使用Web服务对摘要进行iText7签名PDF文件

如何解决使用Web服务对摘要进行iText7签名PDF文件

我需要对PDF文档进行数字签名。要签名，我需要将摘要发送到另一个Web服务。

Firs步骤是创建带有空签名的新PDF文件。之后，我需要提取该文件的摘要，然后将其发送到Web服务进行签名。

 public static string GetBytesToSign(string unsignedPdf,string tempPdf,string signatureFieldName)

        {

            if (File.Exists(tempPdf))
                File.Delete(tempPdf);

            using (PdfReader reader = new PdfReader(unsignedPdf))
            {
                using (FileStream os = File.OpenWrite(tempPdf))
                {
                    StampingProperties sp = new StampingProperties();
                    sp.UseAppendMode();

                    PdfSigner pdfSigner = new PdfSigner(reader,os,sp);
                    pdfSigner.SetFieldName(signatureFieldName);

                    PdfSignatureAppearance appearance = pdfSigner.GetSignatureAppearance();
                    appearance.SetPageNumber(1);
                    appearance.SetPageRect(new Rectangle(100,100));
                    appearance.SetLocation("Varazdin");

                    //Creating container for emty signature,with atrivute where digest is calculated.
                    //ExternalHashingSignatureContainer external = new ExternalHashingSignatureContainer(PdfName.Adobe_PPKLite,PdfName.Adbe_pkcs7_detached);
                    //pdfSigner.SignExternalContainer(external,8192);
                    //hash = external.Hash;

                    //Creating container for empty signature.
                    IExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.Adobe_PPKLite,PdfName.Adbe_x509_rsa_sha1);
                    pdfSigner.SignExternalContainer(external,8192);

                    //Digest from created new temporary PDF with empty space for signature.
                    FileStream oso = File.OpenRead(temp);
                    hash = DigestAlgorithms.Digest(oso,DigestAlgorithms.SHA256);

                    return Convert.ToBase64String(hash);
                }
            }
        }

然后Web服务发送此签名：

T0Am92hjg5Q+CrMdrq8hQEiJ80UUyBWyAeo0JQoLGSq0LdK/ZhZB2Mk9feCiohiDpA5Qyp2TCoa2FWbk6jK48ajAHKxmEc5wO4Niv2DlaWh8fv2lBj+OxpVrUDm7Y26o4ITQzC8VECLNHE6jQL7O6VCmCdhaOrSpnvHaCZctrFfrxWlTT4d6tQBtR/Znp/eam4PxQtoUxYkan8FYxxMacfOm3hEPdUJ2Wp2chiWDou4bn+Mc6JERLqErmZcNrzASifiuvMn5padc/qNN71tJYCi1XlAgzrWgZqShgRTNxXD8CBbJD+2vCGkLJlrEuiCqyvMeFNc911zw/Ln1P8ZCDw==

然后我尝试将该签名添加到带有空签名的临时pdf文件中。我使用以下代码：

public static void EmbedSignature(string tempPdf,string signedPdf,string signatureFieldName,string signature,string tbs)
        {            
            //Convert given data from web service to bytes
            byte[] signedBytes = ConvertToBytes(signature);
            byte[] toBeSigned = ConvertToBytes(tbs);

            using (PdfReader reader = new PdfReader(tempPdf))
            {
                using (FileStream os = File.OpenWrite(signedPdf))
                {
                    PdfSigner signer = new PdfSigner(reader,new StampingProperties());
                    IExternalSignatureContainer external = new MyExternalSignatureContainer(signedBytes,GetChains(),toBeSigned);
                    PdfSigner.SignDeferred(signer.GetDocument(),signatureFieldName,external);
                }
            }
        }



class MyExternalSignatureContainer : IExternalSignatureContainer
{
    protected X509Certificate[] chain;
    protected byte[] signedHash;
    protected byte[] hash;

    public MyExternalSignatureContainer(byte[] signedHash,X509Certificate[] chain,byte[] hash)
    {
        this.signedHash = signedHash;
        this.chain = chain;
        this.hash = hash;
    }

    public byte[] Sign(Stream inputStream)
    {
        try
        {
            String hashAlgorithm = DigestAlgorithms.SHA256;
            PdfPKCS7 sgn = new PdfPKCS7(null,chain,hashAlgorithm,false);
            sgn.SetExternalDigest(signedHash,null,"RSA");

            return sgn.GetEncodedPKCS7(hash,PdfSigner.CryptoStandard.CMS,null);
        }
        catch (IOException ioe)
        {
            throw new Exception(ioe.Message);
        }
    }

    public void ModifySigningDictionary(PdfDictionary signDic)
    {
    }
}

此后，所有消息均已成功导入，但在Adobe Reader中是此消息：

文档签名后被更改或损坏。