This scenario describes issuing a wildcard certificate (*.example.com). Wildcard certificates secure all subdomains of a domain with a single certificate.

Advantages:

• One certificate for all subdomains
• Easier management
• More cost-effective

Disadvantages:

• Higher risk if compromised
• Does not cover root domain
• Only one level

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
 
using var ctx = PqCryptoContext.Initialize();
 
var caCert = ctx.LoadCertificate("intermediate-ca.crt.pem");
var caKey = ctx.LoadPrivateKey("intermediate-ca.key.pem", "CaPassword!");
 
// Key pair for wildcard
using var wildcardKey = ctx.GenerateKeyPair(PqAlgorithm.MlDsa65);
 
var dn = new DnBuilder()
    .AddCN("*.example.com")
    .AddO("Example GmbH")
    .AddC("DE")
    .Build();
 
// Create CSR
var csr = ctx.CreateCertificateRequest(
    wildcardKey, dn,
    new ExtBuilder()
        // Wildcard + root domain
        .SubjectAlternativeName(new[] {
            "dns:*.example.com",
            "dns:example.com"
        })
        .Build()
);
 
// Issue wildcard certificate
var wildcardCert = ctx.IssueCertificate(
    csr,
    issuerCert: caCert,
    issuerKey: caKey,
    serialNumber: ctx.GenerateSerialNumber(),
    validDays: 365,
    extensions: new ExtBuilder()
        .BasicConstraints(ca: false, critical: true)
        .KeyUsage(KeyUsageFlags.DigitalSignature | KeyUsageFlags.KeyEncipherment)
        .ExtendedKeyUsage(ExtKeyUsage.ServerAuth)
        .SubjectKeyIdentifier(csr.PublicKey)
        .AuthorityKeyIdentifier(caCert)
        .CrlDistributionPoint("http://crl.example.com/intermediate.crl")
        .Build()
);
 
wildcardCert.ToPemFile("wildcard.crt.pem");
wildcardKey.ToEncryptedPemFile("wildcard.key.pem", "SecurePassword!");

For multiple subdomain levels, combine multiple wildcards:

.SubjectAlternativeName(new[] {
    "dns:example.com",
    "dns:*.example.com",          // www, api, app, etc.
    "dns:*.dev.example.com",      // dev1.dev, dev2.dev, etc.
    "dns:*.staging.example.com"   // staging environments
})

Best Practices:

• Store private key centrally (HSM)
• Short validity (max. 1 year)
• Separate wildcard certificates for Prod/Dev/Staging
• Monitoring for all subdomains

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