~~NOTOC~~
====== Scenario 8.3: Timestamp ======

<WRAP round info>
**Category:** [[.:start|Digital Signatures]] \\
**Complexity:** ⭐⭐⭐ (Medium) \\
**Prerequisites:** Signature available, TSA access \\
**Estimated Time:** 10-15 Minutes
</WRAP>

----

===== Description =====

This scenario describes **adding timestamps** to digital signatures (RFC 3161). Timestamps prove that a signature existed at a specific point in time and enable:

  * **Long-term validation** - Signature remains valid after certificate expiration
  * **Provability** - Exact signature time documented
  * **Compliance** - Required for eIDAS qualified signatures

**Concept:**
  * Signature hash is sent to Timestamp Authority (TSA)
  * TSA signs the hash with time information
  * Timestamp token is attached to the signature

----

===== Workflow =====

<mermaid>
flowchart LR
    SIG[Signature] --> HASH[Hash of Signature]
    HASH --> REQ[TSA Request]
    REQ -->|HTTP POST| TSA[Timestamp Authority]
    TSA --> TOKEN[Timestamp Token]
    TOKEN --> ATTACH[Attach to Signature]
    ATTACH --> OUTPUT[Signature with Timestamp]

    style TSA fill:#e8f5e9
    style OUTPUT fill:#e3f2fd
</mermaid>

----

===== Code Example: RFC 3161 Timestamp Request =====

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

using var ctx = PqCryptoContext.Initialize();

// Load signature
var signature = File.ReadAllBytes("document.sig");

// Calculate hash of signature
var signatureHash = SHA256.HashData(signature);

// Create timestamp request (RFC 3161)
var tsRequest = new Rfc3161TimestampRequest(
    messageHash: signatureHash,
    hashAlgorithm: HashAlgorithmName.SHA256,
    requestedPolicyId: null,  // Default Policy
    nonce: GenerateNonce(),
    requestSignerCertificates: true
);

// Send request to TSA
using var http = new HttpClient();
var content = new ByteArrayContent(tsRequest.Encode());
content.Headers.ContentType = new MediaTypeHeaderValue("application/timestamp-query");

var response = await http.PostAsync("http://timestamp.digicert.com", content);
var tsResponseBytes = await response.Content.ReadAsByteArrayAsync();

// Parse response
var tsResponse = Rfc3161TimestampToken.Decode(tsResponseBytes, out int bytesConsumed);

// Validate
if (!tsResponse.VerifySignatureForHash(signatureHash, HashAlgorithmName.SHA256, out var signerCert, null))
{
    throw new CryptographicException("Timestamp signature invalid");
}

// Save token
File.WriteAllBytes("document.sig.tst", tsResponse.AsSignedCms().Encode());

Console.WriteLine("Timestamp received:");
Console.WriteLine($"  Time: {tsResponse.TokenInfo.Timestamp}");
Console.WriteLine($"  TSA: {signerCert.Subject}");
Console.WriteLine($"  Policy: {tsResponse.TokenInfo.PolicyId}");
</code>

----

===== Code Example: Add Timestamp to CMS Signature =====

<code csharp>
public class TimestampService
{
    private readonly string _tsaUrl;
    private readonly HttpClient _http;

    public TimestampService(string tsaUrl)
    {
        _tsaUrl = tsaUrl;
        _http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) };
    }

    public async Task<SignedCms> AddTimestamp(SignedCms signedCms)
    {
        foreach (var signerInfo in signedCms.SignerInfos)
        {
            // Signature hash for timestamp
            var signatureHash = SHA256.HashData(signerInfo.GetSignature());

            // Timestamp request
            var tsRequest = new Rfc3161TimestampRequest(
                signatureHash,
                HashAlgorithmName.SHA256,
                requestedPolicyId: null,
                nonce: GenerateNonce(),
                requestSignerCertificates: true
            );

            // Send to TSA
            var content = new ByteArrayContent(tsRequest.Encode());
            content.Headers.ContentType = new MediaTypeHeaderValue("application/timestamp-query");

            var response = await _http.PostAsync(_tsaUrl, content);
            response.EnsureSuccessStatusCode();

            var tsResponseBytes = await response.Content.ReadAsByteArrayAsync();
            var tsToken = Rfc3161TimestampToken.Decode(tsResponseBytes, out _);

            // Add as unsigned attribute
            var timestampAttr = new AsnEncodedData(
                new Oid("1.2.840.113549.1.9.16.2.14"),  // id-aa-timeStampToken
                tsToken.AsSignedCms().Encode()
            );

            signerInfo.AddUnsignedAttribute(timestampAttr);
        }

        return signedCms;
    }

    private byte[] GenerateNonce()
    {
        var nonce = new byte[8];
        RandomNumberGenerator.Fill(nonce);
        return nonce;
    }
}
</code>

----

===== Validate Timestamp Token =====

<code csharp>
public class TimestampValidator
{
    public TimestampValidationResult Validate(
        byte[] timestampToken,
        byte[] originalSignature,
        X509Certificate2Collection? trustedTsaCerts = null)
    {
        var result = new TimestampValidationResult();

        try
        {
            var tsToken = Rfc3161TimestampToken.Decode(timestampToken, out _);
            result.Timestamp = tsToken.TokenInfo.Timestamp;
            result.PolicyId = tsToken.TokenInfo.PolicyId?.Value;

            // Verify hash
            var expectedHash = SHA256.HashData(originalSignature);
            if (!tsToken.TokenInfo.GetMessageHash().ReadOnlySpan.SequenceEqual(expectedHash))
            {
                result.IsValid = false;
                result.Error = "Hash does not match";
                return result;
            }

            // Verify signature
            if (!tsToken.VerifySignatureForHash(expectedHash, HashAlgorithmName.SHA256, out var tsaCert, trustedTsaCerts))
            {
                result.IsValid = false;
                result.Error = "TSA signature invalid";
                return result;
            }

            result.TsaCertificate = tsaCert;

            // Validate TSA certificate chain
            var chain = new X509Chain();
            chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;

            if (trustedTsaCerts != null)
            {
                chain.ChainPolicy.CustomTrustStore.AddRange(trustedTsaCerts);
                chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;
            }

            result.IsValid = chain.Build(tsaCert);
            if (!result.IsValid)
            {
                result.Error = "TSA certificate chain invalid";
            }
        }
        catch (Exception ex)
        {
            result.IsValid = false;
            result.Error = ex.Message;
        }

        return result;
    }
}

public class TimestampValidationResult
{
    public bool IsValid { get; set; }
    public DateTimeOffset? Timestamp { get; set; }
    public string? PolicyId { get; set; }
    public X509Certificate2? TsaCertificate { get; set; }
    public string? Error { get; set; }
}
</code>

----

===== Timestamp Authorities (TSAs) =====

^  Provider  ^  URL  ^  Qualified (eIDAS)  ^
| DigiCert | http://timestamp.digicert.com | No |
| Sectigo | http://timestamp.sectigo.com | No |
| GlobalSign | http://timestamp.globalsign.com | No |
| D-TRUST | http://zeitstempel.2.1.3.bundesdruckerei.de | **Yes** |
| SwissSign | http://tsa.swisscom.com/tsa | **Yes** |
| Bundesdruckerei | http://ts.2.bundesdruckerei.de | **Yes** |

<WRAP round tip>
**eIDAS:** A qualified TSA is required for qualified electronic signatures.
</WRAP>

----

===== Long-Term Archival (LTA) =====

<code csharp>
public class LongTermArchive
{
    public async Task ExtendValidation(
        SignedCms signedCms,
        string tsaUrl,
        X509Certificate2Collection validationData)
    {
        // 1. Embed validation data (certificates, CRLs, OCSP)
        AddValidationData(signedCms, validationData);

        // 2. Add archive timestamp
        var timestampService = new TimestampService(tsaUrl);
        await timestampService.AddArchiveTimestamp(signedCms);

        // This process can be repeated periodically
        // to keep the signature validatable long-term
    }
}
</code>

----

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

^  Application  ^  Timestamp Required?  ^  Qualified?  ^
| **Code Signing** | Required | No |
| **eIDAS QES** | Required | Yes |
| **PDF/A Archival** | Recommended | Depending on requirements |
| **S/MIME** | Optional | No |
| **Healthcare** | Required | Depending on country |

----

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

^  Relationship  ^  Scenario  ^  Description  ^
| **Prerequisite** | [[.:dokument_signieren|8.1 Sign Document]] | Create signature |
| **Prerequisite** | [[.:code_signieren|8.2 Sign Code]] | Code signature |
| **Next Step** | [[.:signatur_verifizieren|8.4 Verify Signature]] | Validation |

----

<< [[.:code_signieren|← 8.2 Sign Code]] | [[.:start|↑ Signatures Overview]] | [[.:signatur_verifizieren|8.4 Verify Signature →]] >>

{{tag>scenario signature timestamp tsa rfc3161 longterm}}

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