AGENT SELF-RENEWAL:
- Agent checks certificate expiry on startup
- If < 30 days remaining, requests renewal from server
- Server generates new certificate and updates database
- Agent installs new cert, updates config, and restarts
SERVER-SIDE (/api/endpoints/certificate/renew):
- Requires authentication via existing (expiring) client certificate
- Rate limited: 1 renewal per asset per 24 hours
- Generates 2-year self-signed certificate with cryptography library
- Computes SHA1 fingerprint (matches nginx $ssl_client_fingerprint)
- Updates assets table with new fingerprint and expiration
- Returns PFX/P12 certificate with random password to agent
- Audit logs all renewal operations
AGENT-SIDE (CertificateRenewal.cs):
- CheckAndRenewIfNeeded() called on service startup before host starts
- Finds current certificate by thumbprint in LocalMachine\My store
- Checks expiry: if < 30 days, POST to /api/endpoints/certificate/renew
- Authenticates renewal request with expiring certificate (mTLS)
- Receives PFX + password in JSON response
- Imports new certificate to LocalMachine\My store
- Grants BUILTIN\Users read access to private key (for tray app)
- Updates appsettings.json with new thumbprint
- Exits with code 0 so service manager restarts agent with new cert
CERTIFICATE GENERATION (certificate_renewal.py):
- Uses cryptography library to generate RSA 2048-bit key pairs
- Self-signed certificates with CN=hostname, 2-year validity
- Extended key usage: CLIENT_AUTH
- Subject Alternative Name: hostname as DNS name
- Exports as password-protected PFX/P12 for Windows import
- SHA1 fingerprint computed from DER encoding (nginx compatible)
STARTUP FLOW:
1. Agent service starts (Program.cs RunHostAsync)
2. CertificateRenewal.CheckAndRenewIfNeeded() runs BEFORE host starts
3. If cert expires in < 30 days, requests renewal from server
4. If renewal succeeds, agent exits (Environment.Exit(0))
5. Windows Service Manager automatically restarts the service
6. New process picks up updated thumbprint from appsettings.json
7. Agent continues with renewed certificate
SECURITY:
- Renewal requires valid (not expired) existing certificate
- Rate limit prevents renewal loop abuse
- Server validates cert not revoked before renewal
- Audit logging tracks all renewal operations
- PFX password is random, used once during import, never stored
PERFORMANCE:
- Renewal check adds ~100-200ms to service startup
- Only triggers when expiry < 30 days (rare operation)
- No impact on steady-state agent performance
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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