PDF Signing with Self-Signed Certificate and TSA - Technical Specification

1. Introduction:

The technical specification outlines the process and requirements for signing PDF documents using a self-signed certificate and integrating a Trusted Timestamping Service (TSA) in a .NET Core environment.

2. Tools and Libraries:

• iText 7 Library: Used for PDF manipulation and digital signatures.
• Bouncy Castle Library: Provides cryptographic operations for handling certificates and keys.

3. Sample Code Implementation

using System;
using System.IO;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Pkcs;
using Org.BouncyCastle.Security;
using iText.Kernel.Pdf;
using iText.Signatures;
using iText.Signatures.Pdf;

public class PdfSigner
{
    public void SignPdf(string pdfPath, string outputPath, string certificatePath, string certificatePassword, string tsaUrl)
    {
        // Load the PDF document
        PdfReader reader = new PdfReader(pdfPath);
        PdfSigner signer = new PdfSigner(reader, new FileStream(outputPath, FileMode.Create), new StampingProperties());

        // Load the certificate
        Pkcs12Store store = new Pkcs12Store(new FileStream(certificatePath, FileMode.Open), certificatePassword.ToCharArray());
        string alias = null;
        foreach (string t in store.Aliases)
        {
            if (store.IsKeyEntry(t) && store.GetKey(t).Key.IsPrivate)
            {
                alias = t;
                break;
            }
        }
        AsymmetricKeyEntry key = store.GetKey(alias);
        X509CertificateEntry[] chain = store.GetCertificateChain(alias);

        // Create the appearance
        PdfSignatureAppearance appearance = signer.GetSignatureAppearance();
        appearance.SetReason("Reason for signing");
        appearance.SetLocation("Location");
        appearance.SetPageNumber(1);
        appearance.SetReuseAppearance(false);

        // Define the signature
        IExternalSignature pks = new PrivateKeySignature(key.Key, DigestAlgorithms.SHA256);
        IExternalDigest digest = new BouncyCastleDigest();

        // Create the signature dictionary
        signer.SetFieldName("Signature");
        signer.SignDetached(digest, pks, chain, null, null, null, 0, PdfSigner.CryptoStandard.CMS);

        // Timestamp the signature
        TSAClientBouncyCastle tsaClient = new TSAClientBouncyCastle(tsaUrl, null, null, 4096, "SHA-256");
        signer.Timestamp(tsaClient, null);

        signer.Close();
        reader.Close();
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        string pdfPath = "example.pdf";
        string outputPath = "signed_example.pdf";
        string certificatePath = "certificate.pfx";
        string certificatePassword = "your_password";
        string tsaUrl = "http://your.tsa.url";

        PdfSigner pdfSigner = new PdfSigner();
        pdfSigner.SignPdf(pdfPath, outputPath, certificatePath, certificatePassword, tsaUrl);

        Console.WriteLine("PDF signed successfully!");
    }
}

4. Process Overview:

1. Load the PDF document to be signed.
2. Load the self-signed certificate for digital signing.
3. Create the appearance and define the signature parameters.
4. Sign the PDF document using the provided certificate.
5. Timestamp the signature using the Trusted Timestamping Service (TSA).
6. Save the signed PDF document to the specified output path.

5. Code Implementation:

• Define a PdfSigner class responsible for signing PDF documents.
• Implement a SignPdf method within the PdfSigner class to carry out the signing process.
• The SignPdf method accepts parameters such as paths to input and output PDF files, certificate path, certificate password, and TSA URL.
• Load the PDF document using PdfReader and initialize the PdfSigner object.
• Load the self-signed certificate and define the signature appearance.
• Create the signature using the provided certificate and TSA URL.
• Timestamp the signature using the TSA.
• Save the signed PDF document to the output path.

6. Main Program:

• Implement a Main method within a Program class to orchestrate the PDF signing process.
• Instantiate a PdfSigner object and call the SignPdf method with appropriate parameters.
• Replace placeholder values with actual file paths, certificate details, and TSA URL.

7. Error Handling:

• Implement error handling mechanisms to handle exceptions and error cases during PDF signing.
• Provide appropriate error messages and logging to aid in troubleshooting.

8. Deployment Considerations:

• Ensure that the .NET Core runtime environment is installed on the deployment system.
• Deploy the required certificate and TSA URL configuration to the deployment environment.
• Test the PDF signing process in the deployment environment to ensure functionality and reliability.

9. Conclusion:

The technical specification provides a detailed guide for implementing PDF signing with a self-signed certificate and Trusted Timestamping Service in a .NET Core environment. By following the outlined process and requirements, users can ensure the integrity and authenticity of digitally signed PDF documents.