~~NOTOC~~
====== Scenario 8.4: Verify Signature ======

<WRAP round info>
**Category:** [[.:start|Digital Signatures]] \\
**Complexity:** ⭐⭐⭐ (Medium) \\
**Prerequisites:** Signed data, signature certificate \\
**Estimated Time:** 10-15 Minutes
</WRAP>

----

===== Description =====

This scenario describes the **complete verification of digital signatures**. Proper verification includes:

  * **Signature verification** - Cryptographic validation
  * **Certificate verification** - Chain, revocation, validity
  * **Timestamp verification** - If present
  * **Policy verification** - Intended usage

----

===== Workflow =====

<mermaid>
flowchart TD
    SIG[Signature] --> CRYPTO[Crypto Verification]
    CRYPTO --> CERT[Verify Certificate]
    CERT --> CHAIN[Chain Building]
    CHAIN --> REV[Revocation Check]
    REV --> TIME[Time Verification]
    TIME --> TS{Timestamp?}
    TS -->|Yes| TS_CHECK[Verify Timestamp]
    TS -->|No| POLICY[Verify Policy]
    TS_CHECK --> POLICY
    POLICY --> RESULT{All OK?}
    RESULT -->|Yes| VALID[Signature VALID]
    RESULT -->|No| INVALID[Signature INVALID]

    style VALID fill:#e8f5e9
    style INVALID fill:#ffebee
</mermaid>

----

===== Code Example: Complete Verification =====

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

using var ctx = PqCryptoContext.Initialize();

public class SignatureVerifier
{
    public VerificationResult Verify(
        byte[] data,
        byte[] signature,
        X509Certificate2 signerCert,
        VerificationOptions options)
    {
        var result = new VerificationResult();

        try
        {
            // 1. Cryptographic signature verification
            result.SignatureValid = ctx.VerifyData(
                data: data,
                signature: signature,
                certificate: signerCert,
                hashAlgorithm: options.HashAlgorithm,
                mode: options.CryptoMode
            );

            if (!result.SignatureValid)
            {
                result.Error = "Cryptographic signature invalid";
                return result;
            }

            // 2. Validate certificate chain
            var chain = new X509Chain();
            ConfigureChainPolicy(chain.ChainPolicy, options);

            result.CertificateValid = chain.Build(signerCert);
            result.ChainStatus = chain.ChainStatus
                .Select(s => s.Status.ToString())
                .ToList();

            if (!result.CertificateValid)
            {
                result.Error = $"Certificate invalid: {string.Join(", ", result.ChainStatus)}";
                return result;
            }

            // 3. Verify Extended Key Usage
            if (options.RequiredEku != null)
            {
                result.EkuValid = HasExtendedKeyUsage(signerCert, options.RequiredEku);
                if (!result.EkuValid)
                {
                    result.Error = $"Required EKU ({options.RequiredEku}) missing";
                    return result;
                }
            }

            // 4. Verify signature time
            var signatureTime = options.SignatureTime ?? DateTimeOffset.UtcNow;
            result.TimeValid = signatureTime >= signerCert.NotBefore &&
                               signatureTime <= signerCert.NotAfter;

            if (!result.TimeValid)
            {
                result.Error = "Certificate was not valid at signature time";
                return result;
            }

            result.IsValid = true;
            result.SignerSubject = signerCert.Subject;
            result.SignatureTime = signatureTime;
        }
        catch (Exception ex)
        {
            result.IsValid = false;
            result.Error = ex.Message;
        }

        return result;
    }
}
</code>

----

===== Code Example: Verify CMS/PKCS#7 Signature =====

<code csharp>
public VerificationResult VerifyCms(byte[] signedData, byte[]? originalContent = null)
{
    var result = new VerificationResult();

    try
    {
        // Parse CMS
        var signedCms = new SignedCms();

        if (originalContent != null)
        {
            // Detached signature
            var contentInfo = new ContentInfo(originalContent);
            signedCms = new SignedCms(contentInfo, true);
        }

        signedCms.Decode(signedData);

        // Verify each signature
        foreach (var signerInfo in signedCms.SignerInfos)
        {
            // Cryptographic verification
            signerInfo.CheckSignature(verifySignatureOnly: false);

            // Signer certificate
            var cert = signerInfo.Certificate;
            result.SignerSubject = cert?.Subject;

            // Verify timestamp (if present)
            var timestampAttr = signerInfo.UnsignedAttributes
                .Cast<CryptographicAttributeObject>()
                .FirstOrDefault(a => a.Oid.Value == "1.2.840.113549.1.9.16.2.14");

            if (timestampAttr != null)
            {
                var tsToken = Rfc3161TimestampToken.Decode(
                    timestampAttr.Values[0].RawData, out _);

                result.TimestampPresent = true;
                result.TimestampTime = tsToken.TokenInfo.Timestamp;

                // Validate timestamp
                var signatureBytes = signerInfo.GetSignature();
                var signatureHash = SHA256.HashData(signatureBytes);

                result.TimestampValid = tsToken.VerifySignatureForHash(
                    signatureHash,
                    HashAlgorithmName.SHA256,
                    out _,
                    extraCandidates: null
                );
            }

            // Signing Time attribute
            var signingTimeAttr = signerInfo.SignedAttributes
                .Cast<CryptographicAttributeObject>()
                .FirstOrDefault(a => a.Oid.Value == "1.2.840.113549.1.9.5");

            if (signingTimeAttr != null)
            {
                var pkcs9Time = new Pkcs9SigningTime(signingTimeAttr.Values[0].RawData);
                result.SignatureTime = pkcs9Time.SigningTime;
            }
        }

        result.IsValid = true;
        result.SignatureValid = true;
        result.CertificateValid = true;
    }
    catch (CryptographicException ex)
    {
        result.IsValid = false;
        result.Error = ex.Message;
    }

    return result;
}
</code>

----

===== Verification Result =====

<code csharp>
public class VerificationResult
{
    public bool IsValid { get; set; }
    public bool SignatureValid { get; set; }
    public bool CertificateValid { get; set; }
    public bool TimeValid { get; set; }
    public bool EkuValid { get; set; } = true;
    public bool TimestampPresent { get; set; }
    public bool TimestampValid { get; set; }

    public string? SignerSubject { get; set; }
    public DateTimeOffset? SignatureTime { get; set; }
    public DateTimeOffset? TimestampTime { get; set; }
    public List<string> ChainStatus { get; set; } = new();
    public string? Error { get; set; }

    public override string ToString()
    {
        var sb = new StringBuilder();
        sb.AppendLine($"Signature valid: {IsValid}");
        sb.AppendLine($"  Crypto verification: {SignatureValid}");
        sb.AppendLine($"  Certificate: {CertificateValid}");
        sb.AppendLine($"  Time verification: {TimeValid}");

        if (SignerSubject != null)
            sb.AppendLine($"  Signer: {SignerSubject}");

        if (SignatureTime.HasValue)
            sb.AppendLine($"  Signature time: {SignatureTime:yyyy-MM-dd HH:mm:ss}");

        if (TimestampPresent)
        {
            sb.AppendLine($"  Timestamp: {(TimestampValid ? "valid" : "invalid")}");
            if (TimestampTime.HasValue)
                sb.AppendLine($"  Timestamp time: {TimestampTime:yyyy-MM-dd HH:mm:ss}");
        }

        if (Error != null)
            sb.AppendLine($"  ERROR: {Error}");

        return sb.ToString();
    }
}
</code>

----

===== Batch Verification =====

<code csharp>
public class BatchVerifier
{
    public async Task<Dictionary<string, VerificationResult>> VerifyDirectory(
        string directory,
        X509Certificate2Collection trustedCerts)
    {
        var results = new Dictionary<string, VerificationResult>();
        var verifier = new SignatureVerifier();

        // Find all .sig files
        var sigFiles = Directory.GetFiles(directory, "*.sig", SearchOption.AllDirectories);

        foreach (var sigFile in sigFiles)
        {
            // Find original file
            var originalFile = sigFile.Replace(".sig", "");
            if (!File.Exists(originalFile))
            {
                results[sigFile] = new VerificationResult
                {
                    IsValid = false,
                    Error = "Original file not found"
                };
                continue;
            }

            // Find certificate
            var certFile = sigFile.Replace(".sig", ".crt");
            X509Certificate2 signerCert;

            if (File.Exists(certFile))
            {
                signerCert = new X509Certificate2(certFile);
            }
            else
            {
                // Search in trusted certs (simplified)
                signerCert = trustedCerts[0];
            }

            // Verify
            var data = await File.ReadAllBytesAsync(originalFile);
            var signature = await File.ReadAllBytesAsync(sigFile);

            results[sigFile] = verifier.Verify(
                data,
                signature,
                signerCert,
                new VerificationOptions
                {
                    HashAlgorithm = HashAlgorithmName.SHA256,
                    CryptoMode = CryptoMode.Hybrid
                }
            );
        }

        return results;
    }
}
</code>

----

===== Error Codes and Resolution =====

^  Error  ^  Cause  ^  Solution  ^
| **SignatureInvalid** | Data or signature tampered | Check originals |
| **CertificateExpired** | Certificate expired | Use timestamp |
| **CertificateRevoked** | Certificate revoked | New certificate |
| **UntrustedRoot** | Root not trusted | Check trust store |
| **ChainIncomplete** | Intermediate missing | Complete chain |
| **TimestampInvalid** | Timestamp signature invalid | Check TSA certificate |

----

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

^  Relationship  ^  Scenario  ^  Description  ^
| **Prerequisite** | [[.:dokument_signieren|8.1 Sign Document]] | Create signature |
| **Prerequisite** | [[.:timestamp|8.3 Timestamp]] | Add timestamp |
| **Related** | [[en:int:pqcrypt:szenarien:validierung:chain_validation|5.2 Chain Validation]] | Certificate verification |

----

<< [[.:timestamp|← 8.3 Timestamp]] | [[.:start|↑ Signatures Overview]] | [[en:int:pqcrypt:szenarien:start|→ All Scenarios]] >>

{{tag>scenario signature verification validation cms}}

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