Build certificate chain in BouncyCastle in C#

Question

I have a bunch of root and intermediate certificates given as byte arrays, and I also have end user certificate. I want to build a certificate chain for given end user certificate. In .NET framework I can do it like this:

using System.Security.Cryptography.X509Certificates;

static IEnumerable<X509ChainElement>
    BuildCertificateChain(byte[] primaryCertificate, IEnumerable<byte[]> additionalCertificates)
{
    X509Chain chain = new X509Chain();
    foreach (var cert in additionalCertificates.Select(x => new X509Certificate2(x)))
    {
        chain.ChainPolicy.ExtraStore.Add(cert);
    }

    // You can alter how the chain is built/validated.
    chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
    chain.ChainPolicy.VerificationFlags = X509VerificationFlags.IgnoreWrongUsage;

    // Do the preliminary validation.
    var primaryCert = new X509Certificate2(primaryCertificate);
    if (!chain.Build(primaryCert))
        throw new Exception("Unable to build certificate chain");

    return chain.ChainElements.Cast<X509ChainElement>();
}

How to do it in BouncyCastle? I tried with code below but I get PkixCertPathBuilderException: No certificate found matching targetContraints:

using Org.BouncyCastle;
using Org.BouncyCastle.Pkix;
using Org.BouncyCastle.Utilities.Collections;
using Org.BouncyCastle.X509;
using Org.BouncyCastle.X509.Store;

static IEnumerable<X509Certificate> BuildCertificateChainBC(byte[] primary, IEnumerable<byte[]> additional)
{
    X509CertificateParser parser = new X509CertificateParser();
    PkixCertPathBuilder builder = new PkixCertPathBuilder();

    // Separate root from itermediate
    List<X509Certificate> intermediateCerts = new List<X509Certificate>();
    HashSet rootCerts = new HashSet();

    foreach (byte[] cert in additional)
    {
        X509Certificate x509Cert = parser.ReadCertificate(cert);

        // Separate root and subordinate certificates
        if (x509Cert.IssuerDN.Equivalent(x509Cert.SubjectDN))
            rootCerts.Add(new TrustAnchor(x509Cert, null));
        else
            intermediateCerts.Add(x509Cert);
    }

    // Create chain for this certificate
    X509CertStoreSelector holder = new X509CertStoreSelector();
    holder.Certificate = parser.ReadCertificate(primary);

    // WITHOUT THIS LINE BUILDER CANNOT BEGIN BUILDING THE CHAIN
    intermediateCerts.Add(holder.Certificate);

    PkixBuilderParameters builderParams = new PkixBuilderParameters(rootCerts, holder);
    builderParams.IsRevocationEnabled = false;

    X509CollectionStoreParameters intermediateStoreParameters =
        new X509CollectionStoreParameters(intermediateCerts);

    builderParams.AddStore(X509StoreFactory.Create(
        "Certificate/Collection", intermediateStoreParameters));

    PkixCertPathBuilderResult result = builder.Build(builderParams);

    return result.CertPath.Certificates.Cast<X509Certificate>();
}

Edit: I added the line that fixed my problem. It's commented with all caps. Case closed.

Answer 1

The code below does not answer your question (it's a pure Java solution). I only just realized now after typing out everything that it doesn't answer your question! I forgot BouncyCastle has a C# version! Oops.

It still might help you roll your own chain builder. You probably don't need any libraries or frameworks.

Good luck!

http://juliusdavies.ca/commons-ssl/src/java/org/apache/commons/ssl/X509CertificateChainBuilder.java

/**
 * @param startingPoint the X509Certificate for which we want to find
 *                      ancestors
 *
 * @param certificates  A pool of certificates in which we expect to find
 *                      the startingPoint's ancestors.
 *
 * @return Array of X509Certificates, starting with the "startingPoint" and
 *         ending with highest level ancestor we could find in the supplied
 *         collection.
 */
public static X509Certificate[] buildPath(
  X509Certificate startingPoint, Collection certificates
) throws NoSuchAlgorithmException, InvalidKeyException,
         NoSuchProviderException, CertificateException {

    LinkedList path = new LinkedList();
    path.add(startingPoint);
    boolean nodeAdded = true;
    // Keep looping until an iteration happens where we don't add any nodes
    // to our path.
    while (nodeAdded) {
        // We'll start out by assuming nothing gets added.  If something
        // gets added, then nodeAdded will be changed to "true".
        nodeAdded = false;
        X509Certificate top = (X509Certificate) path.getLast();
        if (isSelfSigned(top)) {
            // We're self-signed, so we're done!
            break;
        }

        // Not self-signed.  Let's see if we're signed by anyone in the
        // collection.
        Iterator it = certificates.iterator();
        while (it.hasNext()) {
            X509Certificate x509 = (X509Certificate) it.next();
            if (verify(top, x509.getPublicKey())) {
                // We're signed by this guy!  Add him to the chain we're
                // building up.
                path.add(x509);
                nodeAdded = true;
                it.remove(); // Not interested in this guy anymore!
                break;
            }
            // Not signed by this guy, let's try the next guy.
        }
    }
    X509Certificate[] results = new X509Certificate[path.size()];
    path.toArray(results);
    return results;
}

Requires these two additional methods:

isSelfSigned():

public static boolean isSelfSigned(X509Certificate cert)
    throws CertificateException, InvalidKeyException,
    NoSuchAlgorithmException, NoSuchProviderException {

    return verify(cert, cert.getPublicKey());
}

And verify():

public static boolean verify(X509Certificate cert, PublicKey key)
    throws CertificateException, InvalidKeyException,
    NoSuchAlgorithmException, NoSuchProviderException {

    String sigAlg = cert.getSigAlgName();
    String keyAlg = key.getAlgorithm();
    sigAlg = sigAlg != null ? sigAlg.trim().toUpperCase() : "";
    keyAlg = keyAlg != null ? keyAlg.trim().toUpperCase() : "";
    if (keyAlg.length() >= 2 && sigAlg.endsWith(keyAlg)) {
        try {
            cert.verify(key);
            return true;
        } catch (SignatureException se) {
            return false;
        }
    } else {
        return false;
    }
}

Answer 2

I've done this in Java a number of times. Given that the API seems to be a straight port of the Java one I'll take a stab.

1. I'm pretty sure when you add the store to the builder, that collection is expected to contain all certs in the chain to be built, not just intermediate ones. So rootCerts and primary should be added.
2. If that doesn't solve the problem on its own I would try also specifying the desired cert a different way. You can do one of two things:
   • Implement your own Selector that always only matches your desired cert (primary in the example).
   • Instead of setting holder.Certificate, set one or more criteria on holder. For instance, setSubject, setSubjectPublicKey, setIssuer.

Those are the two most common problems I had with PkixCertPathBuilder.