CompTIA Security+ Preparation · lesson 6 of 45 · 25 min
Public Key Infrastructure (PKI)
PKI is a framework of policies, procedures, hardware, software, and people that creates, manages, distributes, uses, stores, and revokes digital certificates. It enables trust in digital communications.
Digital certificate — an electronic document that binds a public key to an identity (person, server, organization). Signed by a Certificate Authority (CA). Based on the X.509 standard.
Certificate contents: subject (owner), issuer (CA), public key, validity period (not before/not after), serial number, signature algorithm, extensions (key usage, SAN).
Certificate Authority (CA) — a trusted entity that issues and signs digital certificates. Vouches for the identity of certificate holders.
Root CA — the top of the trust hierarchy. Self-signed. Kept offline for security. Root CA certificates are pre-installed in browsers and operating systems.
Intermediate CA (Subordinate CA) — signed by the root CA. Issues end-entity certificates. If compromised, only its certificates are revoked — the root CA is protected.
Certificate chain (chain of trust) — the path from an end-entity certificate up through intermediate CAs to the root CA. Each certificate is signed by the one above it.
Certificate Revocation
Certificates can be revoked before their expiration date if the private key is compromised, the owner leaves the organization, or the certificate was issued incorrectly.
CRL (Certificate Revocation List) — a list of revoked certificate serial numbers published by the CA. Clients download the CRL periodically. Drawback: CRLs can be large and become stale between updates.
OCSP (Online Certificate Status Protocol) — allows real-time certificate status checking. Client sends the certificate serial number to an OCSP responder; receives “good,” “revoked,” or “unknown.” Faster and more current than CRL.
OCSP Stapling — the server fetches its own OCSP response and includes (“staples”) it in the TLS handshake. Reduces latency and privacy concerns of client-to-OCSP-responder queries.
Certificate Types
DV (Domain Validation) — verifies domain ownership only. Fastest and cheapest. Used for basic HTTPS.
OV (Organization Validation) — verifies domain ownership and organization identity. More trust than DV.
EV (Extended Validation) — most rigorous vetting. Previously showed green bar in browsers. Highest trust level.
Wildcard certificate — covers a domain and all its subdomains: *.example.com covers mail.example.com, www.example.com, etc. Cannot cover sub-subdomains.
SAN (Subject Alternative Name) certificate — covers multiple different domain names in one certificate. More flexible than wildcard.
Self-signed certificate — signed by the same entity that owns it (no CA). Not trusted by browsers by default. Used for internal systems or testing.
Code signing certificate — used to sign software to verify it hasn’t been tampered with and identify the publisher.
Key Terms
Key Terms
Knowledge Check — 12 Questions
Security+ — PKI & Certificates
A digital certificate binds a public key to:
- A private key
- An identity (person, server, or organization)
- A symmetric key
- A hash value
- Answer: An identity (person, server, or organization)
Certificates bind a public key to an identity — the CA vouches that the public key belongs to the named entity.
Why is the root CA kept offline?
- To save power
- To protect it from compromise — if the root CA is compromised, all certificates in the hierarchy are untrusted
- To reduce network traffic
- Because it does not issue certificates directly
Answer: To protect it from compromise — if the root CA is compromised, all certificates in the hierarchy are untrusted
The root CA is the ultimate trust anchor. Keeping it offline (air-gapped) protects it from network-based attacks.
OCSP provides what advantage over CRL?
- Larger revocation lists
- Real-time certificate status; CRLs can be stale
- Offline operation
- No network required
Answer: Real-time certificate status; CRLs can be stale
OCSP provides real-time status — CRLs are published periodically and may not reflect recent revocations.
*A wildcard certificate for .example.com covers:** example.com only All subdomains: mail.example.com, www.example.com, etc.
- Sub-subdomains: a.b.example.com
- Any domain ending in .com
Answer: All subdomains: mail.example.com, www.example.com, etc. Wildcards cover one level of subdomains. *.example.com covers mail.example.com but NOT a.b.example.com. An EV (Extended Validation) certificate differs from DV because:
- EV is cheaper
- EV requires the most rigorous identity vetting
- EV only covers one domain
- EV does not require a CA
- Answer: EV requires the most rigorous identity vetting
EV certificates require extensive verification of the organization — highest trust level, used by banks and e-commerce. OCSP Stapling means:
- The CA staples the certificate to the server
- The server fetches its own OCSP response and includes it in the TLS handshake
- The client checks OCSP directly
- The certificate is pinned to the browser
- Answer: The server fetches its own OCSP response and includes it in the TLS handshake
Stapling reduces latency and privacy concerns — the server pre-fetches the OCSP response so clients do not need to contact the OCSP responder. A self-signed certificate is:
- Signed by a trusted CA
- Signed by the same entity that owns it — not trusted by browsers by default
- Always used for public websites
- More secure than CA-signed certificates
- Answer: Signed by the same entity that owns it — not trusted by browsers by default
Self-signed certificates have no third-party trust — browsers show warnings. Used for internal systems or testing. A SAN (Subject Alternative Name) certificate differs from a wildcard because:
- SAN is cheaper
- SAN can cover multiple different domain names; wildcard covers subdomains of one domain
- Wildcard covers more domains
- They are identical
Answer: SAN can cover multiple different domain names; wildcard covers subdomains of one domain SAN certificates list specific domain names (example.com, example.org, mail.example.net) — more flexible than wildcards. The certificate chain of trust ends at:
- The intermediate CA
- The end-entity certificate
- The root CA
- The OCSP responder
- Answer: The root CA
The chain of trust goes: end-entity → intermediate CA(s) → root CA. The root CA is self-signed and is the ultimate trust anchor. A code signing certificate is used to:
- Encrypt email
- Sign software to verify it has not been tampered with
- Authenticate web servers
- Encrypt disk drives
- Answer: Sign software to verify it has not been tampered with
Code signing certificates let users verify that software came from a known publisher and has not been modified since signing.
The X.509 standard defines the format for digital _____.
Answer: certificates
X.509 is the standard format for digital certificates used in PKI, TLS/SSL, and code signing.
Why use an intermediate CA instead of having the root CA issue all certificates?
- Intermediate CAs are faster
- If an intermediate CA is compromised, only its certificates are revoked — the root CA is protected
- Intermediate CAs are cheaper
- Root CAs cannot issue certificates
Answer: If an intermediate CA is compromised, only its certificates are revoked — the root CA is protected
The intermediate CA acts as a buffer — compromise is contained. The root CA remains trusted and offline.
Try it: PKI terms
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