~~NOTOC~~
====== Scenario 5.1: Chain Building ======

<WRAP round info>
**Category:** [[.:start|Validation & Trust]] \\
**Complexity:** *** (Medium) \\
**Prerequisites:** Certificates, Trust Store \\
**Estimated Time:** 10-15 minutes
</WRAP>

----

===== Description =====

This scenario describes **building a certificate chain** (Chain Building) from an end-entity certificate to the trust anchor. Chain building is the first step in certificate validation.

**Process:**
  - End-entity certificate -> Find issuer
  - Issuer -> Find its issuer
  - Repeat until trust anchor is reached

----

===== Workflow =====

<mermaid>
flowchart BT
    EE[End-Entity Certificate] --> INT[Intermediate CA]
    INT --> ROOT[Root CA]
    ROOT --> TRUST{In Trust Store?}
    TRUST -->|Yes| OK[Chain complete]
    TRUST -->|No| FAIL[Chain incomplete]

    style OK fill:#e8f5e9
    style FAIL fill:#ffebee
</mermaid>

----

===== Code Example (C#) =====

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

using var ctx = PqCryptoContext.Initialize();

// Load certificate to validate
var serverCert = ctx.LoadCertificate("server.crt.pem");

// Configure trust store
using var trustStore = new X509Store(StoreName.Root, StoreLocation.LocalMachine);
trustStore.Open(OpenFlags.ReadOnly);

// Intermediate certificates (optional)
var intermediates = new X509Certificate2Collection();
intermediates.Add(ctx.LoadCertificate("intermediate-ca.crt.pem"));

// Configure chain builder
var chain = new X509Chain();
chain.ChainPolicy.ExtraStore.AddRange(intermediates);
chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;  // Only chain building first
chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllFlags;  // Only build chain

// Build chain
bool built = chain.Build(serverCert);

Console.WriteLine($"Chain Building: {(built ? "Successful" : "Failed")}");
Console.WriteLine($"Chain length: {chain.ChainElements.Count}");

foreach (var element in chain.ChainElements)
{
    Console.WriteLine($"  {element.ChainElementStatus.Length} Status");
    Console.WriteLine($"    Subject: {element.Certificate.Subject}");
    Console.WriteLine($"    Issuer: {element.Certificate.Issuer}");
}
</code>

----

===== Automatic AIA Fetching =====

Authority Information Access (AIA) enables automatic downloading of missing intermediate certificates:

<code csharp>
public X509Certificate2Collection FetchIntermediatesViaAia(X509Certificate2 cert)
{
    var intermediates = new X509Certificate2Collection();
    var current = cert;

    while (true)
    {
        // Read AIA extension
        var aiaUrl = GetCaIssuersUrl(current);
        if (string.IsNullOrEmpty(aiaUrl)) break;

        // Download issuer certificate
        using var http = new HttpClient();
        var certData = http.GetByteArrayAsync(aiaUrl).Result;

        var issuer = new X509Certificate2(certData);
        intermediates.Add(issuer);

        // Self-signed = Root reached
        if (issuer.Subject == issuer.Issuer) break;

        current = issuer;
    }

    return intermediates;
}

private string? GetCaIssuersUrl(X509Certificate2 cert)
{
    var aia = cert.Extensions["1.3.6.1.5.5.7.1.1"];  // AIA OID
    if (aia == null) return null;

    // Parse AIA (simplified)
    var asnData = new AsnReader(aia.RawData, AsnEncodingRules.DER);
    // ... Extract CA Issuers URL
    return null;  // Implementation depends on ASN.1 structure
}
</code>

----

===== Chain Building with PQ Certificates =====

<code csharp>
public class PqChainBuilder
{
    public X509Chain BuildPqChain(X509Certificate2 endEntity, PqCryptoContext ctx)
    {
        var chain = new X509Chain();

        // Enable PQ-specific validation
        chain.ChainPolicy.ApplicationPolicy.Add(new Oid("1.3.6.1.5.5.7.3.1"));  // serverAuth
        chain.ChainPolicy.CertificatePolicy.Add(new Oid("2.5.29.32.0"));  // anyPolicy

        // Custom trust store with PQ root CAs
        var pqTrustStore = new X509Certificate2Collection();
        pqTrustStore.Add(ctx.LoadCertificate("pq-root-ca.crt.pem"));
        chain.ChainPolicy.CustomTrustStore.AddRange(pqTrustStore);
        chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;

        chain.Build(endEntity);
        return chain;
    }
}
</code>

----

===== Chain Building Troubleshooting =====

^  Status  ^  Meaning  ^  Solution  ^
| **PartialChain** | Intermediate missing | Use AIA or add manually |
| **UntrustedRoot** | Root not in trust store | Add root to trust store |
| **NotSignatureValid** | Signature invalid | Certificate corrupted/wrong |
| **RevocationStatusUnknown** | CRL/OCSP unreachable | Adjust revocation check |

<code csharp>
// Analyze chain status
foreach (var element in chain.ChainElements)
{
    foreach (var status in element.ChainElementStatus)
    {
        Console.WriteLine($"  {element.Certificate.Subject}");
        Console.WriteLine($"    Status: {status.Status}");
        Console.WriteLine($"    Info: {status.StatusInformation}");
    }
}
</code>

----

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

^  Industry  ^  Chain Depth  ^  Trust Store  ^  Specifics  ^
| **WebPKI** | Max. 3 | Browser Root Store | CA/B Forum Rules |
| **Enterprise** | Variable | Custom Root Store | Own hierarchy |
| **Energy/SCADA** | 2-3 | Offline Store | Air-gap systems |
| **Healthcare** | 3-4 | Qualified TSP | eIDAS-compliant |

----

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

^  Relationship  ^  Scenario  ^  Description  ^
| **Next Step** | [[.:chain_validation|5.2 Chain Validation]] | Complete validation |
| **Prerequisite** | [[en:int:pqcrypt:szenarien:pki:trust_store_konfigurieren|1.4 Trust Store]] | Trust anchors |
| **Related** | [[.:revocation_check|5.3 Revocation Check]] | Revocation verification |

----

<< [[.:start|<- Validation Overview]] | [[en:int:pqcrypt:szenarien:start|^ Scenarios]] | [[.:chain_validation|5.2 Chain Validation ->]] >>

{{tag>scenario validation chain-building x509 trust}}

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