~~NOTOC~~
====== Scenario 5.2: Chain Validation ======

<WRAP round info>
**Category:** [[.:start|Validation & Trust]] \\
**Complexity:** **** (High) \\
**Prerequisites:** Built certificate chain \\
**Estimated Time:** 15-20 minutes
</WRAP>

----

===== Description =====

This scenario describes **complete validation of a certificate chain** according to RFC 5280. The validation checks:

  * **Signature validity** - Each certificate signed by issuer
  * **Time validity** - All certificates within validity period
  * **Key Usage** - Permitted usage purposes
  * **Path Length** - BasicConstraints pathLenConstraint
  * **Policy Constraints** - Certificate policies
  * **Name Constraints** - Permitted/excluded names

----

===== Workflow =====

<mermaid>
flowchart TD
    BUILD[Chain Building] --> SIG[Verify signature]
    SIG --> TIME[Time validity]
    TIME --> KU[Key Usage]
    KU --> PATH[Path Length]
    PATH --> POLICY[Policy Constraints]
    POLICY --> NAME[Name Constraints]
    NAME --> REV[Revocation Check]
    REV --> OK{All OK?}
    OK -->|Yes| VALID[Certificate valid]
    OK -->|No| INVALID[Certificate invalid]

    style VALID fill:#e8f5e9
    style INVALID 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
var serverCert = ctx.LoadCertificate("server.crt.pem");

// Complete chain validation
var chain = new X509Chain();
chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
chain.ChainPolicy.RevocationFlag = X509RevocationFlag.EntireChain;
chain.ChainPolicy.UrlRetrievalTimeout = TimeSpan.FromSeconds(30);
chain.ChainPolicy.VerificationTime = DateTime.UtcNow;

// Additional intermediate certificates
chain.ChainPolicy.ExtraStore.Add(ctx.LoadCertificate("intermediate-ca.crt.pem"));

// Build and validate chain
bool isValid = chain.Build(serverCert);

// Evaluate result
Console.WriteLine($"Chain validation: {(isValid ? "VALID" : "INVALID")}");

if (!isValid)
{
    foreach (var element in chain.ChainElements)
    {
        foreach (var status in element.ChainElementStatus)
        {
            Console.WriteLine($"  {element.Certificate.Subject}");
            Console.WriteLine($"    Error: {status.Status}");
            Console.WriteLine($"    Details: {status.StatusInformation}");
        }
    }
}
</code>

----

===== Detailed Validation =====

<code csharp>
public class ChainValidator
{
    public ValidationResult ValidateChain(X509Certificate2 certificate, ValidationOptions options)
    {
        var result = new ValidationResult { IsValid = true };

        using var chain = new X509Chain();
        ConfigureChainPolicy(chain.ChainPolicy, options);

        if (!chain.Build(certificate))
        {
            result.IsValid = false;
            result.Errors = ExtractErrors(chain);
        }

        // Additional PQ checks
        if (options.RequirePostQuantum)
        {
            ValidatePqRequirements(chain, result);
        }

        return result;
    }

    private void ConfigureChainPolicy(X509ChainPolicy policy, ValidationOptions options)
    {
        policy.RevocationMode = options.CheckRevocation
            ? X509RevocationMode.Online
            : X509RevocationMode.NoCheck;

        policy.RevocationFlag = X509RevocationFlag.EntireChain;
        policy.VerificationTime = options.ValidationTime ?? DateTime.UtcNow;

        // Application Policies (Extended Key Usage)
        if (options.RequiredEku != null)
        {
            policy.ApplicationPolicy.Add(options.RequiredEku);
        }

        // Certificate Policies
        if (options.RequiredPolicies != null)
        {
            foreach (var policyOid in options.RequiredPolicies)
            {
                policy.CertificatePolicy.Add(policyOid);
            }
        }

        // Custom Trust Store
        if (options.CustomTrustStore != null)
        {
            policy.CustomTrustStore.AddRange(options.CustomTrustStore);
            policy.TrustMode = X509ChainTrustMode.CustomRootTrust;
        }
    }

    private void ValidatePqRequirements(X509Chain chain, ValidationResult result)
    {
        foreach (var element in chain.ChainElements)
        {
            var cert = element.Certificate;
            var algorithm = cert.PublicKey.Oid.Value;

            // Check if PQ algorithm is used
            if (!IsPqAlgorithm(algorithm))
            {
                result.Warnings.Add($"{cert.Subject}: No PQ algorithm ({algorithm})");
            }
        }
    }

    private bool IsPqAlgorithm(string oid)
    {
        return oid switch
        {
            "2.16.840.1.101.3.4.3.17" => true,  // ML-DSA-44
            "2.16.840.1.101.3.4.3.18" => true,  // ML-DSA-65
            "2.16.840.1.101.3.4.3.19" => true,  // ML-DSA-87
            _ => false
        };
    }
}
</code>

----

===== Validation Options =====

<code csharp>
public class ValidationOptions
{
    // Validation time
    public DateTime? ValidationTime { get; set; }

    // Revocation check
    public bool CheckRevocation { get; set; } = true;

    // Extended Key Usage
    public Oid? RequiredEku { get; set; }

    // Certificate Policies
    public List<Oid>? RequiredPolicies { get; set; }

    // Custom Trust Store
    public X509Certificate2Collection? CustomTrustStore { get; set; }

    // PQ requirements
    public bool RequirePostQuantum { get; set; }

    // Timeout for online checks
    public TimeSpan UrlTimeout { get; set; } = TimeSpan.FromSeconds(30);
}

// Example: TLS server validation
var tlsOptions = new ValidationOptions
{
    RequiredEku = new Oid("1.3.6.1.5.5.7.3.1"),  // serverAuth
    CheckRevocation = true,
    RequirePostQuantum = true
};

// Example: Code signing validation
var codeSigningOptions = new ValidationOptions
{
    RequiredEku = new Oid("1.3.6.1.5.5.7.3.3"),  // codeSigning
    CheckRevocation = true,
    ValidationTime = signatureTimestamp  // Time of signature
};
</code>

----

===== Chain Status Codes =====

^  Status  ^  Meaning  ^  Critical?  ^  Solution  ^
| **NoError** | No errors | No | - |
| **NotTimeValid** | Expired/not yet valid | Yes | Renewal |
| **NotTimeNested** | Time periods don't overlap | Yes | Correct chain |
| **Revoked** | Revoked | Yes | New certificate |
| **NotSignatureValid** | Signature invalid | Yes | Check chain |
| **NotValidForUsage** | Wrong usage purpose | Yes | Correct certificate |
| **UntrustedRoot** | Root not trusted | Yes | Trust Store |
| **RevocationStatusUnknown** | CRL/OCSP unreachable | Warning | Offline check |
| **PartialChain** | Chain incomplete | Yes | Add intermediates |

----

===== Industry-Specific Validation =====

^  Industry  ^  Additional Checks  ^  Policy OIDs  ^
| **WebPKI** | CT Logs, EV Policies | CA/B Forum |
| **eIDAS** | QC Statements, TSL Check | 0.4.0.194121.1.* |
| **Healthcare** | Professional attributes | gematik OIDs |
| **Automotive** | ECU Policies | V2X-specific |

----

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

^  Relationship  ^  Scenario  ^  Description  ^
| **Prerequisite** | [[.:chain_building|5.1 Chain Building]] | Build chain |
| **Next Step** | [[.:revocation_check|5.3 Revocation Check]] | Revocation verification |
| **Related** | [[.:policy_validation|5.4 Policy Validation]] | Policy verification |

----

<< [[.:chain_building|<- 5.1 Chain Building]] | [[.:start|^ Validation Overview]] | [[.:revocation_check|5.3 Revocation Check ->]] >>

{{tag>scenario validation chain x509 rfc5280}}

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