# Simple PDF PAdES Signature Verification Technical Specification

## 1. Introduction
This technical specification outlines a simple process for verifying PAdES signatures in PDF documents using the iText library. PAdES (PDF Advanced Electronic Signatures) is an ISO standard for electronic signatures in PDF documents, ensuring long-term validation and compliance.

## 2. Overview
The process involves opening a PDF document, retrieving signature information, and verifying the integrity and authenticity of each signature.

## 3. Dependencies
- iText library: A comprehensive PDF library for creating, manipulating, and extracting content from PDF documents.
- BouncyCastle library: A cryptography library providing support for various cryptographic operations.

## 4. Inputs
- Path to the PDF document to be verified.

## 5. Outputs
- Verification status of each signature in the document.
- Certificate information associated with each signature.

## 6. Process
1. Open the PDF document specified by the input path.
2. Retrieve the list of signature names from the document.
3. For each signature name:
a. Read the signature data.
b. Verify the signature integrity and authenticity.
c. Retrieve information about the signing certificate.
d. Print verification status and certificate information.
4. Close the PDF document.

## 7. Sample Code Implementation

using iText.Commons.Bouncycastle.Cert;
using iText.Forms;
using iText.Kernel.Pdf;
using iText.Kernel.Pdf.Canvas.Parser;
using iText.Signatures;

string originalPdfPath = "dummy.pdf";
string signedPdfPath = "dummy_sign.pdf";

bool isSignatureValid = VerifySignature(signedPdfPath);
if (!isSignatureValid)
{
    Console.WriteLine("Signature verification failed.");
}

string originalContent = ExtractText(originalPdfPath);
string signedContent = ExtractText(signedPdfPath);

if (originalContent == signedContent)
{
    Console.WriteLine("PDF files are identical.");
}
else
{
    Console.WriteLine("PDF files are different.");
}
Console.ReadLine();

bool VerifySignature(string signedPdfPath)
{
    using (PdfReader reader = new PdfReader(signedPdfPath))
    {
        using (PdfDocument pdfDoc = new PdfDocument(reader))
        {
            PdfAcroForm acroForm = PdfAcroForm.GetAcroForm(pdfDoc, false);
            SignatureUtil signatureUtil = new SignatureUtil(pdfDoc);
            foreach (string signatureName in signatureUtil.GetSignatureNames())
            {
                PdfPKCS7 pkcs7 = signatureUtil.ReadSignatureData(signatureName);
                if (!pkcs7.VerifySignatureIntegrityAndAuthenticity())
                {
                    return false; // Invalid certificate
                }
                else
                {
                    foreach (IX509Certificate certificate in pkcs7.GetCertificates())
                    {
                        // Check if certificate is valid from ca server
                        Console.WriteLine($"Serial Number: {certificate.GetSerialNumber().ToString()}");
                        Console.WriteLine($"Issuer DN: {certificate.GetIssuerDN().ToString()}");
                        Console.WriteLine($"Subject DN: {certificate.GetSubjectDN().ToString()}");
                    }
                }
            }
        }
    }
    return true; 
}


string ExtractText(string pdfPath)
{
    PdfDocument pdfDoc = new PdfDocument(new PdfReader(pdfPath));
    StringWriter output = new StringWriter();
    for (int i = 1; i <= pdfDoc.GetNumberOfPages(); i++)
    {
        output.WriteLine(PdfTextExtractor.GetTextFromPage(pdfDoc.GetPage(i)));
    }
    pdfDoc.Close();
    return output.ToString();
}

## 8. Code Explanation
- The code defines a class PAdESSignatureVerifier responsible for verifying PAdES signatures.
- The VerifySignatures method opens the specified PDF document, retrieves signature information, and iterates through each signature.
- For each signature, the VerifySignature method reads the signature data, verifies its integrity, and prints the verification status and certificate information.
- The main method Main invokes the VerifySignatures method with the path to the PDF document to be verified.
## 9. Conclusion
This technical specification provides a straightforward guide for verifying PAdES signatures in PDF documents using the iText library. It outlines the steps involved in the process and provides sample code for implementation.