如何使用iText IExternalSignatureContainer提前创建pdf哈希

Question

我正在使用iText7对pdf文档应用签名。我还使用自己的IExternalSignatureContainer实现将证书集成到PKCS7 CMS中，因为签名服务只返回PKCS1签名。

签名过程是异步的（用户必须进行身份验证），我希望执行以下操作：

• 准备文档（PDF阅读器）
• 将文档的哈希值返回给用户
• 扔掉文档（PDF阅读器）
• 让用户进行身份验证（与iText签名过程没有直接关系）并创建签名（PKCS1）
• 如果用户已通过身份验证，请重新准备文档并应用签名

这样做的原因是，我不需要把准备好的文档保存在内存中，也不需要批量签名。

我的问题是，创建的哈希值总是不同的。（即使我通过pdfSigner.SetSignDate将日期/时间设置为相同的值）或每个PdfReader/pdfSigner实例。

            //Create the hash of of the pdf document 
            //Part of my IExternalSignatureContainer Sign method
            //Called from iText pdfSigner.SignExternalContainer
            //The produced hash is always different
            byte[] hash = DigestAlgorithms.Digest(pdfStream, DigestAlgorithms.GetMessageDigest(hashAlgorithm));

问题：有没有办法

• 在PdfReader的一个实例上“提前”生成pdf文档的哈希值

附件是该过程的完整示例（包括实际上需要由不同服务完成的签名创建）

using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using iText.Kernel.Pdf;
using iText.Signatures;
using Org.BouncyCastle.X509;
using X509Certificate = Org.BouncyCastle.X509.X509Certificate;

namespace SignExternalTestManuel
{
    class Program
    {
        const string filePath = @"c:\temp\pdfsign\";
        public static string pdfToSign = Path.Combine(filePath, @"test.pdf");
        public static string destinationFile = Path.Combine(filePath, "test_signed.pdf");
        public static string LocalUserCertificatePublicKey = Path.Combine(filePath, "BITSignTestManuel5Base64.cer");
        public static string LocalCaCertificatePublicKey = Path.Combine(filePath, "BITRoot5Base64.cer");
        public static string privateKeyFile = Path.Combine(filePath, "BITSignTestManuel5.pfx");
        public static string privateKeyPassword = "test";

        public static void Main(String[] args)
        {
            PdfReader reader = new PdfReader(pdfToSign);
            using (FileStream os = new FileStream(destinationFile, FileMode.OpenOrCreate))
            {
                
                StampingProperties stampingProperties = new StampingProperties();
                stampingProperties.UseAppendMode();
                PdfSigner pdfSigner = new PdfSigner(reader, os, stampingProperties);
                pdfSigner.SetCertificationLevel(PdfSigner.NOT_CERTIFIED);

                IExternalSignatureContainer external = new GsSignatureContainer(
                    PdfName.Adobe_PPKLite,
                    PdfName.Adbe_pkcs7_detached);

                pdfSigner.SetSignDate(new DateTime(2021, 2, 22, 10, 0, 0));

                pdfSigner.SetFieldName("MySignatureField");
                pdfSigner.SignExternalContainer(external, 32000);
            }
        }
    }


    public class GsSignatureContainer : IExternalSignatureContainer
    {
        private PdfDictionary sigDic;


        public GsSignatureContainer(PdfName filter, PdfName subFilter)
        {
            sigDic = new PdfDictionary();
            sigDic.Put(PdfName.Filter, filter);
            sigDic.Put(PdfName.SubFilter, subFilter);
        }

        /// <summary>
        /// Implementation based on https://kb.itextpdf.com/home/it7kb/examples/how-to-use-a-digital-signing-service-dss-such-as-globalsign-with-itext-7#HowtouseaDigitalSigningService(DSS)suchasGlobalSign,withiText7-Examplecode
        /// </summary>
        /// <param name="pdfStream"></param>
        /// <returns></returns>
        public byte[] Sign(Stream pdfStream)
        {
            //Create the certificate chaing since the signature is just a PKCS1, the certificates must be added to the signature
            X509Certificate[] chain = null;


            string cert = System.IO.File.ReadAllText(Program.LocalUserCertificatePublicKey);
            string ca = System.IO.File.ReadAllText(Program.LocalCaCertificatePublicKey);
            chain = CreateChain(cert, ca);

            X509CrlParser p = new X509CrlParser();

            String hashAlgorithm = DigestAlgorithms.SHA256;
            PdfPKCS7 pkcs7Signature = new PdfPKCS7(null, chain, hashAlgorithm, false);

            //Create the hash of of the pdf document 
            //Part of my IExternalSignatureContainer Sign method
            //Called from iText pdfSigner.SignExternalContainer
            //The produced hash is always different
            byte[] hash = DigestAlgorithms.Digest(pdfStream, DigestAlgorithms.GetMessageDigest(hashAlgorithm));

            byte[] signature = null;

            //Create the hash based on the document hash which is suitable for pdf siging with SHA256 and a X509Certificate
            byte[] sh = pkcs7Signature.GetAuthenticatedAttributeBytes(hash, null, null, PdfSigner.CryptoStandard.CMS);
            //Create the signature via own certificate
            signature = CreateSignature(sh, Program.privateKeyFile, Program.privateKeyPassword);
            pkcs7Signature.SetExternalDigest(signature, null, "RSA");
            return pkcs7Signature.GetEncodedPKCS7(hash, null, null, null, PdfSigner.CryptoStandard.CMS);
        }

        public void ModifySigningDictionary(PdfDictionary signDic)
        {
            signDic.PutAll(sigDic);
        }

        private static X509Certificate[] CreateChain(String cert, String ca)
        {
            //Note: The root certificate could be omitted and it would still work
            X509Certificate[] chainy = new X509Certificate[2];
            X509CertificateParser parser = new X509CertificateParser();
            chainy[0] = new X509Certificate(parser.ReadCertificate(Encoding.UTF8.GetBytes(cert))
                .CertificateStructure);
            chainy[1] = new X509Certificate(parser.ReadCertificate(Encoding.UTF8.GetBytes(ca))
                .CertificateStructure);
            return chainy;
        }

        #region "Create signature, will be done by an actual service"
        private byte[] CreateSignature(byte[] hash, string privateKeyFile, string privateKeyPassword)
        {
            //Sign data directly with a X509Certificate
            X509Certificate2 rootCertificateWithPrivateKey = new X509Certificate2();
            byte[] rawData = System.IO.File.ReadAllBytes(privateKeyFile);
            rootCertificateWithPrivateKey.Import(rawData, privateKeyPassword, X509KeyStorageFlags.Exportable);

            using (var key = rootCertificateWithPrivateKey.GetRSAPrivateKey())
            {
                return key.SignData(hash, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
            }
        }
        #endregion


    }
}

Answer 1

问题：有没有办法

• 在PdfReader的一个实例上“提前”生成pdf文档的哈希值
• 创建签名
• 在PdfReader的不同实例上应用签名

iText目前不支持此用例，尤其是在每个过程中

• 将生成不同的PDF ID，
• 使用不同的修改时间，并且
• 在AES加密PDF的情况下，用于加密的随机数是不同的

在每一次传递中，可以修补ITEXT使用相同的值，但是在修补库之前，您应该考虑是否可以调整架构以使补丁不必要。

例如，在您的情况下，如果您不能保留原始的PdfSigner实例，另一种方法可以是让原始的PdfSigner在散列后用虚拟签名字节（例如new byte[0]）存储其结果文件）。然后，在检索签名容器后，您可以使用PdfSigner.signDeferred将其注入到不同服务中的存储文件中，只要两个服务都可以访问共享存储（或者第一个服务至少可以将文件转发到第二服务的存储）。