使用iTextsharp进行外部签名PDF（3）

时间：2020-01-15 12:50:18

标签： pdf itext digital-signature

需要通过使用外部Web服务对文档哈希进行签名来对PDF进行签名，该过程必须分两步完成，并使用一个临时的空签名。

继What's the best practice to keep all the constants in Flutter?和Priyanka question并阅读该帖子的mkl答案后，即使添加了像Grazina这样的哈希前缀，我当前的签名也无效。

iTextSharp版本：5.5.13.1

该程序是Grazina question的另一种方法。 当前代码（编译并开始调用 SignPDF 方法）：

public class PDFSigner
{
    private const string SIG_FIELD_NAME = "sigField1";

    private void SignPDF(string pdfFilePath, string userId)
    {
        var preparedSigPdfFilePath = $"{pdfFilePath}.tempsig.pdf";
        var signedPdfFilePath = $"{pdfFilePath}.signed.pdf";

        //Get certificates chain from webservice
        var certificatesChain = this.GetUserCertificates(userId);

        byte[] hash = this.CreatePDFEmtySignature(pdfFilePath, preparedSigPdfFilePath, certificatesChain);

        //Get signature from webservice
        byte[] signedHash = this.GetSignature(hash, userId);

        CreateFinalSignature(preparedSigPdfFilePath, signedPdfFilePath, hash, signedHash, certificatesChain);
    }

    private byte[] CreatePDFEmtySignature(string pdfFilePath, string preparedSigPdfFilePath, List<Org.BouncyCastle.X509.X509Certificate> certificatesChain)
    {
        byte[] hash = null;

        using (PdfReader reader = new PdfReader(pdfFilePath))
        {
            using (FileStream baos = File.OpenWrite(preparedSigPdfFilePath))
            {
                PdfStamper pdfStamper = PdfStamper.CreateSignature(reader, baos, '\0', null, true);
                PdfSignatureAppearance sap = pdfStamper.SignatureAppearance;
                sap.SetVisibleSignature(new iTextSharp.text.Rectangle(36, 720, 160, 780), 1, SIG_FIELD_NAME);

                //TODO: check how to select the correct certificate, have 3 items on list, selected leaf after debug (first one)
                sap.Certificate = certificatesChain.First();

                var externalEmptySigContainer = new MyExternalEmptySignatureContainer(PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED, preparedSigPdfFilePath);

                MakeSignature.SignExternalContainer(sap, (IExternalSignatureContainer)externalEmptySigContainer, 8192);

                hash = externalEmptySigContainer.PdfHash;
            }
        }
        return hash;
    }

    private void CreateFinalSignature(string preparedSigPdfFilePath, string signedPdfFilePath, 
        byte[] hash, byte[] signedHash, List<Org.BouncyCastle.X509.X509Certificate> certificatesChain)
    {
        using (PdfReader reader = new PdfReader(preparedSigPdfFilePath))
        {
            using (FileStream baos = File.OpenWrite(signedPdfFilePath))
            {
                IExternalSignatureContainer externalSigContainer = new MyExternalSignatureContainer(signedPdfFilePath, hash, signedHash, certificatesChain);
                MakeSignature.SignDeferred(reader, SIG_FIELD_NAME, baos, externalSigContainer);
            }
        }
    }

    public class MyExternalEmptySignatureContainer : ExternalBlankSignatureContainer
    {
        public string PdfTempFilePath { get; set; }
        public byte[] PdfHash { get; private set; }

        public MyExternalEmptySignatureContainer(PdfName filter, PdfName subFilter, string pdfTempFilePath) : base(filter, subFilter)
        {
            this.PdfTempFilePath = pdfTempFilePath;
        }

        override public byte[] Sign(Stream data)
        {
            byte[] sigContainer = base.Sign(data);

            //Get the hash
            IDigest messageDigest = DigestUtilities.GetDigest("SHA-256");
            byte[] messageHash = DigestAlgorithms.Digest(data, messageDigest);

            #region Log
            var messageHashFilePath = $"{this.PdfTempFilePath}.messageHash-b64.txt";
            File.WriteAllText(messageHashFilePath, Convert.ToBase64String(messageHash));
            #endregion Log

            //Add hash prefix
            byte[] sha256Prefix = { 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20 };
            byte[] digestInfo = new byte[sha256Prefix.Length + messageHash.Length];
            sha256Prefix.CopyTo(digestInfo, 0);
            messageHash.CopyTo(digestInfo, sha256Prefix.Length);

            #region Log
            var messageHashWithPrefixFilePath = $"{this.PdfTempFilePath}.messageHash-with-prefix-b64.txt";
            File.WriteAllText(messageHashWithPrefixFilePath, Convert.ToBase64String(digestInfo));
            #endregion Log

            this.PdfHash = digestInfo;

            return sigContainer;
        }
    }

    public class MyExternalSignatureContainer : IExternalSignatureContainer
    {
        public byte[] Hash { get; set; }
        public byte[] SignedHash { get; set; }
        public List<Org.BouncyCastle.X509.X509Certificate> CertificatesList { get; set; }

        public MyExternalSignatureContainer(string signedPdfFilePath, byte[] hash, byte[] signedHash, List<Org.BouncyCastle.X509.X509Certificate> certificatesList)
        {
            this.Hash = hash;
            this.SignedHash = signedHash;
            this.CertificatesList = certificatesList;
        }

        public byte[] Sign(Stream data)
        {
            PdfPKCS7 sgn = new PdfPKCS7(null, this.CertificatesList, "SHA256", false);
            sgn.SetExternalDigest(this.SignedHash, null, "RSA");
            return sgn.GetEncodedPKCS7(this.Hash, null, null, null, CryptoStandard.CMS);         
        }

        public void ModifySigningDictionary(PdfDictionary signDic)  {   }
    }

    public byte[] GetSignature(byte[] hash, string userId)
    {
        // Request signature for hash value messageHash and return signature bytes               
        byte[] signature = null;

        //CALL WEBSERVICE:
        //signature = WEBSERVICE_CALL_TO_GET_SIGNED_HASH(hash, userId);

        return signature;
    }

    private List<Org.BouncyCastle.X509.X509Certificate> GetUserCertificates(string userId)
    {
        List<Org.BouncyCastle.X509.X509Certificate> certChain = null;

        //CALL WEBSERVICE:
        //certChain = WEBSERVICE_CALL_TO_GET_SIGNED_CERTIFICATES(userId);

        return certChain;
    }
}

获得结果

Check image from Acrobat reader signature validity check

邮件哈希（BASE 64）：

lA2cMByHLkuNdd+aHJRDy3GD2VIeIpVtzlgQGsq3cJw=

带前缀的邮件哈希（BASE 64）：

MDEwDQYJYIZIAWUDBAIBBQAEIJQNnDAchy5LjXXfmhyUQ8txg9lSHiKVbc5YEBrKt3Cc

签名哈希（基于64）：

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

更新 使用my previous question测试了PDF签名，获得了以下结果：

Certificates:
Subject: [REMOVED]
Issuer: [REMOVED]
Serial: 7590871326079402939
Subject: [REMOVED]
Issuer: [REMOVED]
Serial: 1738456118788016053
Subject: [REMOVED]
Issuer: [REMOVED]
Serial: 8957244856106358046

Attribute Certificates: none

CRLs: none

SignerInfo: [REMOVED] 8957244856106358046
Certificate: [REMOVED]
Signed attribute 1.2.840.113549.1.9.4 (PKCS 9 - Message Digest)

Digest: 
3031300D060960864801650304020105000420940D9C301C872E4B8D75DF9A1
C9443CB7183D9521E22956DCE58101ACAB7709C

Signed attribute 1.2.840.113549.1.9.3 (PKCS 9 - Content Type)

Signed Attributes Hash: 
08767823328F202C1C3E5DB543785ED591C6D84D23DAF3DCBB83684B987008CB
Signed Attributes Hash Hash: 
1E2D10B23CD772D16987126182E51BD4D827DB58C497BA4129BB533A576E3548
!!! Decrypted RSA signature is not PKCS1 padded: Decryption error
Decrypted signature bytes: [REMOVED]
!!! Decrypted RSA signature does not end with the PSS 0xbc byte either

!!! Signature does not validate with certificate

将尝试将填充添加到签名中，不胜感激。

1 个答案:

答案 0 :(得分：1)

签名值与签名者证书不符

首先，根据结果PDF ，尽管使用了GetUserCertificates ，但似乎没有返回GetSignature 的签名的签名者证书相同的userId值。

这可以通过粘贴到问题中的AnalyzeSignatures输出来表明：

SignerInfo: [REMOVED] 8957244856106358046
Certificate: [REMOVED]

签名容器中单个SignerInfo中的SID与所包含的证书集中的单个证书匹配。

!!! Decrypted RSA signature is not PKCS1 padded: Decryption error
Decrypted signature bytes: [REMOVED]
!!! Decrypted RSA signature does not end with the PSS 0xbc byte either

该证书的公共密钥是RSA密钥，并且SignerInfo签名值的长度与密钥长度匹配，但是使用该密钥解密该值既不会返回填充了PKCS＃1 v1.5的内容，也不会返回PSS结构。因此，“签名值”不是根本不是RSA签名值，或者是使用与所称签名者证书中的公钥不匹配的私钥生成的签名。

因此，要做的第一件事就是分析

    //CALL WEBSERVICE:
    //signature = WEBSERVICE_CALL_TO_GET_SIGNED_HASH(hash, userId);

和

    //CALL WEBSERVICE:
    //certChain = WEBSERVICE_CALL_TO_GET_SIGNED_CERTIFICATES(userId);

代码的隐藏部分并进行修复（如果存在问题，则修复Web服务），然后将其固定集成到PDF签名框中。

PDF签名代码框架的错误

此处的PDF签名框架包含一些错误。上一个问题"External signing PDF with iText (2) [closed]"中的代码看起来更正确，因此在这里找到对签名Web服务的正确使用之后，我建议在该代码的基础上采用下一种方法。

尽管如此，这里还是对两个错误的解释，这些错误很快引起了我的注意：

byte[] hash = this.CreatePDFEmtySignature(pdfFilePath, preparedSigPdfFilePath, certificatesChain);

//Get signature from webservice
byte[] signedHash = this.GetSignature(hash, userId);

CreateFinalSignature(preparedSigPdfFilePath, signedPdfFilePath, hash, signedHash, certificatesChain);

发送了用于签名到Web服务的错误哈希值

CreatePDFEmtySignature返回的哈希值是类MyExternalEmptySignatureContainer试图确定有符号字节范围的哈希值的结果。

但是通常在CMS签名容器的上下文中（最原始的类型除外），不是直接为文档字节创建实际的签名字节，而是为属性结构（即所谓的“签名属性”或“经过身份验证的”）创建属性”）；文档字节的哈希仅仅是这些属性之一的值。

因此，signedHash在hash中通过GetSignature返回的CreateFinalSignature的{​​{1}}的签名值MyExternalSignatureContainer.Sign被注入到PKCS＃7 / CMS签名容器SignerInfo中其经过身份验证的属性具有完全不同的哈希值。

使用iText PdfPKCS7类生成签名容器时，必须使用与后面的{{ 1}}通话。

因此，对这样签名的PDF的实际签名字节的验证必须失败。

用DigestInfo包裹的散列应该放在裸露的地方

由PdfPKCS7.GetAuthenticatedAttributeBytes返回的哈希值包装在PdfPKCS7.GetEncodedPKCS7结构中（通过在CreatePDFEmtySignature中相应地预先放置一些字节）。

通过DigestInfo中的MyExternalEmptySignatureContainer.Sign，稍后将其作为有符号字节的哈希值提供给CreateFinalSignature。但是，这里期望它是裸露的，没有包裹在MyExternalSignatureContainer.Sign结构中。

因此，对于像这样签名的PDF，签名文档字节的哈希值验证必须失败。

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