This scenario describes Hybrid TLS - the combination of classical and Post-Quantum algorithms in the TLS handshake for maximum security during the transition phase.

Hybrid Approaches:

• Hybrid Key Exchange - X25519 + ML-KEM-768
• Hybrid Signature - ECDSA + ML-DSA-65
• Dual Certificates - Classical + PQ certificate

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Runtime.InteropServices;
 
// OpenSSL 3.6 Hybrid Groups for Key Exchange
public static class HybridTlsConfig
{
    // Hybrid Key Exchange Groups (OpenSSL 3.6+)
    public const string HYBRID_X25519_MLKEM768 = "x25519_mlkem768";
    public const string HYBRID_SECP384R1_MLKEM768 = "secp384r1_mlkem768";
    public const string HYBRID_SECP256R1_MLKEM512 = "secp256r1_mlkem512";
 
    [DllImport("libssl")]
    private static extern int SSL_CTX_set1_groups_list(IntPtr ctx, string groups);
 
    public static void ConfigureHybridKeyExchange(IntPtr sslContext)
    {
        // Configure hybrid groups (preference order)
        var groups = $"{HYBRID_X25519_MLKEM768}:{HYBRID_SECP384R1_MLKEM768}:X25519:secp384r1";
 
        int result = SSL_CTX_set1_groups_list(sslContext, groups);
        if (result != 1)
        {
            throw new CryptographicException("Failed to configure hybrid key exchange");
        }
    }
}

using Microsoft.AspNetCore.Server.Kestrel.Core;
using System.Security.Authentication;
 
var builder = WebApplication.CreateBuilder(args);
 
builder.WebHost.ConfigureKestrel(options =>
{
    options.ListenAnyIP(443, listenOptions =>
    {
        listenOptions.UseHttps(httpsOptions =>
        {
            // Hybrid certificate (ML-DSA + ECDSA)
            httpsOptions.ServerCertificate = LoadHybridCertificate();
 
            // Enforce TLS 1.3 (prerequisite for hybrid)
            httpsOptions.SslProtocols = SslProtocols.Tls13;
 
            // OpenSSL configuration for hybrid key exchange
            httpsOptions.OnAuthenticate = (context, sslOptions) =>
            {
                // Cipher suites for TLS 1.3
                sslOptions.CipherSuitesPolicy = new CipherSuitesPolicy(new[]
                {
                    TlsCipherSuite.TLS_AES_256_GCM_SHA384,
                    TlsCipherSuite.TLS_CHACHA20_POLY1305_SHA256
                });
 
                // Application-Layer Protocol Negotiation
                sslOptions.ApplicationProtocols = new List<SslApplicationProtocol>
                {
                    SslApplicationProtocol.Http2,
                    SslApplicationProtocol.Http11
                };
            };
        });
 
        listenOptions.Protocols = HttpProtocols.Http1AndHttp2;
    });
});
 
X509Certificate2 LoadHybridCertificate()
{
    using var ctx = PqCryptoContext.Initialize();
 
    // Dual certificate with both algorithms
    var cert = ctx.LoadCertificateWithPrivateKey(
        "server-hybrid.crt.pem",
        "server-hybrid.key.pem",
        "Password!"
    );
 
    return cert;
}

public class HybridCertificateBuilder
{
    public X509Certificate2 CreateHybridServerCertificate(
        X509Certificate2 caCert,
        AsymmetricAlgorithm caKey,
        string[] dnsNames)
    {
        using var ctx = PqCryptoContext.Initialize();
 
        // Dual Key: ECDSA + ML-DSA
        using var ecdsaKey = ECDsa.Create(ECCurve.NamedCurves.nistP384);
        using var mlDsaKey = ctx.GenerateKeyPair(PqAlgorithm.MlDsa65);
 
        var dn = new DnBuilder()
            .AddCN(dnsNames[0])
            .AddO("Example GmbH")
            .Build();
 
        // CSR with both keys
        var csr = ctx.CreateHybridCertificateRequest(
            classicalKey: ecdsaKey,
            pqKey: mlDsaKey,
            subject: dn,
            extensions: new ExtBuilder()
                .SubjectAlternativeName(dnsNames.Select(d => $"dns:{d}").ToArray())
                .Build()
        );
 
        // Issue hybrid certificate
        var cert = ctx.IssueHybridCertificate(
            csr,
            issuerCert: caCert,
            issuerKey: caKey,
            validDays: 365,
            extensions: new ExtBuilder()
                .BasicConstraints(ca: false, critical: true)
                .KeyUsage(KeyUsageFlags.DigitalSignature | KeyUsageFlags.KeyEncipherment)
                .ExtendedKeyUsage(ExtKeyUsage.ServerAuth)
                .SubjectKeyIdentifier(mlDsaKey.PublicKey)
                .AuthorityKeyIdentifier(caCert)
                // Hybrid-specific extension
                .AddCustomExtension(
                    oid: "1.3.6.1.4.1.99999.1.1",  // Example OID
                    critical: false,
                    value: "hybrid:ecdsa-p384+ml-dsa-65"
                )
                .Build()
        );
 
        return cert;
    }
}

server {
    listen 443 ssl http2;
    server_name www.example.com;
 
    # Hybrid certificate
    ssl_certificate     /etc/nginx/ssl/server-hybrid-chain.pem;
    ssl_certificate_key /etc/nginx/ssl/server-hybrid.key.pem;
 
    # TLS 1.3 only
    ssl_protocols TLSv1.3;
 
    # Hybrid Key Exchange Groups (OpenSSL 3.6+)
    ssl_ecdh_curve x25519_mlkem768:secp384r1_mlkem768:X25519:secp384r1;
 
    # Signature Algorithms (Hybrid)
    ssl_conf_command SignatureAlgorithms ed25519:ecdsa_secp384r1_sha384:mldsa65:hybrid_mldsa65_ecdsa_p384;
 
    # OCSP Stapling
    ssl_stapling on;
    ssl_stapling_verify on;
 
    location / {
        proxy_pass http://backend:8080;
    }
}

public HttpClient CreateHybridTlsClient()
{
    var handler = new SocketsHttpHandler
    {
        SslOptions = new SslClientAuthenticationOptions
        {
            EnabledSslProtocols = SslProtocols.Tls13,
 
            // Cipher suites (TLS 1.3 default good)
            CipherSuitesPolicy = new CipherSuitesPolicy(new[]
            {
                TlsCipherSuite.TLS_AES_256_GCM_SHA384,
                TlsCipherSuite.TLS_CHACHA20_POLY1305_SHA256
            }),
 
            RemoteCertificateValidationCallback = (sender, certificate, chain, errors) =>
            {
                if (errors != SslPolicyErrors.None)
                {
                    Console.WriteLine($"TLS Error: {errors}");
                    return false;
                }
 
                // Verify hybrid signature
                if (certificate is X509Certificate2 cert2)
                {
                    var algorithm = cert2.SignatureAlgorithm.FriendlyName;
                    Console.WriteLine($"Server Signature Algorithm: {algorithm}");
 
                    // Check if PQ algorithm is used
                    if (!algorithm.Contains("ML-DSA") && !algorithm.Contains("hybrid"))
                    {
                        Console.WriteLine("WARNING: Server is not using PQ certificate");
                    }
                }
 
                return true;
            }
        }
    };
 
    return new HttpClient(handler);
}

public class HybridTlsClientWithFallback
{
    public async Task<HttpResponseMessage> GetWithFallback(string url)
    {
        // Attempt 1: Hybrid TLS
        try
        {
            using var hybridClient = CreateHybridTlsClient();
            return await hybridClient.GetAsync(url);
        }
        catch (AuthenticationException ex) when (IsHybridNotSupported(ex))
        {
            Console.WriteLine("Hybrid TLS not supported, falling back to classic...");
        }
 
        // Fallback: Classic TLS 1.3
        using var classicClient = CreateClassicTlsClient();
        return await classicClient.GetAsync(url);
    }
 
    private bool IsHybridNotSupported(Exception ex)
    {
        return ex.Message.Contains("no common signature algorithm") ||
               ex.Message.Contains("unsupported group");
    }
 
    private HttpClient CreateClassicTlsClient()
    {
        var handler = new HttpClientHandler
        {
            SslProtocols = SslProtocols.Tls13
        };
        return new HttpClient(handler);
    }
}

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