ECDsa 签名的 PDF 在 Adobe Reader 中显示“文档自签名以来已被更改或损坏”错误

Question

我已使用 ECDSA (SHA-256) 证书使用 iText 8.0.2 签署了 PDF 文档。不幸的是，Adobe Reader 说签名无效，原因是“文档自签名以来已被更改或损坏”。由于数字签名主题不是我的专长，因此我无法弄清楚如何以及在哪里查找错误。有人可以指导我吗？

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); // byte tömbben a pfx file tartalmát adjuk be
                X509Certificate2 signatureCert = new X509Certificate2(cert);
                ECDsa pk = signatureCert.GetECDsaPrivateKey();
                IExternalSignature signature = new EcdsaSignature(pk, DigestAlgorithms.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 _encryptionAlgorithm;
        private readonly string _hashAlgorithm;
        private readonly ECDsa _pk;

        public EcdsaSignature(ECDsa pk, string hashAlgorithm)
        {
            _pk = pk;
            _hashAlgorithm = DigestAlgorithms.GetDigest(DigestAlgorithms.GetAllowedDigest(hashAlgorithm));
            _encryptionAlgorithm = "ECDSA";
        }

        public string GetDigestAlgorithmName()
        {
            return "SHA-256";
        }

        public virtual string GetEncryptionAlgorithm()
        {
            return _encryptionAlgorithm;
        }

        public virtual string GetHashAlgorithm()
        {
            return _hashAlgorithm;
        }

        public string GetSignatureAlgorithmName()
        {
            return "ECDSA";
        }

        public ISignatureMechanismParams GetSignatureMechanismParameters()
        {
            return null;
        }

        public virtual byte[] Sign(byte[] message)
        {
            return _pk.SignData(message, new HashAlgorithmName(_hashAlgorithm));
        }
    }
}

我删除了所有附加功能，例如 TSA 和 LTV，预计可能是这些步骤之一导致了错误。我还尝试使用从 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

在这里

。

Answer 1

ECDsa.SignData

返回 PLAIN 格式的签名值，而 iText 使用 DER 格式的 ECDSA 签名的 OID。因此，在从

EcdsaSignature.Sign

返回签名值之前，您必须将其从 PLAIN 转换为 DER 格式，例如像这样：

using Org.BouncyCastle.Asn1;
using Org.BouncyCastle.Math;

[...]

public virtual byte[] Sign(byte[] message)
{
    return PlainToDer(_pk.SignData(message, new HashAlgorithmName(_hashAlgorithm)));
}

byte[] PlainToDer(byte[] plain)
{
    int valueLength = plain.Length / 2;
    BigInteger r = new BigInteger(1, plain, 0, valueLength);
    BigInteger s = new BigInteger(1, plain, valueLength, valueLength);
    return new DerSequence(new DerInteger(r), new DerInteger(s)).GetEncoded(Asn1Encodable.Der);
}

（

SignWithEcdsaCert类EcdsaSignature方法）

PlainToDer

辅助方法借用自 iText 知识库文章

使用 iText 进行数字签名

/第一部分 - 概述和简单案例

/

使用 .NET 进行签名的

X509Certificate2Signature

实现IExternalSignature CryptoAPI/CNG 密钥.