Certificates & PKI

// X.509 Certificate fields (simplified):
// ┌────────────────────────────────────────────────────────────────┐
// │ Version         : 3                                           │
// │ Serial Number   : 0x4A2B...  (unique per CA)                  │
// │ Signature Alg   : sha256WithRSAEncryption                     │
// │ Issuer          : CN=Let's Encrypt R3, O=Let's Encrypt        │
// │ Validity        : 2026-01-01 → 2026-04-01  (90-day cert)     │
// │ Subject         : CN=aicancode.org                            │
// │ Public Key      : RSA 2048-bit / EC P-256                     │
// │ Subject Alt Names (SAN): aicancode.org, www.aicancode.org    │
// │ Key Usage       : Digital Signature, Key Encipherment         │
// │ CA Signature    : <Let's Encrypt signs the above with its key>│
// └────────────────────────────────────────────────────────────────┘

// Inspect a certificate from command line:
openssl s_client -connect aicancode.org:443 -showcerts
openssl x509 -in cert.pem -text -noout

// Certificate chain for aicancode.org:
//
//   Root CA (built into OS / browser trust store)
//   └─ DigiCert Global Root CA  [self-signed, 20-year validity]
//       └─ DigiCert TLS RSA SHA256 2020 CA1  [intermediate, 5-year]
//           └─ aicancode.org  [end-entity, 1-year]
//
// Verification steps (client):
// 1. Hash the end-entity cert content
// 2. Decrypt intermediate's signature with intermediate's public key → compare hash
// 3. Hash intermediate cert content
// 4. Decrypt root's signature with root's public key → compare hash
// 5. Check root is in trust store → TRUSTED

// If any step fails → certificate error (NET::ERR_CERT_AUTHORITY_INVALID)

// CRL (Certificate Revocation List):
// CA publishes a signed list of revoked serial numbers at a URL (CDP extension)
// Client downloads CRL and checks if cert serial is in the list
// Problem: CRL can be megabytes in size and is cached — not real-time

// OCSP (Online Certificate Status Protocol):
// Client sends: OCSP Request { serial: 0x4A2B... } → CA OCSP responder
// CA responds:  OCSP Response { status: good | revoked | unknown, thisUpdate, nextUpdate }
// Problem: extra round-trip per connection; privacy leak (CA sees every connection)

// OCSP Stapling (best practice):
// Server periodically fetches OCSP response and "staples" it to TLS handshake
// Client gets status without extra round-trip or privacy leak
// Nginx config:
ssl_stapling on;
ssl_stapling_verify on;
resolver 8.8.8.8;

// ACME HTTP-01 challenge flow:
// 1. Client (certbot) requests cert for aicancode.org from Let's Encrypt
// 2. LE sends challenge token
// 3. certbot writes file:
//    /.well-known/acme-challenge/Kxyz...abc
//    content: Kxyz...abc.fingerprint
// 4. LE fetches http://aicancode.org/.well-known/acme-challenge/Kxyz...abc
// 5. Content matches → domain ownership proved → cert issued

// Auto-renewal (cron):
0 3 * * * certbot renew --quiet --post-hook "systemctl reload nginx"

// Wildcard cert requires DNS-01 (add TXT record _acme-challenge.aicancode.org)
certbot certonly --dns-cloudflare --dns-cloudflare-credentials ~/.cloudflare.ini   -d "*.aicancode.org" -d "aicancode.org"