PKI Fundamentals

PKI Fundamentals

This document covers the foundational concepts of Public Key Infrastructure (PKI). If you’re new to PKI or need a refresher, start here before diving into QPKI operations.

1. What is PKI?

Public Key Infrastructure (PKI) is a framework for managing digital identities and securing communications using cryptographic keys and certificates.

PKI answers three fundamental questions:

• Who are you? (Authentication)
• Can I trust you? (Trust chain verification)
• Is this message really from you? (Digital signatures)

1.1 The Problem PKI Solves

When two parties communicate over the internet, they face a challenge: how can Alice be sure she’s really talking to Bob, and not an imposter?

PKI solves this by introducing a trusted third party (the Certificate Authority) that vouches for identities through digital certificates.

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2. Certificates

2.1 What is a Certificate?

A digital certificate is an electronic document that binds a public key to an identity (person, server, organization). Think of it as a digital passport issued by a trusted authority.

A certificate states: “This public key belongs to this identity, and I (the CA) vouch for it.”

2.2 X.509 Structure

Certificates follow the X.509 standard (RFC 5280). Key fields:

Field	Description	Example
Subject	Who the certificate is issued to	CN=www.example.com, O=ACME Corp
Issuer	Who signed the certificate	CN=ACME Root CA
Validity	When the certificate is valid	Not Before: 2025-01-01, Not After: 2026-01-01
Public Key	The subject’s public key	ECDSA P-256, RSA 2048, ML-DSA-65…
Serial Number	Unique identifier within the CA	0x03
Extensions	Additional constraints and information	Key Usage, SANs, CRL URLs…
Signature	CA’s digital signature over all fields	Proves authenticity

2.3 Certificate Types

Type	Description	Example
Root CA	Self-signed, trust anchor	Corporate Root CA
Intermediate/Issuing CA	Signed by Root, issues end-entity certs	Issuing CA
End-Entity	The final certificate for a service/user	TLS server, code signing

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3. Keys

3.1 Public and Private Keys

PKI uses asymmetric cryptography with key pairs:

Key	Who Has It	Purpose
Private Key	Only the owner (kept secret)	Sign data, decrypt messages
Public Key	Anyone (in certificate)	Verify signatures, encrypt messages

The mathematical relationship between the keys ensures:

• Only the private key can create valid signatures
• Only the private key can decrypt messages encrypted with the public key
• The private key cannot be derived from the public key

3.2 Key Types

Type	Purpose	Algorithms
Signature	Sign certificates, documents, code	ECDSA, RSA, Ed25519, ML-DSA, SLH-DSA
Key Encapsulation	Establish shared secrets for encryption	ECDH, RSA, ML-KEM

3.3 Key Storage

Private keys must be protected. Common storage options:

Storage	Security	Use Case
File (PEM/DER)	Low	Development, testing
Encrypted file (PKCS#8)	Medium	Production with passphrase
HSM (PKCS#11)	High	Enterprise, compliance

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4. Certificate Authority (CA)

4.1 What is a CA?

A Certificate Authority (CA) is a trusted entity that issues and signs certificates. The CA vouches for the identity of certificate subjects.

4.2 CA Hierarchy

Most PKIs use a hierarchical structure:

Root CA (offline, trust anchor)
    │
    ├── Issuing CA A (online, issues TLS certs)
    │       ├── www.example.com
    │       └── api.example.com
    │
    └── Issuing CA B (online, issues code signing certs)
            └── Developer certificate

CA Type	Description	Typical Validity
Root CA	Self-signed, kept offline, trust anchor	20+ years
Issuing CA	Signed by Root, issues end-entity certs	5-10 years

4.3 Why Hierarchies?

• Security: Root CA stays offline (air-gapped), protected from compromise
• Flexibility: Multiple Issuing CAs for different purposes
• Revocation: Compromise of Issuing CA doesn’t require replacing the Root

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5. Certificate Signing Request (CSR)

5.1 What is a CSR?

A Certificate Signing Request (CSR) is a message sent to a CA to request a certificate. It contains:

1. The subject’s public key
2. The requested identity (subject name, SANs)
3. A signature proving the requester owns the private key

5.2 CSR Workflow

1. Generate key pair         →  private.key + public key
2. Create CSR                →  request.csr (includes public key + identity)
3. Submit CSR to CA          →  CA verifies identity
4. CA issues certificate     →  certificate.crt (signed by CA)
5. Deploy certificate        →  Use with private key

The private key never leaves the requester’s system.

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6. Trust Chain

6.1 Chain of Trust

A trust chain connects an end-entity certificate back to a trusted root:

End-Entity Certificate (www.example.com)
    ↓ signed by
Issuing CA Certificate
    ↓ signed by
Root CA Certificate (trusted)

6.2 Certificate Verification

When verifying a certificate, the verifier:

1. Checks the signature on the end-entity cert (using Issuing CA’s public key)
2. Checks the signature on the Issuing CA cert (using Root CA’s public key)
3. Confirms the Root CA is in the local trust store
4. Validates dates, extensions, and revocation status

If any step fails, the certificate is rejected.

6.3 Trust Stores

Operating systems and browsers maintain trust stores containing trusted Root CA certificates:

Platform	Trust Store Location
macOS	Keychain
Windows	Certificate Store
Linux	/etc/ssl/certs/
Browsers	Built-in + OS trust store

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7. Certificate Lifecycle

7.1 Lifecycle Stages

   Issue           Use             Renew/Revoke         Expire
     │               │                   │                 │
     ▼               ▼                   ▼                 ▼
┌─────────┐    ┌─────────┐         ┌─────────┐       ┌─────────┐
│ Created │───▶│ Active  │────────▶│ Revoked │   or  │ Expired │
└─────────┘    └─────────┘         └─────────┘       └─────────┘

7.2 Revocation

When a certificate must be invalidated before expiry (key compromise, employee departure):

Method	Description	Pros	Cons
CRL	Signed list of revoked serial numbers	Simple, offline verification	Can grow large, update delay
OCSP	Real-time status check via HTTP	Current status, small responses	Requires online responder

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8. File Formats

8.1 Encoding Formats

Format	Description	Use Case
PEM	Base64 with -----BEGIN/END----- headers	Text files, human-readable
DER	Binary ASN.1	Compact, machine processing

8.2 Common Extensions

Extension	Content	Format
.crt, .cer, .pem	Certificate	PEM or DER
.key	Private key	PEM
.csr	Certificate Signing Request	PEM
.p12, .pfx	Certificate + private key bundle	PKCS#12 (binary)
.crl	Certificate Revocation List	PEM or DER

8.3 PEM Examples

Certificate:

-----BEGIN CERTIFICATE-----
MIIBkTCB+wIJAKHBfpegPqAOMA0GCSqGSIb3DQEBCwUAMBExDzAN...
-----END CERTIFICATE-----

Private Key:

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg...
-----END PRIVATE KEY-----

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See Also

• Post-Quantum Cryptography - PQC algorithms and hybrid certificates
• Glossary - PKI and PQC terminology
• CA Operations - CA initialization and management
• Keys & CSR - Key generation and CSR operations