Add a Timestamp to a PDF File in C# (RFC 3161)

A document time-stamp (RFC 3161, ETSI.RFC3161 signature) proves that a PDF document existed at a given moment and that it was not changed afterwards. No signing certificate is needed: the time-stamp is signed by the Time Stamping Authority (TSA).

How it works

• Load the document, set the time-stamping server in TimeStamping.ServerUrl and call ApplyTimestampSignature.
• Servers that require authentication are supported: UserName and Password (HTTP basic authentication) or AuthenticationCertificate (TLS client certificate).
• The document time-stamps are listed with the signatures of the document, in DocumentProperties.DigitalSignatures.

C# code

/// <summary>
/// Adds a document time-stamp to a PDF file.
/// </summary>
static void AddTimestampToPdfFile(string unsignedPdfFile, string timestampedPdfFile)
{
    PdfSignature pdfSignature = new PdfSignature(LibrarySerialNumber);

    pdfSignature.LoadPdfDocument(unsignedPdfFile);

    // The time-stamping server. A server that requires authentication can be used with the properties UserName and
    // Password (HTTP basic authentication) or AuthenticationCertificate (TLS client certificate).
    pdfSignature.TimeStamping.ServerUrl = TimestampServerUrl;

    // The hash algorithm of the time-stamp request (SHA-256 is the default).
    pdfSignature.TimeStamping.HashAlgorithm = SignLib.HashAlgorithm.SHA256;

    // No signing certificate is needed. The result is the PDF document with the time-stamp added.
    File.WriteAllBytes(timestampedPdfFile, pdfSignature.ApplyTimestampSignature());

    Console.WriteLine("The PDF document time-stamp was created." + Environment.NewLine);
}

/// <summary>
/// Displays the document time-stamps of a PDF file.
/// </summary>
static void PrintPdfTimestamps(string timestampedPdfFile)
{
    PdfSignature pdfSignature = new PdfSignature(LibrarySerialNumber);

    pdfSignature.LoadPdfDocument(timestampedPdfFile);

    // The document time-stamps are listed together with the digital signatures of the document.
    foreach (PdfSignatureInfo signatureInfo in pdfSignature.DocumentProperties.DigitalSignatures)
    {
        Console.WriteLine("Signature name: " + signatureInfo.SignatureName);
        Console.WriteLine("Signature type: " + signatureInfo.SignatureType);
        Console.WriteLine("Time-stamp is valid: " + signatureInfo.SignatureIsValid);
        Console.WriteLine("Time-stamp time: " + signatureInfo.SignatureTime.ToLocalTime());
        Console.WriteLine("Time-stamping authority certificate: " + signatureInfo.SignatureCertificate.Subject);
        Console.WriteLine();
    }

    Console.WriteLine("Done PDF time-stamp verification." + Environment.NewLine);
}

The free server used by the sample is for tests. For legal effect in the European Union use a qualified time-stamping service, such as the eIDAS qualified timestamps of qtsa.eu; to run your own server, see TSA Server.

Frequently asked questions