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Version: 1.3

Sign a PDF document

The Pdftools SDK lets you apply a document approval signature to a PDF document. This type of signature records the identity of the signer, and confirms that the content of the document has not changed after the signature is applied.

Quick start

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In this example, the Built-in cryptographic provider is used to apply a document approval signature to a PDF document. The PFX certificate is read from a local file. Information required for long-term signature validation (LTV) is embedded in the output PDF.

note

Any of the cryptographic providers supported by the Pdftools SDK can be used to apply a document approval signature.

Steps to sign a document:

  1. Initialize the cryptographic provider.
  2. Read the PFX or P12 certificate.
  3. (Optional) Add long-term validation information
  4. Open and sign the document.
Before you begin

You need to initialize the library.

Initializing the cryptographic provider​

When using the Built-in cryptographic provider, you start the digital signing process by instantiating the Provider object. The Provider object exposes the methods of the cryptographic provider. The cryptographic provider manages certificates and private keys, and implements cryptographic algorithms.

// Create a session to the built-in cryptographic provider
using var session = new BuiltIn.Provider();

Reading the PFX or P12 certificate​

Using the Built-in cryptographic provider, PFX certificate files can be loaded directly into the cryptographic provider from the file system. The certificate file is opened as a stream and loaded into the provider to prepare it to apply a digital signature.

// Create signature configuration from PFX (or P12) file
using var pfxStr = File.OpenRead(certificateFile);
var signature = session.CreateSignatureFromCertificate(pfxStr, password);

Adding long-term validation information​

As an optional step, long-term validation information can be added to the output document. It embeds revocation information such as online certificate status response and certificate revocation lists. Revocation information is provided by a validation service at the time of signing and acts as proof that the certificate was valid at the time of signing.

// Embed validation information to enable the long-term validation (LTV) of the signature
signature.ValidationInformation = PdfTools.Crypto.ValidationInformation.EmbedInDocument;

Opening and signing the document​

After instantiating the Provider and preparing the signature configuration, you are ready to apply the digital signature to a document.

The input and output PDF documents are created as streams (in this example, as file streams). The Signer object is used to apply the digital document signature.

Non-critical processing errors raise a Warning event. It is recommended to listen for these events, and review the WarningCategory to determine if further action is needed.

// Open the input document
using var inStr = File.OpenRead(inPath);
using var inDoc = Document.Open(inStr);

// Create a stream for the output file
using var outStr = File.Create(outPath);

// Create the Signer object
Signer signer = new Signer();

// (optional) Create an event listener to listen for warning events that are raised and write them to console
signer.Warning += (s, e) => Console.WriteLine("Warning - {0}: {1}: {2}", e.Category, e.Context, e.Message);

// Sign the output document
using var outDoc = signer.Sign(inDoc, signature, outStr);

Full example​

// Create a session to the built-in cryptographic provider
using var session = new BuiltIn.Provider();

// Create signature configuration from PFX (or P12) file
using var pfxStr = File.OpenRead(certificateFile);
var signature = session.CreateSignatureFromCertificate(pfxStr, password);

// Embed validation information to enable the long-term validation (LTV) of the signature (default)
signature.ValidationInformation = PdfTools.Crypto.ValidationInformation.EmbedInDocument;

// Open input document
using var inStr = File.OpenRead(inPath);
using var inDoc = Document.Open(inStr);

// Create stream for output file
using var outStr = File.Create(outPath);

// Create the Signer object
Signer signer = new Signer();

// Create an event listener to listen for warning events that are raised and write them to console
signer.Warning += (s, e) => Console.WriteLine("Warning - {0}: {1}: {2}", e.Category, e.Context, e.Message);

// Sign the output document
using var outDoc = signer.Sign(inDoc, signature, outStr);
Signing PDF/A documents

During the conversion process from PDF to PDF/A, any signatures are removed from the file before it is converted to PDF/A for archiving. Therefore, files that require archiving should be archived to PDF/A format before any digital signatures are applied.