This scenario describes the renewal of an expiring certificate while retaining the existing key pair. Renewal is appropriate when the key has not been compromised and the key strength is still sufficient.

Renewal vs. Rekey:

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
 
using var ctx = PqCryptoContext.Initialize();
 
// Load existing certificate and key
var oldCert = ctx.LoadCertificate("server.crt.pem");
var existingKey = ctx.LoadPrivateKey("server.key.pem", "KeyPassword!");
 
// Load CA
var caCert = ctx.LoadCertificate("intermediate-ca.crt.pem");
var caKey = ctx.LoadPrivateKey("intermediate-ca.key.pem", "CaPassword!");
 
// Check if renewal makes sense
var daysUntilExpiry = (oldCert.NotAfter - DateTime.UtcNow).Days;
if (daysUntilExpiry > 90)
{
    Console.WriteLine($"Certificate still valid for {daysUntilExpiry} days.");
}
 
// Create CSR with existing key
var csr = ctx.CreateCertificateRequest(
    existingKey,
    oldCert.SubjectName,  // Preserve subject
    ctx.GetExtensions(oldCert)  // Preserve extensions
);
 
// Issue new certificate
var newCert = ctx.IssueCertificate(
    csr,
    issuerCert: caCert,
    issuerKey: caKey,
    serialNumber: ctx.GenerateSerialNumber(),  // New serial!
    validFrom: DateTime.UtcNow.AddDays(-1),    // Overlap
    validDays: 365,
    extensions: new ExtBuilder()
        .BasicConstraints(ca: false, critical: true)
        .KeyUsage(KeyUsageFlags.DigitalSignature | KeyUsageFlags.KeyEncipherment)
        .ExtendedKeyUsage(ExtKeyUsage.ServerAuth)
        .SubjectKeyIdentifier(existingKey.PublicKey)  // Same SKI!
        .AuthorityKeyIdentifier(caCert)
        .CrlDistributionPoint("http://crl.example.com/intermediate.crl")
        .Build()
);
 
// Save
newCert.ToPemFile("server-renewed.crt.pem");
 
Console.WriteLine("Certificate renewed:");
Console.WriteLine($"  Old Serial: {oldCert.SerialNumber}");
Console.WriteLine($"  New Serial: {newCert.SerialNumber}");
Console.WriteLine($"  New Expiry Date: {newCert.NotAfter:yyyy-MM-dd}");

// Check all certificates in store
public IEnumerable<X509Certificate2> GetExpiringCertificates(
    X509Store store,
    int warningDays = 30)
{
    store.Open(OpenFlags.ReadOnly);
    try
    {
        return store.Certificates
            .Where(c => c.NotAfter <= DateTime.UtcNow.AddDays(warningDays))
            .Where(c => c.NotAfter > DateTime.UtcNow)  // Not yet expired
            .OrderBy(c => c.NotAfter)
            .ToList();
    }
    finally
    {
        store.Close();
    }
}
 
// Usage
using var store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
var expiring = GetExpiringCertificates(store, warningDays: 30);
 
foreach (var cert in expiring)
{
    Console.WriteLine($"WARNING: {cert.Subject} expires on {cert.NotAfter:d}");
}

« ← Management Overview | ↑ Scenarios | 4.2 Rekey → »

Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional