C#和vb.net实现PDF 添加可视化和不可见数字签名

C# 和 VB.net 都可以使用 iTextSharp 库来实现 PDF 文件添加数字签名。数字签名可以是可视化的，也可以是不可见的。

以下是实现 PDF 添加数字签名的完整攻略：

步骤 1：引入 iTextSharp 库

在项目中引入 iTextSharp 库。通常会从 NuGet 软件包管理器中安装该库，或者从官方网站 https://github.com/itext/itextsharp 下载。

步骤 2：创建签名类

首先，创建一个签名类，该类继承了 IExternalSignature 接口，实现了 Sign 方法。在 Sign 方法中，我们可以使用数字证书对 PDF 签名，并将签名数据写入 PDF 文件。

public class MyExternalSignature : IExternalSignature
{
    private string _pkcs12FilePath;
    private string _password;

    public MyExternalSignature(string pkcs12FilePath, string password)
    {
        _pkcs12FilePath = pkcs12FilePath;
        _password = password;
    }

    public string GetHashAlgorithm()
    {
        return DigestAlgorithms.SHA256;
    }

    public string GetEncryptionAlgorithm()
    {
        return EncryptionAlgorithm.RSA;
    }

    public byte[] Sign(byte[] message)
    {
        X509Certificate2 pkcs12 = new X509Certificate2(_pkcs12FilePath, _password);
        RSACryptoServiceProvider key = (RSACryptoServiceProvider)pkcs12.PrivateKey;
        Org.BouncyCastle.X509.X509Certificate[] chain = new Org.BouncyCastle.X509.X509Certificate[] { DotNetUtilities.FromX509Certificate(pkcs12) };
        IExternalSignature signature = new PrivateKeySignature(key, GetHashAlgorithm());
        return signature.Sign(message);
    }
}

步骤 3：创建 PDF 签名

接下来，我们需要创建 PDF 签名。以下是一个简单的方法，它接受输入文件名，数字证书和输出文件名。其中，数字证书作为参数传递给 MyExternalSignature 类的构造函数。

private void SignPdf(string inputFilePath, string certFilePath, string certPassword, string outputFilePath)
{
    var reader = new PdfReader(inputFilePath);
    var stamper = PdfStamper.CreateSignature(reader, new FileStream(outputFilePath, FileMode.Create), '\0');
    var appearance = stamper.SignatureAppearance;
    appearance.SetVisibleSignature("Signed by me");
    appearance.Reason = "Signed by me";
    appearance.Location = "Shanghai";
    var externalSignature = new MyExternalSignature(certFilePath, certPassword);
    MakeSignature.SignDetached(appearance, externalSignature, new Org.BouncyCastle.X509.X509Certificate[] { });
}

示例 1: 可视化签名

以下是一个多页 PDF 文件的示例。此示例在第一页顶部添加了可视化签名。

var inputFilePath = "input.pdf";
var outputFilePath = "output.pdf";
var certFilePath = "cert.pfx";
var certPassword = "password";

var reader = new PdfReader(inputFilePath);
var stamper = PdfStamper.CreateSignature(reader, new FileStream(outputFilePath, FileMode.Create), '\0');

var signaturePage = 1;
var pageSize = reader.GetPageSizeWithRotation(signaturePage);
var x = pageSize.Left + 10;
var y = pageSize.Top - 10;
var signatureField = PdfFormField.CreateSignature(stamper.Writer);
signatureField.FieldName = "Signature1";
signatureField.SetWidget(new iTextSharp.text.Rectangle(x, y, x + 200, y + 50), PdfAnnotation.HighlightInvert);
signatureField.SetPage(signaturePage);
signatureField.MKBorderColor = BaseColor.GRAY;
signatureField.MKBackgroundColor = BaseColor.LIGHT_GRAY;
stamper.AddAnnotation(signatureField, signaturePage);

var appearance = stamper.SignatureAppearance;
appearance.SetVisibleSignature(new Rectangle(x, y, x + 200, y + 50), signaturePage, "Signed by me");
appearance.SignatureRenderingMode = PdfSignatureAppearance.RenderingMode.GRAPHIC;
appearance.CertificationLevel = PdfSignatureAppearance.CERTIFIED_NO_CHANGES_ALLOWED;
appearance.Reason = "Signed by me";
appearance.Location = "Shanghai";

var externalSignature = new MyExternalSignature(certFilePath, certPassword);
MakeSignature.SignDetached(appearance, externalSignature, new Org.BouncyCastle.X509.X509Certificate[] { });
stamper.Close();

此示例首先创建了一个 signatureField 对象并将其添加到第一页的指定位置。然后，它设置了可视化签名的外观，并使用 MakeSignature.SignDetached 方法进行签名。

示例 2: 不可视化签名

以下示例使用不可视化签名签署 PDF 文件。

var inputFilePath = "input.pdf";
var outputFilePath = "output.pdf";
var certFilePath = "cert.pfx";
var certPassword = "password";

var externalSignature = new MyExternalSignature(certFilePath, certPassword);

using (Stream input = new FileStream(inputFilePath, FileMode.Open, FileAccess.Read, FileShare.Read))
using (Stream output = new FileStream(outputFilePath, FileMode.Create, FileAccess.Write, FileShare.None))
{
    var signer = new PdfSigner(new PdfReader(input), output, new StampingProperties().UseAppendMode());
    signer.SetCertificationLevel(PdfSigner.NOT_CERTIFIED);
    signer.SignExternalContainer(externalSignature, 8192);
    signer.Document.Close();
}

此示例使用 PdfSigner 类来签署文档。它设置了不可视化签名，并最终使用 externalSignature 对象来签署 PDF 文件。

希望这个攻略对您有所帮助！