使用存储在证书存储中并使用 BouncyCastle 和 C# 标记为不可导出的 X509Certificate2 的私钥

Question

I'm trying to build a small certificate authority using bouncycastle.

The certificate is generated by using BouncyCastle and is stored in the Certificate Store of the current user, along with the private key.

However for security reasons the key is marked as non-exportable. I know that it is easy to circumvent this, but it's meant to be an additional barrier.

Now if a new certificate should be signed, I need to access the private key of the certificate. I tried using this code:

X509Certificate2 caCert = GetRootKey(); // Fetches the certificate from the store
AsymmetricCipherKeyPair caPrivKey = DotNetUtilities.GetKeyPair(caCert.PrivateKey);

Unfortunately I get an CryptographicException - because the private key is marked as not exportable, and this is what I intended ;-)

My question is: How can I use the private key of the certificate without accessing it? I'm pretty sure I'm missing some important point as it would not make any sense to have a private key stored without any way to access it...

I'm searching now for hours for this, but don't find anything on stackoverflow or Google.

Can anyone tell me please what I do fundamentally wrong? Is there another approach to sign something?!

Thanks in advance!

Chris

Answer 1

Short answer: Not with BouncyCastle.

You can't put the key into the BouncyCastle (or OpenSSL or any other) library and use it from there. There is no way to do this without circumventing the "no export" flag and exporting it, which as you point out is possible if you are an administrator and have the requisite Fu.

What you can do is get a CryptoAPI handle to the key, and ask CryptoAPI to do the signing or encryption operations for you. The DotNet crypto classes encapsulate CyptoAPI.

So essentially what you need is a tutorial on how to use the DotNet crypto classes. There are many, for example this MSDN Magazine article has an example of digital signatures about three quarters of the way down:

http://msdn.microsoft.com/en-us/magazine/cc163454.aspx

Answer 2

This question is exactly what I need, but the answer has a link that is not working. After searching for some time I found this implementation using NCrypt library: https://github.com/Microsoft/Windows-classic-samples/blob/master/Samples/Win7Samples/security/cryptoapi/CertSign/CPP/Sign.cpp

Unfortunately this is written in C, while I need a C# implementation. So I created this C++\\CLI wrapper (SignerWrapper.h) that can be used in managed C# code: https://github.com/DmitriNymi/StoreCertificateSigner

I also wrote a custom ISignatureFactory to be used in BouncyCastle, see PksAsn1SignatureFactory.cs in the same repo. It is there only to have my custom ISigner to be created (my class: PksEcdsaSigner ).

Now you can pass PksAsn1SignatureFactory as ISignatureFactory to BouncyCastle for creating new certificates for example, see unit test PksAsn1SignatureFactoryTest.cs - explicit call to ISignatureFactory and CertificateIssuerTest.cs - using helper class to create new certificates signed by private keys from another certificate in the store

Answer 3

I didn't want to add C++/CLI to my project, and so came up with a pure-c# solution using System.Security.Pkcs.SignedCms. Similar to Searcher's answer, this provides an implementation of ISignatureFactory that can be used to sign certificates in BouncyCastle.

using Org.BouncyCastle.Asn1.X509;
using Org.BouncyCastle.Cms;
using Org.BouncyCastle.Crypto;
using System.IO;
using System.Linq;
using System.Security.Cryptography.X509Certificates;

public class WindowsSignatureFactory : ISignatureFactory
{
    public AlgorithmIdentifier Algorithm { get; }
    public X509Certificate2 Cert { get; }

    public object AlgorithmDetails => Algorithm;

    public WindowsSignatureFactory(AlgorithmIdentifier algorithm,
            X509Certificate2 cert)
    {
        Algorithm = algorithm;
        Cert = cert;
    }

    public IStreamCalculator CreateCalculator()
    {
        return new WindowsStreamCalculator(Algorithm, Cert);
    }

    private class WindowsStreamCalculator : IStreamCalculator
    {
        public AlgorithmIdentifier Algorithm { get; }
        public X509Certificate2 Cert { get; }
        private MemoryStream MemoryStream { get; } = new MemoryStream();
        public Stream Stream => MemoryStream;

        public WindowsStreamCalculator(AlgorithmIdentifier algorithm,
                X509Certificate2 cert)
        {
            Algorithm = algorithm;
            Cert = cert;
        }

        public object GetResult()
        {
            var signer = new System.Security.Cryptography.Pkcs.CmsSigner(Cert);
            // This maps e.g. "rsa with sha256" (OID 1.2.840.113549.1.1.11) to
            // "sha256" (OID 2.16.840.1.101.3.4.1)
            var hashAlgorithm = new DefaultDigestAlgorithmIdentifierFinder().find(Algorithm);
            var hashOid = hashAlgorithm.Algorithm.Oid;
            signer.DigestAlgorithm = new System.Security.Cryptography.Oid(hashOid);

            var dataToSign = MemoryStream.ToArray();
            var info = new System.Security.Cryptography.Pkcs.ContentInfo(dataToSign);
            var cms = new System.Security.Cryptography.Pkcs.SignedCms(info, true);
            cms.ComputeSignature(signer, true);

            // This is the tricky part: usually you would call cms.Export() to get a signed
            // message, but we only want the signature
            var cmsSigner = cms.SignerInfos
                .Cast<System.Security.Cryptography.Pkcs.SignerInfo>().ToArray().Single();
            var signatureData = cmsSigner.GetSignature();

            return new SimpleBlockResult(signatureData);
        }
    }
}

It can be used as a substitute for Asn1SignatureFactory as follows:

public static Org.BouncyCastle.X509.X509Certificate SignCertificate(
        X509V3CertificateGenerator generator,
        X509Certificate2 nonExportableSigningCertificate)
{
    var signer = new WindowsSignatureFactory(
        new AlgorithmIdentifier(X9ObjectIdentifiers.ECDsaWithSha256),
        nonExportableSigningCertificate);

    var signed = generator.Generate(signer);

    // Make sure it worked
    var signingCertBouncy = DotNetUtilities.FromX509Certificate(nonExportableSigningCertificate);
    signed.Verify(signingCertBouncy.GetPublicKey());

    return signed;
}