PDF Document API v20.1 - Multiple Signatures and Timestamps

Our PDF Document API (v20.1) allows you to apply multiple signatures to a document without existing signatures and to apply new signatures to a document that has yet to be signed.

This new PDF Signature API allows you to execute the following actions:

  • Sign an existing signature field
  • Sign a new field
  • Apply a signature with a timestamp

The PdfDocumentSigner class is an entry point to the Signature API. Use it to save a document with applied signatures. You can pass a single signature or an array of signatures to the PdfDocumentSigner.SaveDocument method.

For a complete sample project, please refer to the following in our GitHub repository: How to Apply Multiple Signatures

Sign an Existing Signature Field

Use the PdfSignatureBuilder class to apply a signature to an existing form field. The Pkcs7Signer class allows you to specify signature validation data (certificate, hash algorithm, etc.). The PDF Document API supports PKCS#7 signatures with X.509 certificates.

//Load a document to sign:
using (var signer = new PdfDocumentSigner("Document.pdf"))
    //Create a PKCS#7 signature:
    Pkcs7Signer pkcs7Signature = new Pkcs7Signer("Documents/testcert.pfx", "123", 

    //Apply the signature to an existing form field with the name "Sign":
    var signature = new PdfSignatureBuilder(pkcs7Signature, "Sign");

    //Sign and save the document:
    signer.SaveDocument("SignedDocument.pdf", signature);

Sign a New Field

Use the PdfSignatureFieldInfo class to specify information about a new form field (name, location on the page, etc.) and pass it to the PdfSignatureBuilder, as shown below:

//Load a document to sign
using (var signer = new PdfDocumentSigner("Document.pdf"))
    //Create a PKCS#7 signature:
    Pkcs7Signer pkcs7Signature = new Pkcs7Signer("Documents/testcert.pfx", "123", 

    //Specify the name and location of a new signature field
    var signatureFieldInfo = new PdfSignatureFieldInfo(1);
    signatureFieldInfo.Name = "SignatureField";
    signatureFieldInfo.SignatureBounds = new PdfRectangle(10, 10, 150, 150);
    signatureFieldInfo.RotationAngle = PdfAcroFormFieldRotation.Rotate90;

    //Apply the signature with a new signature field created above:
    var signature = new PdfSignatureBuilder(pkcs7Signature, signatureFieldInfo);

    //Sign and save the document
    signer.SaveDocument("SignedDocument.pdf", signature);

Apply a Timestamp to a Signature

The TsaClient class allows you to generate a timestamp. Pass its instance to the Pkcs7Signer constructor to apply the timestamp to a signature.

//Create a timestamp
ITsaClient tsaClient = new PdfTsaClient(new Uri(@"https://freetsa.org/tsr"), PdfHashAlgorithm.SHA256);

//Create a PKCS#7 signature with a timestamp:
Pkcs7Signer pkcs7Signature = new Pkcs7Signer("Documents/testcert.pfx", "123", PdfHashAlgorithm.SHA256, tsaClient);

Create a Custom Signer and a Timestamp Client

If you want to implement a custom signer, you can declare a descendant of the Pkcs7SignerBase class.

Implement the ITsaClient interface to use a custom timestamp client. Sample projects with complete code are available on GitHub:

Use PdfDocumentProcessor to Sign Documents

We enhanced our old approach, so you can use PdfDocumentProcessor to sign a document with a PKCS#7 signature.

  using (PdfDocumentProcessor processor = new PdfDocumentProcessor())
      //Create a PKCS#7 signature
      Pkcs7Signer pkcs7Signature = new Pkcs7Signer("Documents/certificate.pfx", "123", PdfHashAlgorithm.SHA256);

      //Specify the signature's image data and location parameters:
      byte[] imageData = File.ReadAllBytes("Documents/JohnSmith.jpg");
      int pageNumber = 1;
      PdfOrientedRectangle signatureBounds = new PdfOrientedRectangle(new PdfPoint(0, 460), 250, 90);

      //Pass all instances created above to the PdfSignature constructor:
      PdfSignature signature = new PdfSignature(pkcs7Signature, imageData, pageNumber, signatureBounds);

      //Save the signed document:
      processor.SaveDocument("SignedDocument.pdf", new PdfSaveOptions()
      { Signature = signature });

Please note that the PdfDocumentProcessor class has the following limitations:

  • You can sign a document only once.
  • Existing signatures are removed from a document when saved. However, if you use PdfDocumentProcessor to apply a signature, it is retained.


As always, we welcome your thoughts. Please comment below and let us know what you think about this new feature.

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