如何使用ITextSharp.NET将相同的数字签名放置到PDF中的多个位置

Question

我已经使用iTextSharp Dll实现了数字签名，使用单个签名对PDF文件进行签名，创建空签名字段，并使用签名哈希更新签名字段。现在，我想在PDF的每一页都放置相同的数字签名。这是我的客户要求。

我正在使用以下代码：

public class MyExternalSignatureContainer : IExternalSignatureContainer
{
    private readonly byte[] signedBytes;

    public MyExternalSignatureContainer(byte[] signedBytes)
    {
        this.signedBytes = signedBytes;
    }

    public byte[] Sign(Stream data)
    {
        return signedBytes;
    }

    public void ModifySigningDictionary(PdfDictionary signDic)
    {
    }
}

PdfReader reader = new PdfReader(unsignedPdf);
FileStream os = File.OpenWrite(tempPdf);
PdfStamper stamper = PdfStamper.CreateSignature(reader, os, '\0');
PdfSignatureAppearance appearance = stamper.SignatureAppearance;

appearance.Reason = "Reason1";
appearance.Contact = "";
appearance.Location = "Location1";
appearance.Acro6Layers = false;
appearance.Image = null;
appearance.SignatureRenderingMode = PdfSignatureAppearance.RenderingMode.DESCRIPTION;
appearance.SetVisibleSignature(new iTextSharp.text.Rectangle(36, 748, 144, 780), 1, null);
for (int i = 1; i < 8; i++)
{
    var signatureField = PdfFormField.CreateSignature(stamper.Writer);
    var signatureRect = new Rectangle(200, 200, 100, 100);
    signatureField.Put(PdfName.T, new PdfString("ClientSignature_"+i.ToString()));
    PdfIndirectReference PRef = stamper.Writer.PdfIndirectReference;
    signatureField.Put(PdfName.V, PRef);
    signatureField.Put(PdfName.F, new PdfNumber("132"));
    signatureField.SetWidget(signatureRect, null);
    signatureField.Put(PdfName.SUBTYPE, PdfName.WIDGET);

    PdfDictionary xobject1 = new PdfDictionary();
    PdfDictionary xobject2 = new PdfDictionary();
    xobject1.Put(PdfName.N, appearance.GetAppearance().IndirectReference);
    xobject2.Put(PdfName.AP, xobject1);
    signatureField.Put(PdfName.AP, xobject1);
    signatureField.SetPage();
    PdfDictionary xobject3 = new PdfDictionary();
    PdfDictionary xobject4 = new PdfDictionary();
    xobject4.Put(PdfName.FRM, appearance.GetAppearance().IndirectReference);
    xobject3.Put(PdfName.XOBJECT, xobject4);
    signatureField.Put(PdfName.DR, xobject3);

    stamper.AddAnnotation(signatureField, i);
}

IExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.ADOBE_PPKMS, PdfName.ADBE_PKCS7_DETACHED);
MakeSignature.SignExternalContainer(appearance, external, 8192);
stamper.Close();

byte[] SignedHash =  DoEsign(SHA256Managed.Create().ComputeHash(appearance.GetRangeStream());
os.close();
reader.close();

reader = new PdfReader(tempPdf))
os = File.OpenWrite(signedPdf)

IExternalSignatureContainer external1 = new MyExternalSignatureContainer(SignedHash);
MakeSignature.SignDeferred(reader, signatureFieldName, os, external1);
os.close();
reader.close();

Answer 1

若要为所有签名字段赋予包装新创建的签名容器的相同单个值，它们都必须引用相同的间接对象作为value。不幸的是，iText只有在应用程序代码有机会添加它的附加字段之后才为签名值创建间接对象，而附加字段又需要对该签名值对象的引用。因此，应用程序代码必须预测间接对象将具有的对象号。

为了使这更容易，应用程序代码应该尽可能晚地添加那些带有其签名值引用的签名字段，以便在iText创建值间接对象之前，创建的其他新间接对象尽可能少。

PdfReader reader = new PdfReader(SRC);
FileStream os = new FileStream(DEST, FileMode.Create, FileAccess.Write);
PdfStamper stamper = PdfStamper.CreateSignature(reader, os, '\0');
PdfSignatureAppearance appearance = stamper.SignatureAppearance;

appearance.Reason = "Reason1";
appearance.Contact = "";
appearance.Location = "Location1";
appearance.Acro6Layers = false;
appearance.Image = null;
appearance.SignatureRenderingMode = PdfSignatureAppearance.RenderingMode.DESCRIPTION;
appearance.SetVisibleSignature(new iTextSharp.text.Rectangle(10, 10, 100, 100), reader.NumberOfPages, null);

IExternalSignature externalSignature = new PrivateKeySignature(cp, "SHA-256");
AllPagesSignatureContainer allPagesContainer = new AllPagesSignatureContainer(appearance, externalSignature, chain);
MakeSignature.SignExternalContainer(appearance, allPagesContainer, 8192);

public class AllPagesSignatureContainer : IExternalSignatureContainer
{
    public AllPagesSignatureContainer(PdfSignatureAppearance appearance, IExternalSignature externalSignature, ICollection<X509Certificate> chain)
    {
        this.appearance = appearance;
        this.chain = chain;
        this.externalSignature = externalSignature;
    }

    public void ModifySigningDictionary(PdfDictionary signDic)
    {
        signDic.Put(PdfName.FILTER, PdfName.ADOBE_PPKMS);
        signDic.Put(PdfName.SUBFILTER, PdfName.ADBE_PKCS7_DETACHED);

        PdfStamper stamper = appearance.Stamper;
        PdfReader reader = stamper.Reader;
        PdfDictionary xobject1 = new PdfDictionary();
        PdfDictionary xobject2 = new PdfDictionary();
        xobject1.Put(PdfName.N, appearance.GetAppearance().IndirectReference);
        xobject2.Put(PdfName.AP, xobject1);

        PdfIndirectReference PRef = stamper.Writer.PdfIndirectReference;
        PdfLiteral PRefLiteral = new PdfLiteral((PRef.Number + 1 + 2*(reader.NumberOfPages - 1)) + " 0 R");

        for (int i = 1; i < reader.NumberOfPages; i++)
        {
            var signatureField = PdfFormField.CreateSignature(stamper.Writer);

            signatureField.Put(PdfName.T, new PdfString("ClientSignature_" + i.ToString()));
            signatureField.Put(PdfName.V, PRefLiteral);
            signatureField.Put(PdfName.F, new PdfNumber("132"));
            signatureField.SetWidget(appearance.Rect, null);
            signatureField.Put(PdfName.SUBTYPE, PdfName.WIDGET);

            signatureField.Put(PdfName.AP, xobject1);
            signatureField.SetPage();
            Console.WriteLine(signatureField);

            stamper.AddAnnotation(signatureField, i);
        }
    }

    public byte[] Sign(Stream data)
    {
        String hashAlgorithm = externalSignature.GetHashAlgorithm();
        PdfPKCS7 sgn = new PdfPKCS7(null, chain, hashAlgorithm, false);
        IDigest messageDigest = DigestUtilities.GetDigest(hashAlgorithm);
        byte[] hash = DigestAlgorithms.Digest(data, hashAlgorithm);
        byte[] sh = sgn.getAuthenticatedAttributeBytes(hash, null, null, CryptoStandard.CMS);
        byte[] extSignature = externalSignature.Sign(sh);
        sgn.SetExternalDigest(extSignature, null, externalSignature.GetEncryptionAlgorithm());
        return sgn.GetEncodedPKCS7(hash, null, null, null, CryptoStandard.CMS);
    }

    PdfSignatureAppearance appearance;
    ICollection<X509Certificate> chain;
    IExternalSignature externalSignature;
}

PdfIndirectReference PRef = stamper.Writer.PdfIndirectReference;
PdfLiteral PRefLiteral = new PdfLiteral((PRef.Number + 1 + 2*(reader.NumberOfPages - 1)) + " 0 R");

我在这里再次强调这一点，因为例如，当尝试“在单页上放置多个签名”时，这个问题的操作没有考虑到这一点。

stamper.Writer.AddToBody(new PdfNull(), stamper.Writer.PdfIndirectReference, true);

PdfIndirectReference PRef = stamper.Writer.PdfIndirectReference;
PdfLiteral PRefLiteral = new PdfLiteral((PRef.Number + reader.NumberOfPages) + " 0 R");