Automatic-Parking/Source/ProofOfConcept/Utilities/CertificateAuthority.cs

namespace ProofOfConcept.Utilities;

using System.Net;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

public static class CertificateAuthority
{
    /// <summary>
    /// Create a self-signed Root CA certificate.
    /// </summary>
    /// <param name="subjectName">Distinguished Name (e.g., "CN=My Root CA, O=MyOrg, C=HU").</param>
    /// <param name="validYears">Validity in years.</param>
    /// <returns>X509Certificate2 with private key.</returns>
    public static X509Certificate2 CreateRootCA(string subjectName, int validYears = 10)
    {
        using var key = ECDsa.Create(ECCurve.NamedCurves.nistP256);

        var req = new CertificateRequest(
            new X500DistinguishedName(subjectName),
            key,
            HashAlgorithmName.SHA256);

        req.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, true, 1, true));
        req.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.KeyCertSign | X509KeyUsageFlags.CrlSign, true));
        req.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(req.PublicKey, false));

        var notBefore = DateTimeOffset.UtcNow.AddMinutes(-5);
        var notAfter = notBefore.AddYears(validYears);

        var cert = req.CreateSelfSigned(notBefore, notAfter);

        // Attach private key so we can export
        return cert.CopyWithPrivateKey(key);
    }

    /// <summary>
    /// Create a TLS/HTTPS certificate for a domain, signed by the given Root CA.
    /// </summary>
    /// <param name="rootCA">Root CA certificate (must include private key).</param>
    /// <param name="subjectName">Common Name (e.g., "CN=myapp.local").</param>
    /// <param name="dnsNames">DNS SAN entries.</param>
    /// <param name="ipAddresses">Optional IP SAN entries.</param>
    /// <param name="validDays">Validity in days (max ~397 for Apple clients).</param>
    /// <returns>X509Certificate2 with private key.</returns>
    public static X509Certificate2 CreateHttpsCertificate(X509Certificate2 rootCA, string subjectName, IEnumerable<string> dnsNames, IEnumerable<IPAddress>? ipAddresses = null, int validDays = 397)
    {
        if (!rootCA.HasPrivateKey)
            throw new ArgumentException("Root CA must have a private key", nameof(rootCA));

        using var leafKey = ECDsa.Create(ECCurve.NamedCurves.nistP256);

        var req = new CertificateRequest(
            new X500DistinguishedName(subjectName),
            leafKey,
            HashAlgorithmName.SHA256);

        // Basic constraints: not a CA
        req.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, true));
        req.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.DigitalSignature | X509KeyUsageFlags.KeyEncipherment, true));
        req.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(
            new OidCollection { new Oid("1.3.6.1.5.5.7.3.1") }, true)); // serverAuth
        req.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(req.PublicKey, false));

        // Subject Alternative Names
        var sanBuilder = new SubjectAlternativeNameBuilder();
        foreach (var dns in dnsNames)
            sanBuilder.AddDnsName(dns);
        if (ipAddresses != null)
        {
            foreach (var ip in ipAddresses)
                sanBuilder.AddIpAddress(ip);
        }
        req.CertificateExtensions.Add(sanBuilder.Build());

        var notBefore = DateTimeOffset.UtcNow.AddMinutes(-5);
        var notAfter = notBefore.AddDays(validDays);

        // Generate random serial
        var serial = new byte[16];
        RandomNumberGenerator.Fill(serial);
        serial[0] &= 0x7F; // positive

        var issued = req.Create(rootCA, notBefore, notAfter, serial);
        return issued.CopyWithPrivateKey(leafKey);
    }
}