Public Key Infrastructure (PKI)

One-liner: Framework of policies, hardware, software, and processes that manage digital certificates and public-key encryption.

🎯 What Is It?

Public Key Infrastructure (PKI) is the foundation of secure digital communications. It enables encryption, digital signatures, and authentication using asymmetric cryptography (public/private key pairs). PKI is essential for HTTPS, email encryption, code signing, and device authentication.

🏽️ PKI Components

┌──────────────────────────────────┐
│    Certificate Authority (CA)  │  ← Root of trust
└────────────┬─────────────────────┘
             │
             ├───────────────────────┐
             │                        │
     ┌───────┴──────┐      ┌─────┴─────┐
     │  Registration   │      │  CRL/OCSP │
     │   Authority (RA) │      │  Repository  │
     └────────────────┘      └────────────┘
             │
     ┌───────┴──────┐
     │  End Entities   │  ← Users, servers, devices
     └────────────────┘

1. Certificate Authority (CA)

Trusted entity that issues and manages digital certificates.

Functions:

• Issue certificates
• Revoke compromised certificates
• Maintain certificate directory
• Generate and manage CA's key pair

Types:

• Root CA — Top of trust hierarchy (e.g., DigiCert, Let's Encrypt)
• Intermediate CA — Issued by Root CA, issues end-entity certificates
• Internal CA — Enterprise PKI (Active Directory Certificate Services)

2. Registration Authority (RA)

Verifies identity before CA issues certificate.

Functions:

• Accept certificate requests
• Verify requestor identity
• Approve/reject requests
• Forward approved requests to CA

3. Certificate Repository

Publicly accessible database of certificates and CRLs.

Contains:

• Issued certificates
• Certificate Revocation Lists (CRLs)
• CA public keys

4. End Entities

Users, devices, or systems that request and use certificates.

Examples:

• Web servers (HTTPS)
• Email clients (S/MIME)
• VPN clients
• Code signing (developers)

📜 Digital Certificate Structure

┌────────────────────────────────┐
│  X.509 Certificate          │
├────────────────────────────────┤
│ Version: 3                  │
│ Serial Number: 123456       │
│ Signature Algorithm: SHA256 │
│ Issuer: CN=DigiCert CA      │
│ Valid From: 2024-01-01      │
│ Valid To: 2025-01-01        │
│ Subject: CN=example.com     │
│ Public Key: RSA 2048-bit    │
│ Extensions:                 │
│   - Key Usage               │
│   - Subject Alternative Name│
│   - CRL Distribution Points │
│ Signature: <CA signature>   │
└────────────────────────────────┘

Key Fields:

• Subject — Certificate owner (CN=Common Name)
• Issuer — CA that signed certificate
• Public Key — Owner's public key
• Validity Period — Start and expiration dates
• Signature — CA's digital signature (proves authenticity)

🔄 Certificate Lifecycle

1. Key Generation

User generates key pair:
  - Private key (kept secret)
  - Public key (sent in certificate request)

2. Certificate Request (CSR)

# Generate CSR
openssl req -new -newkey rsa:2048 -nodes \
  -keyout private.key \
  -out request.csr \
  -subj "/CN=example.com"

3. Identity Verification

RA/CA verifies requestor identity (domain ownership, organization, etc.).

4. Certificate Issuance

CA signs certificate with its private key and issues it.

5. Certificate Deployment

Certificate installed on server/device.

6. Certificate Validation

Clients verify certificate by:

• Checking CA signature
• Verifying validity period
• Checking revocation status (CRL/OCSP)
• Validating trust chain to root CA

7. Key Rotation

Periodically renew certificates before expiration.

8. Key Revocation

If compromised, CA revokes certificate via CRL or OCSP.

9. Key Destruction

Securely delete private key when no longer needed.

🎯 Common PKI Use Cases

1. HTTPS/TLS (Web Security)

Browser → Connects to https://example.com
       → Server presents certificate
       → Browser validates certificate
       → Encrypted connection established

2. Email Encryption (S/MIME)

• Encrypt emails using recipient's public key
• Sign emails with sender's private key

3. Code Signing

• Developers sign software with private key
• Users verify authenticity with certificate

4. VPN Authentication

• Certificate-based authentication (better than passwords)

5. Document Signing

• PDF digital signatures
• Legal validity

6. Device Authentication

• IoT devices
• Client certificates (mutual TLS)

🔒 Trust Hierarchy

        ┌────────────────┐
        │   Root CA       │  ← Self-signed
        │ (DigiCert)     │     (Trusted by browsers)
        └───────┬────────┘
               │
        ┌──────┴──────┐
        │ Intermediate  │  ← Signed by Root CA
        │     CA        │
        └──────┬──────┘
               │
        ┌──────┴──────┐
        │ End-Entity    │  ← example.com certificate
        │  Certificate  │     (Signed by Intermediate)
        └──────────────┘

Browser trusts Root CA → Validates entire chain

Why Intermediate CAs?

• Protect Root CA (kept offline)
• If Intermediate compromised, revoke only that CA
• Root CA remains trusted

⚠️ PKI Security Threats

1. Compromised Private Key

If private key stolen → attacker can impersonate owner.

Mitigation:

• Use Hardware Security Module (HSM) for key storage
• Strong access controls
• Immediate revocation if compromised

2. Man-in-the-Middle (MitM)

Attacker intercepts and presents fake certificate.

Mitigation:

• Certificate pinning
• HSTS (HTTP Strict Transport Security)
• Monitor for suspicious certificates (Certificate Transparency logs)

3. Compromised CA

If CA compromised, attacker can issue fraudulent certificates.

Real-world: DigiNotar (2011), Comodo (2011)

Mitigation:

• Certificate Transparency (CT) logs
• Multiple trust anchors
• CA security audits

4. Certificate Expiration

Expired certificates break services.

Mitigation:

• Automated renewal (Let's Encrypt, ACME protocol)
• Monitoring and alerts

📊 PKI Standards

• X.509 — Certificate format standard
• PKCS (Public Key Cryptography Standards) — RSA Security standards
• CRL — Revoked certificates list
• OCSP (Online Certificate Status Protocol) — Real-time revocation check
• ACME (Automated Certificate Management Environment) — Let's Encrypt protocol

🎭 Example: Checking a Certificate

# View website certificate
openssl s_client -connect example.com:443 -showcerts

# View local certificate file
openssl x509 -in certificate.crt -text -noout

# Verify certificate chain
openssl verify -CAfile ca-bundle.crt certificate.crt

# Check certificate expiration
openssl x509 -in certificate.crt -noout -enddate

🎤 Interview Angles

Q: What is PKI and why is it important?

• Framework for managing digital certificates and public-key encryption
• Enables secure HTTPS, email encryption, code signing
• Based on asymmetric cryptography (public/private keys)
• Certificate Authority (CA) is root of trust
• Critical for secure digital communications and Authentication

Q: How does certificate validation work?

Process:

1. Client receives certificate from server
2. Verifies certificate signature using CA's public key
3. Checks validity period (not expired)
4. Checks revocation status (CRL or OCSP)
5. Validates entire chain up to trusted root CA
6. If all checks pass → trust established

Q: What happens if a certificate is compromised?

STAR Example:
Situation: Server's private key was exposed in a data breach.
Task: Prevent attacker from impersonating the server.
Action:

• Immediately contacted CA to revoke certificate
• CA added certificate to CRL and updated OCSP
• Generated new key pair
• Requested new certificate from CA
• Deployed new certificate to server
  Result: Revoked certificate no longer trusted by clients; new secure certificate deployed within 4 hours.

🔗 Related Concepts

• Certificate Revocation Lists (CRLs) — Revocation mechanism
• Authentication — Primary use case
• Hardware Security Module (HSM) — Secure key storage
• Key Management Lifecycle (KML) — Full key lifecycle
• Asymmetric key distribution — Key exchange