Digital Signatures


Digital Signatures

Digital signatures can be added to a PDF document using PdfDigitalSignatureElement objects. Adding a digital signature requires a PKCS#12 certificate (provided as a .pfx or a .p12 file).

Validating signatures

ValidateSignatures checks every signature of a loaded document and returns a PdfSignatureValidationResult for each: whether it is intact (IsValid), whether its certificate chains to a trusted root (IsTrusted), whether content was added to the document after it (CoversWholeDocument), the signing and time-stamp times, the signer and the certificate, and a Status that sums them up. Pass your own certificate authority to trust it as a root, and set RevocationCheckEnabled to ask the certificate's issuer whether it has been revoked.

Each signature is checked against the bytes it was made over, so a signature followed by a later one, or by a form fill, is reported intact.

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PdfSecurityManager manager = new PdfSecurityManager();
manager.Load("contract-signed.pdf");
manager.RevocationCheckEnabled = true;

// trust the organisation's own certificate authority as well as the system's
PdfDigitalCertificatesCollection roots =
    PdfDigitalCertificatesStore.GetCertificates("our-ca.pfx", "password");
foreach (PdfSignatureValidationResult result in manager.ValidateSignatures(roots))
{
    Console.WriteLine("{0}: {1}, signed by {2} on {3}",
        result.SignatureName, result.Status, result.SignerName, result.SigningTime);
    foreach (string message in result.Messages)
        Console.WriteLine("  " + message);
}
manager.Close();

Dim manager As New PdfSecurityManager()
manager.Load("contract-signed.pdf")
manager.RevocationCheckEnabled = True

' trust the organisation's own certificate authority as well as the system's
Dim roots As PdfDigitalCertificatesCollection =
    PdfDigitalCertificatesStore.GetCertificates("our-ca.pfx", "password")
For Each result As PdfSignatureValidationResult In manager.ValidateSignatures(roots)
    Console.WriteLine("{0}: {1}, signed by {2} on {3}",
        result.SignatureName, result.Status, result.SignerName, result.SigningTime)
    For Each message As String In result.Messages
        Console.WriteLine("  " + message)
    Next
Next
manager.Close()

Sample Code

The following sample code shows how to add a digital signature to a pdf document using Select.Pdf Library for .NET.

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// create a new pdf document
PdfDocument doc = new PdfDocument();

// add a new page to the document
PdfPage page = doc.AddPage();

// get image path 
// the image will be used to display the digital signature over it
string imgFile = @"C:\Path\to\logo.png";

// get certificate path
string certFile = @"C:\Path\to\certificate.pfx";

// define a rendering result object
PdfRenderingResult result;

// create image element from file path 
PdfImageElement img = new PdfImageElement(0, 0, imgFile);
result = page.Add(img);

// get the #PKCS12 certificate from file
PdfDigitalCertificatesCollection certificates = 
    PdfDigitalCertificatesStore.GetCertificates(certFile, "selectpdf");
PdfDigitalCertificate certificate = certificates[0];

// create the digital signature object
PdfDigitalSignatureElement signature = 
    new PdfDigitalSignatureElement(result.PdfPageLastRectangle, certificate);
signature.Reason = "SelectPdf";
signature.ContactInfo = "SelectPdf";
signature.Location = "SelectPdf";
page.Add(signature);

// save pdf document
doc.Save("Sample.pdf");

// close pdf document
doc.Close();

' create a new pdf document
Dim doc As New PdfDocument()

' add a new page to the document
Dim page As PdfPage = doc.AddPage()

' get image path 
' the image will be used to display the digital signature over it
Dim imgFile As String = "C:\Path\to\logo.png"

' get certificate path
Dim certFile As String = "C:\Path\to\certificate.pfx"

' define a rendering result object
Dim result As PdfRenderingResult

' create image element from file path 
Dim img As New PdfImageElement(0, 0, imgFile)
result = page.Add(img)

' get the #PKCS12 certificate from file
Dim certificates As PdfDigitalCertificatesCollection = _
    PdfDigitalCertificatesStore.GetCertificates(certFile, "selectpdf")
Dim certificate As PdfDigitalCertificate = certificates(0)

' create the digital signature object
Dim signature As New PdfDigitalSignatureElement( _
    result.PdfPageLastRectangle, certificate)
signature.Reason = "SelectPdf"
signature.ContactInfo = "SelectPdf"
signature.Location = "SelectPdf"
page.Add(signature)

' save pdf document
doc.Save("Sample.pdf")

' close pdf document
doc.Close()

Reference

PdfDigitalSignatureElement