Csharp 显示文档的ECDsa签名PDF在Adobe Reader中签名后已被更改或损坏

我已经使用ECDSA(SHA-256)证书用iText 8.0.2签署了一个PDF文档.不幸的是，Adobe Reader说签名无效，理由是"Document has been altered or corrupted since it was signed".由于数字签名主题不是我的专长，我不知道如何查找错误以及在哪里查找错误.有谁能给我带路吗？

using iText.Kernel.Pdf;
using iText.Signatures;
using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

namespace PdfSignTest
{   
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 2)
            {
                Console.WriteLine("Usage: PdfSignTest.exe pdfFilePath pfxFilePath");
                Console.WriteLine(" - pdfFilePath: path of an unsigned pdf file which will be signed");
                Console.WriteLine(" - pfxFilePath: path of a pfx file containing the full certificate chain");
                Console.ReadKey();
                return;
            }

            string srcPath = args[0];
            string cerPath = args[1];
            string dstPath = srcPath.Replace(".pdf", ".signed.at." + DateTime.Now.ToString("HH-mm-ss") + ".pdf");

            var signed = SignPdfECC(srcPath, cerPath);
            File.WriteAllBytes(dstPath, signed);

            Console.WriteLine(string.Format("File {0} successfully signed and saved as {1}.", srcPath, dstPath));
            Console.ReadKey();
        }

        private static byte[] SignPdfECC(string unsignedPdfPath, string certificatePfxPath)
        {
            byte[] certificatePfx = File.ReadAllBytes(certificatePfxPath);

            using (PdfReader reader = new PdfReader(unsignedPdfPath))
            using (MemoryStream mem = new MemoryStream())
            {
                PdfSigner signer = new PdfSigner(reader, mem, new StampingProperties().UseAppendMode());
                X509Certificate cert = new X509Certificate(certificatePfx, "password", X509KeyStorageFlags.Exportable);
                X509Certificate2 signatureCert = new X509Certificate2(cert);
                ECDsa pk = signatureCert.GetECDsaPrivateKey();
                IExternalSignature signature = new EcdsaSignature(pk, Digest算法rithms.SHA256);
                iText.Bouncycastle.X509.X509CertificateBC[] chain = new iText.Bouncycastle.X509.X509CertificateBC[] { new iText.Bouncycastle.X509.X509CertificateBC(new Org.BouncyCastle.X509.X509Certificate(signatureCert.GetRawCertData())) };
                signer.SignDetached(signature, chain, null, null, null, 0, PdfSigner.CryptoStandard.CMS);
                return mem.ToArray();
            }
        }
    }

    public class EcdsaSignature : IExternalSignature
    {
        private readonly string _encryption算法rithm;
        private readonly string _hash算法rithm;
        private readonly ECDsa _pk;

        public EcdsaSignature(ECDsa pk, string hash算法rithm)
        {
            _pk = pk;
            _hash算法rithm = Digest算法rithms.GetDigest(Digest算法rithms.GetAllowedDigest(hash算法rithm));
            _encryption算法rithm = "ECDSA";
        }

        public string GetDigest算法rithmName()
        {
            return "SHA-256";
        }

        public virtual string GetEncryption算法rithm()
        {
            return _encryption算法rithm;
        }

        public virtual string GetHash算法rithm()
        {
            return _hash算法rithm;
        }

        public string GetSignature算法rithmName()
        {
            return "ECDSA";
        }

        public ISignatureMechanismParams GetSignatureMechanismParameters()
        {
            return null;
        }

        public virtual byte[] Sign(byte[] message)
        {
            return _pk.SignData(message, new Hash算法rithmName(_hash算法rithm));
        }
    }
}

我删除了所有其他功能，如TSA和LTV，以为可能是这些步骤中的一个导致了错误.我还try 使用从OpenSSL测试证书集合中获取的测试证书，以及从w3.org下载的测试PDF.所有这些都是here.