Extension methods for checking certificate revocation.

namespace WvdS.System.Security.Cryptography.X509Certificates;
 
public static class RevocationExtensions

With existing CRL:

using System.Security.Cryptography.X509Certificates;
using WvdS.System.Security.Cryptography.X509Certificates;
 
var certificate = new X509Certificate2("user.cer");
byte[] crlData = File.ReadAllBytes("ca.crl");
var caCert = new X509Certificate2("ca.cer");
 
// Check with signature verification
RevocationResult result = certificate.IsRevoked(crlData, caCert, CryptoMode.Hybrid);
 
if (result.Success)
{
    if (result.IsRevoked)
    {
        Console.WriteLine($"Certificate revoked on: {result.RevocationDate}");
        Console.WriteLine($"Reason: {result.Reason}");
    }
    else
    {
        Console.WriteLine("Certificate is valid");
    }
}
else
{
    Console.WriteLine($"Error: {result.ErrorMessage}");
}

Automatically load CRL:

// Loads CRL from URL in the certificate's CDP extension
RevocationResult result = await certificate.CheckRevocationAsync(
    caCert,
    mode: CryptoMode.Hybrid);

// Only load CRL, without checking
byte[]? crlData = await certificate.FetchCrlAsync();
 
if (crlData != null)
{
    File.WriteAllBytes("downloaded.crl", crlData);
}

With custom HttpClient:

using var httpClient = new HttpClient();
httpClient.Timeout = TimeSpan.FromSeconds(10);
 
byte[]? crlData = await certificate.FetchCrlAsync(httpClient);

IReadOnlyList<string> crlUrls = certificate.GetCrlDistributionPoints();
 
foreach (var url in crlUrls)
{
    Console.WriteLine($"CRL URL: {url}");
}
 
// Quick check
bool hasCdp = certificate.HasCrlDistributionPoints();

IReadOnlyList<string> ocspUrls = certificate.GetOcspUrls();
 
foreach (var url in ocspUrls)
{
    Console.WriteLine($"OCSP Responder: {url}");
}
 
// Quick check
bool hasOcsp = certificate.HasOcspUrls();

For frequent checks with automatic caching:

using var cache = new CrlCache(defaultCacheDuration: TimeSpan.FromHours(1));
 
// Check with automatic CRL caching
RevocationResult result1 = await cache.CheckRevocationAsync(cert1, caCert);
RevocationResult result2 = await cache.CheckRevocationAsync(cert2, caCert); // CRL from cache
 
// Clear cache
cache.Clear();

With custom HttpClient:

using var httpClient = new HttpClient();
using var cache = new CrlCache(httpClient, TimeSpan.FromMinutes(30));
 
// Check multiple certificates
foreach (var cert in certificates)
{
    var result = await cache.CheckRevocationAsync(cert, caCert, CryptoMode.Hybrid);
    // ...
}

For complete chain validation with CRL checking:

using var chain = new X509Chain();
chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
chain.ChainPolicy.RevocationFlag = X509RevocationFlag.EntireChain;
 
bool valid = chain.Build(certificate, CryptoMode.Hybrid);
 
if (!valid)
{
    foreach (var status in chain.ChainStatus)
    {
        if (status.Status == X509ChainStatusFlags.Revoked)
        {
            Console.WriteLine("Certificate in chain revoked");
        }
    }
}

For air-gapped systems without network access:

// Download CRLs in advance and store
var crlFiles = new Dictionary<string, byte[]>
{
    ["CN=Root CA"] = File.ReadAllBytes("root.crl"),
    ["CN=Issuing CA"] = File.ReadAllBytes("issuing.crl")
};
 
// Offline check
RevocationResult result = certificate.IsRevoked(crlFiles["CN=Issuing CA"], caCert);

• Enable CRL caching: Use CrlCache for repeated checks
• Set timeouts: Configure HttpClient timeouts for network operations
• Implement fallback: Check OCSP when CRL is not available
• Offline copies: Keep local CRL copies for critical systems

• CertificateRevocationListExtensions - Creating CRLs
• X509ChainExtensions - Chain validation
• Revocation Concept

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