This scenario describes cross-platform interoperability of certificates and keys. With PQ cryptography, special care is required as not all systems yet support ML-DSA/ML-KEM.

Interoperability Challenges:

• Format differences - PEM vs. DER vs. PFX
• Algorithm support - Classical vs. PQ vs. Hybrid
• Platform specifics - Windows, Linux, macOS, Java
• Version differences - OpenSSL 1.x vs. 3.x

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
 
public class CertificateConverter
{
    public byte[] ConvertFormat(
        byte[] sourceData,
        CertificateFormat sourceFormat,
        CertificateFormat targetFormat,
        string password = null)
    {
        // 1. Load into .NET object
        var cert = LoadCertificate(sourceData, sourceFormat, password);
 
        // 2. Export in target format
        return ExportCertificate(cert, targetFormat, password);
    }
 
    private X509Certificate2 LoadCertificate(
        byte[] data,
        CertificateFormat format,
        string password)
    {
        return format switch
        {
            CertificateFormat.Der =>
                new X509Certificate2(data),
 
            CertificateFormat.Pem =>
                X509Certificate2.CreateFromPem(Encoding.UTF8.GetString(data)),
 
            CertificateFormat.Pfx =>
                new X509Certificate2(data, password,
                    X509KeyStorageFlags.Exportable),
 
            _ => throw new NotSupportedException($"Format {format} not supported")
        };
    }
 
    private byte[] ExportCertificate(
        X509Certificate2 cert,
        CertificateFormat format,
        string password)
    {
        return format switch
        {
            CertificateFormat.Der =>
                cert.RawData,
 
            CertificateFormat.Pem =>
                Encoding.UTF8.GetBytes(cert.ExportCertificatePem()),
 
            CertificateFormat.Pfx =>
                cert.Export(X509ContentType.Pfx, password),
 
            _ => throw new NotSupportedException($"Format {format} not supported")
        };
    }
}
 
public enum CertificateFormat
{
    Der,
    Pem,
    Pfx,
    P7b
}

public class JavaKeystoreInterop
{
    public void ExportForJava(
        X509Certificate2 certificate,
        X509Certificate2Collection chain,
        string jksPath,
        string storePassword,
        string keyPassword,
        string alias)
    {
        // Create PKCS#12 (Java can import this)
        var collection = new X509Certificate2Collection { certificate };
        foreach (var ca in chain)
        {
            collection.Add(ca);
        }
 
        var pfxBytes = collection.Export(X509ContentType.Pfx, keyPassword);
        var pfxPath = Path.GetTempFileName();
        File.WriteAllBytes(pfxPath, pfxBytes);
 
        try
        {
            // Call keytool
            var process = Process.Start(new ProcessStartInfo
            {
                FileName = "keytool",
                Arguments = $"-importkeystore " +
                    $"-srckeystore \"{pfxPath}\" " +
                    $"-srcstoretype PKCS12 " +
                    $"-srcstorepass \"{keyPassword}\" " +
                    $"-destkeystore \"{jksPath}\" " +
                    $"-deststoretype JKS " +
                    $"-deststorepass \"{storePassword}\" " +
                    $"-destkeypass \"{keyPassword}\" " +
                    $"-alias \"{alias}\"",
                RedirectStandardOutput = true,
                RedirectStandardError = true,
                UseShellExecute = false
            });
 
            process.WaitForExit();
 
            if (process.ExitCode == 0)
            {
                Console.WriteLine($"JKS created: {jksPath}");
            }
            else
            {
                var error = process.StandardError.ReadToEnd();
                throw new Exception($"keytool error: {error}");
            }
        }
        finally
        {
            File.Delete(pfxPath);
        }
    }
 
    public X509Certificate2 ImportFromJks(
        string jksPath,
        string storePassword,
        string keyPassword,
        string alias)
    {
        var pfxPath = Path.GetTempFileName();
 
        try
        {
            // Convert JKS to PKCS#12
            var process = Process.Start(new ProcessStartInfo
            {
                FileName = "keytool",
                Arguments = $"-importkeystore " +
                    $"-srckeystore \"{jksPath}\" " +
                    $"-srcstoretype JKS " +
                    $"-srcstorepass \"{storePassword}\" " +
                    $"-srcalias \"{alias}\" " +
                    $"-destkeystore \"{pfxPath}\" " +
                    $"-deststoretype PKCS12 " +
                    $"-deststorepass \"{keyPassword}\"",
                RedirectStandardOutput = true,
                RedirectStandardError = true,
                UseShellExecute = false
            });
 
            process.WaitForExit();
 
            if (process.ExitCode != 0)
            {
                throw new Exception(process.StandardError.ReadToEnd());
            }
 
            // Load PKCS#12
            return new X509Certificate2(pfxPath, keyPassword,
                X509KeyStorageFlags.Exportable);
        }
        finally
        {
            if (File.Exists(pfxPath))
                File.Delete(pfxPath);
        }
    }
}

public class HybridCertificateFallback
{
    public X509Certificate2 GetCompatibleCertificate(
        X509Certificate2 hybridCert,
        PlatformCapabilities targetCapabilities)
    {
        using var ctx = PqCryptoContext.Initialize();
 
        // Check if hybrid certificate
        var algorithm = hybridCert.GetKeyAlgorithm();
 
        if (!IsHybridAlgorithm(algorithm))
        {
            // Not hybrid - return directly
            return hybridCert;
        }
 
        // Target supports PQ?
        if (targetCapabilities.SupportsPqAlgorithms)
        {
            return hybridCert;
        }
 
        // Fallback: Extract classical part
        Console.WriteLine("Target does not support PQ - using classical fallback");
 
        // Load alternative certificate chain (if available)
        var classicalCert = FindClassicalAlternative(hybridCert);
 
        if (classicalCert != null)
        {
            return classicalCert;
        }
 
        throw new NotSupportedException(
            "Target system supports neither PQ nor is a classical fallback available"
        );
    }
 
    private bool IsHybridAlgorithm(string algorithm)
    {
        return algorithm.Contains("ML-DSA") ||
               algorithm.Contains("ML-KEM") ||
               algorithm.Contains("DILITHIUM") ||
               algorithm.Contains("KYBER");
    }
 
    private X509Certificate2 FindClassicalAlternative(X509Certificate2 hybridCert)
    {
        // Search for certificate with same subject but classical algorithm
        // This requires corresponding infrastructure (Dual-Certificates)
 
        using var store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
        store.Open(OpenFlags.ReadOnly);
 
        var alternatives = store.Certificates
            .Find(X509FindType.FindBySubjectDistinguishedName, hybridCert.Subject, validOnly: true)
            .Where(c => !IsHybridAlgorithm(c.GetKeyAlgorithm()))
            .OrderByDescending(c => c.NotAfter)
            .FirstOrDefault();
 
        return alternatives;
    }
}
 
public class PlatformCapabilities
{
    public bool SupportsPqAlgorithms { get; set; }
    public bool SupportsTls13 { get; set; }
    public string[] SupportedKeyExchanges { get; set; }
    public string[] SupportedSignatures { get; set; }
 
    public static PlatformCapabilities Detect()
    {
        return new PlatformCapabilities
        {
            SupportsPqAlgorithms = CheckPqSupport(),
            SupportsTls13 = CheckTls13Support(),
            SupportedKeyExchanges = GetSupportedKeyExchanges(),
            SupportedSignatures = GetSupportedSignatures()
        };
    }
 
    private static bool CheckPqSupport()
    {
        try
        {
            // Check if OpenSSL 3.6+ with PQ provider is available
            return OpenSslInterop.IsOpenSsl36OrNewer();
        }
        catch
        {
            return false;
        }
    }
 
    private static bool CheckTls13Support()
    {
        return Environment.OSVersion.Platform == PlatformID.Win32NT
            ? Environment.OSVersion.Version >= new Version(10, 0, 17763)  // Windows 1809+
            : true;  // Linux/macOS have TLS 1.3
    }
 
    private static string[] GetSupportedKeyExchanges() =>
        new[] { "X25519", "secp384r1", "secp256r1" };
 
    private static string[] GetSupportedSignatures() =>
        new[] { "RSA", "ECDSA", "Ed25519" };
}

public class CrossPlatformChainValidator
{
    public ValidationResult ValidateForPlatform(
        X509Certificate2 certificate,
        TargetPlatform platform)
    {
        var result = new ValidationResult { Platform = platform };
 
        // 1. Check algorithm compatibility
        var algorithm = certificate.GetKeyAlgorithm();
        result.AlgorithmSupported = IsAlgorithmSupported(algorithm, platform);
 
        if (!result.AlgorithmSupported)
        {
            result.Errors.Add($"Algorithm {algorithm} not supported on {platform}");
        }
 
        // 2. Check key size
        var keySize = certificate.GetRSAPublicKey()?.KeySize
            ?? certificate.GetECDsaPublicKey()?.KeySize
            ?? 0;
 
        result.KeySizeSupported = IsKeySizeSupported(keySize, platform);
 
        // 3. Check extensions
        foreach (var ext in certificate.Extensions)
        {
            if (!IsExtensionSupported(ext.Oid.Value, platform))
            {
                result.Warnings.Add($"Extension {ext.Oid.FriendlyName} may not be supported");
            }
        }
 
        // 4. Validate chain
        using var chain = new X509Chain();
        result.ChainValid = chain.Build(certificate);
 
        result.IsValid = result.AlgorithmSupported &&
                         result.KeySizeSupported &&
                         result.ChainValid &&
                         !result.Errors.Any();
 
        return result;
    }
 
    private bool IsAlgorithmSupported(string algorithm, TargetPlatform platform)
    {
        var pqAlgorithms = new[] { "ML-DSA", "ML-KEM", "DILITHIUM", "KYBER" };
        var isPq = pqAlgorithms.Any(pq => algorithm.Contains(pq));
 
        if (!isPq) return true;  // Classical = supported everywhere
 
        return platform switch
        {
            TargetPlatform.DotNet9 => true,
            TargetPlatform.OpenSsl36 => true,
            TargetPlatform.Java21BouncyCastle => true,
            _ => false
        };
    }
 
    private bool IsKeySizeSupported(int keySize, TargetPlatform platform)
    {
        // Most platforms support RSA 2048-4096, ECDSA P-256/P-384
        return keySize >= 2048;
    }
 
    private bool IsExtensionSupported(string oid, TargetPlatform platform)
    {
        // Standard extensions are supported everywhere
        var standardOids = new[]
        {
            "2.5.29.15",  // Key Usage
            "2.5.29.17",  // SAN
            "2.5.29.19",  // Basic Constraints
            "2.5.29.37"   // Extended Key Usage
        };
 
        return standardOids.Contains(oid);
    }
}
 
public enum TargetPlatform
{
    Windows,
    Linux,
    MacOS,
    Java21BouncyCastle,
    DotNet9,
    OpenSsl36,
    Browser
}
 
public class ValidationResult
{
    public TargetPlatform Platform { get; set; }
    public bool IsValid { get; set; }
    public bool AlgorithmSupported { get; set; }
    public bool KeySizeSupported { get; set; }
    public bool ChainValid { get; set; }
    public List<string> Errors { get; set; } = new();
    public List<string> Warnings { get; set; } = new();
}

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