c# - iTextSharp 中的 Pades LTV 验证抛出公钥不是用于根 CA 证书的证书签名

Question

我得到一个 Org.BouncyCastle.Security.InvalidKeyException使用 LtvVerifier 验证 pdf 时出现错误消息 Public key presented not for certificate signature .

这个问题出现在circumventing an issue with CRL LDAP URIs之后.用于执行验证的代码与上一篇文章相同:

   public static bool Validate(byte[] pdfIn, X509Certificate2 cert)
    {
        using (var reader = new PdfReader(pdfIn))
        {
            var fields = reader.AcroFields;
            var signames = fields.GetSignatureNames();

            if (!signames.Any(n => fields.SignatureCoversWholeDocument(n)))
                throw new Exception("None signature covers all document");

            var verifications = signames.Select(n => fields.VerifySignature(n));

            var invalidSignature = verifications.Where(v => !v.Verify());
            var invalidTimeStamp = verifications.Where(v => !v.VerifyTimestampImprint());

            if (invalidSignature.Any())
                throw new Exception("Invalid signature found");
        }

        using (var reader = new PdfReader(pdfIn))
        {
            var ltvVerifier = new LtvVerifier(reader)
            {
                OnlineCheckingAllowed = false,
                CertificateOption = LtvVerification.CertificateOption.WHOLE_CHAIN,
                Certificates = GetChain(cert).ToList(),
                VerifyRootCertificate = false,
                Verifier = new MyVerifier(null)
            };

            var ltvResult = new List<VerificationOK> { };
            ltvVerifier.Verify(ltvResult);

            if (!ltvResult.Any())
                throw new Exception("Ltv verification failed");
        }
        return true;
   }

从证书链构建 X509Certificates 列表的辅助函数:

    private static X509.X509Certificate[] GetChain(X509Certificate2 myCert)
    {
        var x509Chain = new X509Chain();
        x509Chain.Build(myCert);

        var chain = new List<X509.X509Certificate>();
        foreach(var cert in x509Chain.ChainElements)
        {
            chain.Add(
                DotNetUtilities.FromX509Certificate(cert.Certificate)
                );
        }

        return chain.ToArray();
    }

自定义验证器，刚刚从示例中复制:

 class MyVerifier : CertificateVerifier
{
    public MyVerifier(CertificateVerifier verifier) : base(verifier) { }

    override public List<VerificationOK> Verify(
        X509.X509Certificate signCert, X509.X509Certificate issuerCert, DateTime signDate)
    {
        Console.WriteLine(signCert.SubjectDN + ": ALL VERIFICATIONS DONE");
        return new List<VerificationOK>();
    }
}

我重新实现了LtvVerifier和 CrlVerifier如上一个问题所述。 CRL 验证完成。

证书链包括用于签署 PDF 的证书和 CA 根证书。函数 CrlVerifier.Verify调用下一行时抛出上述异常:

 if (verifier != null)
                result.AddRange(verifier.Verify(signCert, issuerCert, signDate));
            // verify using the previous verifier in the chain (if any)
            return result;

这是 Org.BouncyCastle.Security.InvalidKeyException 的相关堆栈跟踪:

   in Org.BouncyCastle.X509.X509Certificate.CheckSignature(AsymmetricKeyParameter publicKey, ISigner signature)
   in Org.BouncyCastle.X509.X509Certificate.Verify(AsymmetricKeyParameter key)
   in iTextSharp.text.pdf.security.CertificateVerifier.Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime signDate)
   in iTextSharp.text.pdf.security.RootStoreVerifier.Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime signDate)
   in PdfCommon.CrlVerifierSkippingLdap.Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime signDate) in c:\Projects\digit\Fuentes\PdfCommon\CrlVerifierSkippingLdap.cs:line 76
   in iTextSharp.text.pdf.security.OcspVerifier.Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime signDate)
   in PdfCommon.LtvVerifierSkippingLdap.Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime sigDate) in c:\Projects\digit\Fuentes\PdfCommon\LtvVerifierSkippingLdap.cs:line 221
   in PdfCommon.LtvVerifierSkippingLdap.VerifySignature() in c:\Projects\digit\Fuentes\PdfCommon\LtvVerifierSkippingLdap.cs:line 148
   in PdfCommon.LtvVerifierSkippingLdap.Verify(List`1 result) in c:\Projects\digit\Fuentes\PdfCommon\LtvVerifierSkippingLdap.cs:line 112

还有一个 link to the pdf that I try to validate

Answer 1

经过一些调试后发现

iText(Sharp) 5.5.10 LtvVerifier 在验证证书链未以自签名证书结尾的证书时，以观察到的方式失败。

原因

原因很简单:LtvVerifier 建立了一系列 Verifier 实例(OcspVerifier、CrlVerifier、RootStoreVerifier, CertificateVerifier；最后一个通过基类调用链接)。然后它请求相关签名的签名证书的证书链，并为链中的每个证书调用由该证书及其颁发者组成的证书对的 Verifier 序列；如果是链中的最终证书，null 将作为颁发者证书转发。

不幸的是，最终的Verifier，CertificateVerifier，在null颁发者证书的情况下假定要验证的证书是自签名的:

// Check if the signature is valid
if (issuerCert != null) {
    signCert.Verify(issuerCert.GetPublicKey());
}
// Also in case, the certificate is self-signed
else {
    signCert.Verify(signCert.GetPublicKey());
} 

(来自 CertificateVerifier 方法 Verify)

如果最初请求的 LtvVerifier 证书链没有以自签名证书结尾，则最终测试正确地得出观察到的结果

Org.BouncyCastle.Security.InvalidKeyException with error message Public key presented not for certificate signature

OP 的例子

在手头的案例中，我们有一个由

发布的文档时间戳

cn=AUTORIDAD DE SELLADO DE TIEMPO FNMT-RCM, ou=CERES, o=FNMT-RCM, c=ES

颁发者

cn=AC Administración Pública, serialNumber=Q2826004J, ou=CERES, o=FNMT-RCM, c=ES

颁发者

ou=AC RAIZ FNMT-RCM, o=FNMT-RCM, c=ES

这是自签名的。

在这种情况下，中介证书 AC Administración Pública 已经在欧洲信任列表中(参见 the TL manager for Spain，“信任服务提供商”，“Fábrica Nacional de Moneda y Timbre - Real Casa de la Moneda (FNMT-RCM)、“信托服务”、“Certificados reconocidos para su uso en el ámbito de...”、“数字身份”)。

因此，只需要前两个证书即可建立信任，不需要自签名根证书。因此，只有前两个证书嵌入到时间戳中并作为证书链返回给 LtvVerifier，而不是自签名根。

结果是 LtvVerifier 中观察到的错误。

做什么？

好吧，因为我们已经开始在 previous question 中创建我们自己的相关类副本。 ，对它们进行更多更改应该是一种选择。

在这种情况下，应该另外更改 RootStoreVerifier。它的 Verify 方法如下所示:

override public List<VerificationOK> Verify(X509Certificate signCert, X509Certificate issuerCert, DateTime signDate) {
    LOGGER.Info("Root store verification: " + signCert.SubjectDN);
    // verify using the CertificateVerifier if root store is missing
    if (certificates == null)
        return base.Verify(signCert, issuerCert, signDate);
    try {
        List<VerificationOK> result = new List<VerificationOK>();
        // loop over the trusted anchors in the root store
        foreach (X509Certificate anchor in certificates) {
            try {
                signCert.Verify(anchor.GetPublicKey());
                LOGGER.Info("Certificate verified against root store");
                result.Add(new VerificationOK(signCert, this, "Certificate verified against root store."));
                result.AddRange(base.Verify(signCert, issuerCert, signDate));
                return result;
            } catch (GeneralSecurityException) {}
        }
        result.AddRange(base.Verify(signCert, issuerCert, signDate));
        return result;
    } catch (GeneralSecurityException) {
        return base.Verify(signCert, issuerCert, signDate);
    }
}

我们只需要删除标记线

                signCert.Verify(anchor.GetPublicKey());
                LOGGER.Info("Certificate verified against root store");
                result.Add(new VerificationOK(signCert, this, "Certificate verified against root store."));
                // vvv remove
                result.AddRange(base.Verify(signCert, issuerCert, signDate));
                // ^^^ remove
                return result;

在内部 try block 中。由于我们刚刚确定证书 signCert 是由信任 anchor 签署的，因此无论如何都不需要 base.Verify。