~~NOTOC~~
====== Scenario 6.2: OCSP Responder ======

<WRAP round info>
**Category:** [[.:start|Revocation]] \\
**Complexity:** ⭐⭐⭐⭐ (High) \\
**Prerequisites:** CA infrastructure, OCSP signing certificate \\
**Estimated Time:** 30-45 Minutes
</WRAP>

----

===== Description =====

This scenario describes the **implementation of an OCSP Responder** (Online Certificate Status Protocol, RFC 6960). OCSP enables real-time certificate status verification.

**Advantages over CRL:**
  * Real-time status
  * Smaller responses
  * No complete download
  * Privacy (only one certificate checked)

----

===== Workflow =====

<mermaid>
flowchart LR
    CLIENT[Client] -->|OCSP Request| RESPONDER[OCSP Responder]
    RESPONDER --> DB[(Revocation DB)]
    RESPONDER -->|OCSP Response| CLIENT

    subgraph Response
        GOOD[good]
        REVOKED[revoked]
        UNKNOWN[unknown]
    end

    style RESPONDER fill:#e8f5e9
</mermaid>

----

===== Code Example: Create OCSP Request =====

<code csharp>
using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;

using var ctx = PqCryptoContext.Initialize();

// Load certificate to check and issuer
var cert = ctx.LoadCertificate("server.crt.pem");
var issuer = ctx.LoadCertificate("intermediate-ca.crt.pem");

// Create OCSP request
var ocspRequest = ctx.CreateOcspRequest(
    certificate: cert,
    issuer: issuer,
    hashAlgorithm: HashAlgorithmName.SHA256,
    nonce: true  // Replay protection
);

// Request as DER
byte[] requestBytes = ocspRequest.Encode();

// Send request
using var http = new HttpClient();
var content = new ByteArrayContent(requestBytes);
content.Headers.ContentType = new MediaTypeHeaderValue("application/ocsp-request");

var ocspUrl = ctx.GetOcspUrl(cert);  // Read AIA extension
var response = await http.PostAsync(ocspUrl, content);

var responseBytes = await response.Content.ReadAsByteArrayAsync();

// Parse response
var status = ctx.ParseOcspResponse(responseBytes, cert, issuer);

Console.WriteLine($"OCSP Status: {status.Status}");
Console.WriteLine($"  Produced: {status.ProducedAt}");
Console.WriteLine($"  This Update: {status.ThisUpdate}");
Console.WriteLine($"  Next Update: {status.NextUpdate}");
</code>

----

===== Code Example: Implement OCSP Responder =====

<code csharp>
public class OcspResponder
{
    private readonly X509Certificate2 _responderCert;
    private readonly AsymmetricAlgorithm _responderKey;
    private readonly IRevocationDatabase _revocationDb;
    private readonly PqCryptoContext _ctx;

    public OcspResponder(
        X509Certificate2 responderCert,
        AsymmetricAlgorithm responderKey,
        IRevocationDatabase revocationDb)
    {
        _responderCert = responderCert;
        _responderKey = responderKey;
        _revocationDb = revocationDb;
        _ctx = PqCryptoContext.Initialize();
    }

    public byte[] ProcessRequest(byte[] requestBytes)
    {
        try
        {
            // Parse request
            var request = _ctx.ParseOcspRequest(requestBytes);

            // Response builder
            var responseBuilder = new OcspResponseBuilder();

            foreach (var certId in request.CertificateIds)
            {
                // Look up status in database
                var revocationInfo = _revocationDb.GetRevocationStatus(
                    certId.IssuerNameHash,
                    certId.IssuerKeyHash,
                    certId.SerialNumber
                );

                if (revocationInfo == null)
                {
                    // Unknown certificate
                    responseBuilder.AddStatus(certId, OcspCertStatus.Unknown);
                }
                else if (revocationInfo.IsRevoked)
                {
                    // Revoked
                    responseBuilder.AddStatus(
                        certId,
                        OcspCertStatus.Revoked,
                        revocationInfo.RevocationTime,
                        revocationInfo.RevocationReason
                    );
                }
                else
                {
                    // Valid
                    responseBuilder.AddStatus(certId, OcspCertStatus.Good);
                }
            }

            // Sign response
            return responseBuilder.Build(
                responderCert: _responderCert,
                responderKey: _responderKey,
                producedAt: DateTimeOffset.UtcNow,
                nextUpdate: DateTimeOffset.UtcNow.AddMinutes(5),
                nonce: request.Nonce,
                mode: CryptoMode.Hybrid
            );
        }
        catch (Exception ex)
        {
            // Internal Error Response
            return BuildErrorResponse(OcspResponseStatus.InternalError);
        }
    }

    private byte[] BuildErrorResponse(OcspResponseStatus status)
    {
        return _ctx.BuildOcspErrorResponse(status);
    }
}
</code>

----

===== Create OCSP Responder Certificate =====

<code csharp>
// OCSP Responder needs special certificate
using var ocspKey = ctx.GenerateKeyPair(PqAlgorithm.MlDsa65);

var dn = new DnBuilder()
    .AddCN("OCSP Responder - Example CA")
    .AddO("Example GmbH")
    .AddC("DE")
    .Build();

var csr = ctx.CreateCertificateRequest(ocspKey, dn);

var ocspCert = ctx.IssueCertificate(
    csr,
    issuerCert: caCert,
    issuerKey: caKey,
    validDays: 30,  // Short validity for OCSP responder
    extensions: new ExtBuilder()
        .BasicConstraints(ca: false, critical: true)
        // OCSP Signing Key Usage
        .KeyUsage(KeyUsageFlags.DigitalSignature, critical: true)
        // OCSP Signing EKU
        .ExtendedKeyUsage(ExtKeyUsage.OcspSigning)
        // id-pkix-ocsp-nocheck Extension (no revocation check for responder)
        .OcspNoCheck()
        .SubjectKeyIdentifier(ocspKey.PublicKey)
        .AuthorityKeyIdentifier(caCert)
        .Build()
);

ocspCert.ToPemFile("ocsp-responder.crt.pem");
ocspKey.ToEncryptedPemFile("ocsp-responder.key.pem", "OcspPassword!");
</code>

----

===== ASP.NET Core OCSP Endpoint =====

<code csharp>
[ApiController]
[Route("ocsp")]
public class OcspController : ControllerBase
{
    private readonly OcspResponder _responder;

    public OcspController(OcspResponder responder)
    {
        _responder = responder;
    }

    // POST /ocsp
    [HttpPost]
    [Consumes("application/ocsp-request")]
    [Produces("application/ocsp-response")]
    public IActionResult Post()
    {
        using var ms = new MemoryStream();
        Request.Body.CopyTo(ms);
        var requestBytes = ms.ToArray();

        var responseBytes = _responder.ProcessRequest(requestBytes);

        return File(responseBytes, "application/ocsp-response");
    }

    // GET /ocsp/{base64-request} (for OCSP GET)
    [HttpGet("{encodedRequest}")]
    [Produces("application/ocsp-response")]
    public IActionResult Get(string encodedRequest)
    {
        var requestBytes = Convert.FromBase64String(
            Uri.UnescapeDataString(encodedRequest)
        );

        var responseBytes = _responder.ProcessRequest(requestBytes);

        return File(responseBytes, "application/ocsp-response");
    }
}
</code>

----

===== OCSP Response Status =====

^  Status  ^  HTTP  ^  Meaning  ^
| **successful** | 200 | Request processed |
| **malformedRequest** | 200 | Invalid request |
| **internalError** | 200 | Server error |
| **tryLater** | 200 | Temporarily unavailable |
| **sigRequired** | 200 | Signature required |
| **unauthorized** | 200 | Not authorized |

<WRAP round important>
**Important:** OCSP always returns HTTP 200! The status is encoded in the response.
</WRAP>

----

===== Industry-Specific OCSP Requirements =====

^  Industry  ^  Response Caching  ^  Stapling  ^  High Availability  ^
| **WebPKI** | Maximum 10 min | Mandatory | 99.9% |
| **Enterprise** | Maximum 1 hour | Recommended | Per SLA |
| **Financial Sector** | Maximum 5 min | Mandatory | 99.99% |
| **Healthcare** | Maximum 1 hour | Optional | 99.9% |

----

===== Related Scenarios =====

^  Relationship  ^  Scenario  ^  Description  ^
| **Alternative** | [[.:crl_erstellen|6.1 Create CRL]] | Offline verification |
| **Related** | [[en:int:pqcrypt:szenarien:validierung:revocation_check|5.3 Revocation Check]] | Client-side verification |
| **Prerequisite** | [[.:zertifikat_widerrufen|6.4 Revoke Certificate]] | Revocation process |

----

<< [[.:crl_erstellen|← 6.1 Create CRL]] | [[.:start|↑ Revocation Overview]] | [[.:delta_crl|6.3 Delta CRL →]] >>

{{tag>scenario revocation ocsp responder realtime}}

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