Signing PDFs (C# / netframework)
Note
This demo is available in your FlexCel installation at <FlexCel Install Folder>\samples\csharp\VS2022\netframework\25.Printing and Exporting\35.Signing Pdfs and also at https://github.com/tmssoftware/TMS-FlexCel.NET-demos/tree/master/csharp/VS2022/netframework/Modules/25.Printing and Exporting/35.Signing Pdfs
Overview
In this example we will show how to add a visible or invisible signature to a generated PDF file.
Concepts
In order to sign a PDF file you will need a certificate issued by a valid Certificate Authority, or one issued by yourself. In this example we will use a self signed certificate. This certificate will not validate by default when you open it in Acrobat, you need to add it to your trusted list.
The default algorithm for CmsSigner .NET class is SHA-1, which is known to have vulnerabilities and shouldn't be used anymore. So in this example we use SHA512 instead by changing the DigestAlgorithm.
In order to sign a file, FlexCel will write a requirement for Acrobat 8 or newer in the generated files. This is because only Acrobat 8 or newer support SHA512. Older versions of acrobat will still display the pages but will not validate the signature.
We provide a default signing implementation using standard .NET crypto classes**.** You can still create your own signature engine by using a third party cryptography library or by calling CryptoApi in windows via p/invoke. This is explained in the section Signing PDF Files in the PDF exporting guide.
Files
mainForm.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using FlexCel.Render;
using FlexCel.XlsAdapter;
using FlexCel.Pdf;
using System.IO;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Pkcs;
using System.Drawing.Imaging;
using System.Reflection;
using System.Diagnostics;
namespace SigningPdfs
{
public partial class mainForm: Form
{
public mainForm()
{
Application.EnableVisualStyles();
InitializeComponent();
}
private void cbVisibleSignature_CheckedChanged(object sender, EventArgs e)
{
SignaturePicture.Visible = cbVisibleSignature.Checked;
int delta = SignaturePicture.Height + 30;
if (cbVisibleSignature.Checked) this.Height += delta; else this.Height -= delta;
}
private void SignaturePicture_Click(object sender, EventArgs e)
{
if (OpenImageDialog.ShowDialog() != DialogResult.OK) return;
SignaturePicture.Load(OpenImageDialog.FileName);
}
private void btnCreateAndSign_Click(object sender, EventArgs e)
{
//Load the Excel file.
if (OpenExcelDialog.ShowDialog() != DialogResult.OK) return;
XlsFile xls = new XlsFile();
xls.Open(OpenExcelDialog.FileName);
string DataPath = Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location) + @"\..\..\";
//Export it to pdf.
using (FlexCelPdfExport pdf = new FlexCelPdfExport(xls, true))
{
pdf.FontEmbed = TFontEmbed.Embed;
//Load the certificate and create a signer.
//In this example we just have the password in clear. It should be kept in a SecureString.
//Also make sure to set the flag X509KeyStorageFlags.EphemeralKeySet to avoid files created
//on disk: https://snede.net/the-most-dangerous-constructor-in-net/
//As X509KeyStorageFlags.EphemeralKeySet only exists in .NET 4.8 or newer, for older versions we will
//define it as (X509KeyStorageFlags)32. For .NET 4.8 or newer and NET Core, you can use X509KeyStorageFlags.EphemeralKeySet
X509Certificate2 Cert = new X509Certificate2(DataPath + "flexcel.pfx", "password", X509KeyStorageFlags.EphemeralKeySet);
//Note that to use the CmsSigner class you need to add a reference to System.Security dll.
//It is *not* enough to add it to the using clauses, you need to add a reference to the dll.
CmsSigner Signer = new CmsSigner(Cert);
//By default CmsSigner uses SHA1, but SHA1 has known vulnerabilities and it is deprecated.
//So we will use SHA512 instead.
//"2.16.840.1.101.3.4.2.3" is the Oid for SHA512.
Signer.DigestAlgorithm = new System.Security.Cryptography.Oid("2.16.840.1.101.3.4.2.3");
TPdfSignature sig;
if (cbVisibleSignature.Checked)
{
using (MemoryStream fs = new MemoryStream())
{
SignaturePicture.Image.Save(fs, ImageFormat.Png);
byte[] ImgData = fs.ToArray();
//The -1 as "page" parameter means the last page.
sig = new TPdfVisibleSignature(new TBuiltInSignerFactory(Signer),
"Signature", "I have read the document and certify it is valid.", "Springfield", "adrian@tmssoftware.com", -1, new RectangleF(50, 50, 140, 70), ImgData);
}
}
else
{
sig = new TPdfSignature(new TBuiltInSignerFactory(Signer),
"Signature", "I have read the document and certify it is valid.", "Springfield", "adrian@tmssoftware.com");
}
//You must sign the document *BEFORE* starting to write it.
pdf.Sign(sig);
if (savePdfDialog.ShowDialog() != DialogResult.OK) return;
using (FileStream PdfStream = new FileStream(savePdfDialog.FileName, FileMode.Create))
{
pdf.BeginExport(PdfStream);
pdf.ExportAllVisibleSheets(false, "Signed Pdf");
pdf.EndExport();
}
}
if (MessageBox.Show("Do you want to open the generated file?", "Confirm", MessageBoxButtons.YesNo, MessageBoxIcon.Question) != DialogResult.Yes) return;
Process.Start(savePdfDialog.FileName);
}
}
}
mainForm.Designer.cs
namespace SigningPdfs
{
partial class mainForm
{
/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.IContainer components = null;
/// <summary>
/// Clean up any resources being used.
/// </summary>
/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
components.Dispose();
}
base.Dispose(disposing);
}
#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.btnCreateAndSign = new System.Windows.Forms.Button();
this.cbVisibleSignature = new System.Windows.Forms.CheckBox();
this.OpenExcelDialog = new System.Windows.Forms.OpenFileDialog();
this.savePdfDialog = new System.Windows.Forms.SaveFileDialog();
this.SignaturePicture = new System.Windows.Forms.PictureBox();
this.OpenImageDialog = new System.Windows.Forms.OpenFileDialog();
((System.ComponentModel.ISupportInitialize)(this.SignaturePicture)).BeginInit();
this.SuspendLayout();
//
// btnCreateAndSign
//
this.btnCreateAndSign.Image = global::SigningPdfs.Properties.Resources.acroread;
this.btnCreateAndSign.ImageAlign = System.Drawing.ContentAlignment.MiddleLeft;
this.btnCreateAndSign.Location = new System.Drawing.Point(24, 25);
this.btnCreateAndSign.Name = "btnCreateAndSign";
this.btnCreateAndSign.Size = new System.Drawing.Size(155, 30);
this.btnCreateAndSign.TabIndex = 0;
this.btnCreateAndSign.Text = "Create and Sign Pdf";
this.btnCreateAndSign.UseVisualStyleBackColor = true;
this.btnCreateAndSign.Click += new System.EventHandler(this.btnCreateAndSign_Click);
//
// cbVisibleSignature
//
this.cbVisibleSignature.AutoSize = true;
this.cbVisibleSignature.Location = new System.Drawing.Point(24, 78);
this.cbVisibleSignature.Name = "cbVisibleSignature";
this.cbVisibleSignature.Size = new System.Drawing.Size(167, 17);
this.cbVisibleSignature.TabIndex = 1;
this.cbVisibleSignature.Text = "Visible Signature (in last page)";
this.cbVisibleSignature.UseVisualStyleBackColor = true;
this.cbVisibleSignature.CheckedChanged += new System.EventHandler(this.cbVisibleSignature_CheckedChanged);
//
// OpenExcelDialog
//
this.OpenExcelDialog.DefaultExt = "xls";
this.OpenExcelDialog.Filter = "Excel Files|*.xls;*.xlsx;*.xlsm|Excel 97/2003|*.xls|Excel 2007|*.xlsx;*.xlsm|All files|*.*";
this.OpenExcelDialog.Title = "Select Excel file to convert...";
//
// savePdfDialog
//
this.savePdfDialog.DefaultExt = "pdf";
this.savePdfDialog.Filter = "Pdf Files|*.pdf";
this.savePdfDialog.Title = "Select where to save the file...";
//
// SignaturePicture
//
this.SignaturePicture.BorderStyle = System.Windows.Forms.BorderStyle.FixedSingle;
this.SignaturePicture.Image = global::SigningPdfs.Properties.Resources.sign;
this.SignaturePicture.Location = new System.Drawing.Point(24, 110);
this.SignaturePicture.Name = "SignaturePicture";
this.SignaturePicture.Size = new System.Drawing.Size(155, 100);
this.SignaturePicture.SizeMode = System.Windows.Forms.PictureBoxSizeMode.Zoom;
this.SignaturePicture.TabIndex = 2;
this.SignaturePicture.TabStop = false;
this.SignaturePicture.Click += new System.EventHandler(this.SignaturePicture_Click);
//
// OpenImageDialog
//
this.OpenImageDialog.Filter = "Supported Images|*.png;*.bmp*.jpg|All files|*.*";
//
// mainForm
//
this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.ClientSize = new System.Drawing.Size(205, 100);
this.Controls.Add(this.SignaturePicture);
this.Controls.Add(this.cbVisibleSignature);
this.Controls.Add(this.btnCreateAndSign);
this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.Fixed3D;
this.Name = "mainForm";
this.Text = "Signing PDFs";
((System.ComponentModel.ISupportInitialize)(this.SignaturePicture)).EndInit();
this.ResumeLayout(false);
this.PerformLayout();
}
#endregion
private System.Windows.Forms.Button btnCreateAndSign;
private System.Windows.Forms.CheckBox cbVisibleSignature;
private System.Windows.Forms.OpenFileDialog OpenExcelDialog;
private System.Windows.Forms.SaveFileDialog savePdfDialog;
private System.Windows.Forms.PictureBox SignaturePicture;
private System.Windows.Forms.OpenFileDialog OpenImageDialog;
}
}
Program.cs
using System;
using System.Collections.Generic;
using System.Windows.Forms;
namespace SigningPdfs
{
static class Program
{
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new mainForm());
}
}
}