====== RevocationExtensions Class ======

Extension methods for checking certificate revocation.

----

===== Definition =====

<code csharp>
namespace WvdS.System.Security.Cryptography.X509Certificates;

public static class RevocationExtensions
</code>

----

===== Methods =====

^  Method  ^  Description  ^
| IsRevoked | Checks if a certificate is revoked |
| FetchCrlAsync | Downloads CRL from URL (from certificate extension) |
| CheckRevocationAsync | Combined: Load CRL and check revocation |
| GetCrlDistributionPoints | Extracts CRL URLs from certificate |
| GetOcspUrls | Extracts OCSP URLs from certificate |

----

===== Checking Revocation =====

**With existing CRL:**

<code csharp>
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}");
}
</code>

**Automatically load CRL:**

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

----

===== Loading CRL =====

<code csharp>
// Only load CRL, without checking
byte[]? crlData = await certificate.FetchCrlAsync();

if (crlData != null)
{
    File.WriteAllBytes("downloaded.crl", crlData);
}
</code>

**With custom HttpClient:**

<code csharp>
using var httpClient = new HttpClient();
httpClient.Timeout = TimeSpan.FromSeconds(10);

byte[]? crlData = await certificate.FetchCrlAsync(httpClient);
</code>

----

===== Reading CRL Distribution Points =====

<code csharp>
IReadOnlyList<string> crlUrls = certificate.GetCrlDistributionPoints();

foreach (var url in crlUrls)
{
    Console.WriteLine($"CRL URL: {url}");
}

// Quick check
bool hasCdp = certificate.HasCrlDistributionPoints();
</code>

----

===== Reading OCSP URLs =====

<code csharp>
IReadOnlyList<string> ocspUrls = certificate.GetOcspUrls();

foreach (var url in ocspUrls)
{
    Console.WriteLine($"OCSP Responder: {url}");
}

// Quick check
bool hasOcsp = certificate.HasOcspUrls();
</code>

----

===== RevocationResult Class =====

^  Property  ^  Type  ^  Description  ^
| ''IsRevoked'' | bool | Certificate is revoked |
| ''RevocationDate'' | DateTimeOffset? | Time of revocation |
| ''Reason'' | CrlReason? | Revocation reason |
| ''CrlVerified'' | bool | CRL signature was verified |
| ''ErrorMessage'' | string? | Error message (if check failed) |
| ''Success'' | bool | Check was successful |

----

===== CRL Cache =====

For frequent checks with automatic caching:

<code csharp>
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();
</code>

**With custom HttpClient:**

<code csharp>
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);
    // ...
}
</code>

----

===== Integration with X509Chain =====

For complete chain validation with CRL checking:

<code csharp>
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");
        }
    }
}
</code>

----

===== Offline Checking =====

For air-gapped systems without network access:

<code csharp>
// 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);
</code>

----

===== Best Practices =====

  * **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

----

===== See Also =====

  * [[.:certificaterevocationlistextensions|CertificateRevocationListExtensions]] - Creating CRLs
  * [[.:x509chainextensions|X509ChainExtensions]] - Chain validation
  * [[en:int:pqcrypt:konzepte:revocation|Revocation Concept]]

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

{{tag>crl revocation ocsp}}