Enroll an X509Certificate2 without any trace of it being left on the machine

Question

Currently creating Certificate Authorities and Issued Certificates. The Generation of the request, enrollment and validation are all functional, but when I checked my certificate store, I realized, it was placing them in my personal certificate directory. For memory, security and legal reasons, I can't have that.

The certificates are stored in a secure remote database. The certificates may be randomly accessed or generated on a random machine from a collection. If they generate certificates, it will store them on whichever machine created the certificate. Is there a way to generate a certificate enrollment (CX509Enrollment) without any trace of the certificate being left on the machine afterwards?

The portion that controls enrollment is relatively small and straight forward. It can only be ran as an administrator. I assume that's because it's adding certificates to the store.

I'm currently running a separate project file to attempt to debug this issue. Both my certificates are constructed and kept in memory.

static void Main(string[] args)
{
    X509Certificate2 rootCert = CreateSelfSignedCertificate("testRoot");
    X509Certificate2 signedChild = CreateSignedCertificate("testyMcTesterson", rootCert);

    X509Chain chain = new X509Chain();
    chain.ChainPolicy = new X509ChainPolicy()
    {
        RevocationMode = X509RevocationMode.NoCheck,
        VerificationFlags = X509VerificationFlags.AllFlags,
        UrlRetrievalTimeout = new TimeSpan(0, 1, 0)
    };
    chain.ChainPolicy.ExtraStore.Add(rootCert);
    bool isValid = chain.Build(signedChild); //Is True :D
}

The certificates end up in my personal certificate store

My enrollment occurs in this method. It takes a fully contructed and encoded certificate request.

public static CX509Enrollment EnrollCertificateRequest(CX509CertificateRequestCertificate certRequest)
{
    var enroll = new CX509Enrollment();
    enroll.InitializeFromRequest(certRequest);
    string csr = enroll.CreateRequest(); 
    enroll.InstallResponse(InstallResponseRestrictionFlags.AllowUntrustedCertificate,
        csr, EncodingType.XCN_CRYPT_STRING_BASE64, ""); 

    return enroll;
}

EDIT I'm currently limited to .NET 4.5.x. Another problem I'm running into, is that trying to sign a certificate with a root will throw a CRYPT_E_NOT_FOUND exception.

Answer 1

There's probably not a way to do it with CX509Enroll. But you can possibly accomplish your goals with .NET Framework 4.7.2 and the CertificateRequest class.

using (RSA rsa = RSA.Create(2048))
{
    CertificateRequest request = new CertificateRequest(
        "CN=Your Name Here",
        rsa,
        HashAlgorithmName.SHA256,
        RSASignaturePadding.Pkcs1);

    SubjectAlternativeNameBuilder builder = new SubjectAlternativeNameBuilder();
    builder.AddDnsName("your.name.here");
    builder.AddDnsName("your.other.name.here");

    request.CertificateExtensions.Add(builder.Build());

    // Any other extensions you're supposed to request, like not being a CA.
    request.CertificateExtensions.Add(
        new X509BasicConstraintsExtension(false, false, 0, false));

    // TLS Server?
    request.CertificateExtensions.Add(
        new X509EnhancedKeyUsageExtension(
            new OidCollection
            {
                new Oid("1.3.6.1.5.5.7.3.1")
            },
            false));

    byte[] derEncodedRequest = request.CreateSigningRequest();

    X509Certificate2 responseWithPrivateKey;

    using (X509Certificate2 response = SendRequestToServerAndGetResponse(derEncodedRequest))
    {
        responseWithPrivateKey = response.CopyWithPrivateKey(rsa);
    }

    // Use it, save it to a PFX, whatever.
    // At this point, nothing has touched the hard drive.
}

Answer 2

I figured it out without using enrollment and sticking to .NET 4.5.x. I first create a RSACrytpoServiceProvider using CspParameters. I then construct a private and public key from the RSACryptoServiceProvider. I create a certificate request and initiate from the private key. The certificate request is encoded and converted to raw data, then to bytes. The bytes are then used to create an X509Certificate2.

public static X509Certificate2 GenerateCertificate(string subjectName)
{
    var dn = new CX500DistinguishedName();
    dn.Encode("CN=" + subjectName, X500NameFlags.XCN_CERT_NAME_STR_COMMA_FLAG);

    //Create crytpo provider to generate an assymetric key
    int KeyType = (int)X509ProviderType.XCN_PROV_RSA_SCHANNEL;
    CspParameters cspParams = new CspParameters(KeyType);
    cspParams.Flags = CspProviderFlags.UseMachineKeyStore;
    cspParams.KeyContainerName = Guid.NewGuid().ToString();
    var rsa = new RSACryptoServiceProvider(2048, cspParams);
    var CryptoProvider = rsa.CspKeyContainerInfo.ProviderName;

    var keyContainerName = rsa.CspKeyContainerInfo.KeyContainerName;
    CX509PrivateKey privateKey = new CX509PrivateKey();
    privateKey.MachineContext = true;
    privateKey.ProviderName = CryptoProvider;
    privateKey.ContainerName = keyContainerName;
    privateKey.KeyUsage = X509PrivateKeyUsageFlags.XCN_NCRYPT_ALLOW_ALL_USAGES;
    privateKey.Open();

    keyContainerName = privateKey.ContainerName;
    CX509PublicKey publicKey = privateKey.ExportPublicKey();

    var oid = new CObjectId();
    oid.InitializeFromValue("1.3.6.1.5.5.7.3.1"); // SSL server
    var oidlist = new CObjectIds();
    oidlist.Add(oid);
    var eku = new CX509ExtensionEnhancedKeyUsage();
    eku.InitializeEncode(oidlist);

    var hashobj = new CObjectId();
    hashobj.InitializeFromAlgorithmName(ObjectIdGroupId.XCN_CRYPT_HASH_ALG_OID_GROUP_ID,
        ObjectIdPublicKeyFlags.XCN_CRYPT_OID_INFO_PUBKEY_ANY,
        AlgorithmFlags.AlgorithmFlagsNone, "SHA256");

    CX509CertificateRequestCertificate certRequest = new CX509CertificateRequestCertificate();
    certRequest.InitializeFromPrivateKey(
        X509CertificateEnrollmentContext.ContextMachine,
        privateKey,
        "");

    certRequest.Subject = dn;
    certRequest.NotBefore = DateTime.Now;
    certRequest.NotAfter = DateTime.Now.AddYears(1);
    certRequest.HashAlgorithm = hashobj;
    certRequest.X509Extensions.Add((CX509Extension)eku);
    certRequest.Encode();

    return new X509Certificate2(
    Convert.FromBase64String(certRequest.RawData), "",
    X509KeyStorageFlags.Exportable)
    {
        PrivateKey = rsa,
        FriendlyName = subjectName
    };
}

To issue a cert. The same process is followed a CSignerCertificate is initiated and attached. But before that happens, I save the root certificate to the My, Local Machine Certificate Store. I then create a signed certificate using the root that was just added to the store. I then remove the certificate from the store.

Signing a Certificate Request

var dnSigner = new CX500DistinguishedName();
dnSigner.Encode("CN=" + signer.FriendlyName, X500NameFlags.XCN_CERT_NAME_STR_COMMA_FLAG);

string base64Root = Convert.ToBase64String(signer.RawData);
CSignerCertificate certSigner = new CSignerCertificate();

bool useMachineStore = ((ICspAsymmetricAlgorithm)signer.PrivateKey).CspKeyContainerInfo.MachineKeyStore;
certSigner.Initialize(useMachineStore, X509PrivateKeyVerify.VerifyNone, EncodingType.XCN_CRYPT_STRING_BASE64, base64Root);
certRequest.SignerCertificate = certSigner;
certRequest.Issuer = dnSigner;

static void Main(string[] args)
{
    X509Certificate2 rootCert = GenerateCertificate("TEST_ROOT");
    X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
    store.Open(OpenFlags.ReadWrite);
    store.Add(rootCert);
    X509Certificate2 signedChild = GenerateUserCertificate("Testy McTesterson", rootCert);
    store.Remove(rootCert);
    store.Close();
}

A few important things to note: This will only work with certain key usage and key spec flags An X509Chain will still build and validate but it will recognize that the root is untrusted (Easy to bypass).