using the trust anchor (root CA).
- Path processing includes: initialization, cert-by-cert validation (issuer-subject matching, validity, signature), constraints checking, policy evaluation.
- Validation fails if any unsupported critical extension is present.
Name Handling and Internationalization
- DNs and name forms support various encodings; UTF8String is preferred for new names.
- Rules exist for internationalized names in DNs, emails, and domain names.
Security and Interoperability Considerations
- All extensions and fields must be processed according to criticality and type.
- Security risks include circular dependencies (e.g., for CRL distribution) and name ambiguity.
- Transition and backward compatibility are considered for legacy encoding.
Key Terms & Definitions
- X.509 Certificate — Digitally signed structure binding a public key to an identity.
- Certificate Revocation List (CRL) — Signed list of revoked certificate serial numbers issued by a CA.
- Distinguished Name (DN) — Hierarchical naming format for identifying certificate subjects or issuers.
- Extension — ASN.1-encoded field providing extra information or controls in certificates/CRLs.
- Trust Anchor — A trusted CA public key used as the starting point for path validation.
Action Items / Next Steps
- Review ASN.1 certificate and CRL structures in Appendix A.
- Understand critical extensions and their processing rules.
- Study the certification path validation algorithm in detail (Section 6).
- Practice interpreting and constructing X.509 certificates and CRLs using provided examples.