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Is it possible to store RSACryptoServiceProvider in X509Certificate2 or X509Store?

I have keypair provided to me by hardware device in form of RSAParameters
which I can convert to a CryptoServiceProvider using following code and encrypt/decrypt/sign.
RSACryptoServiceProvider RSAPrivKey = new RSACryptoServiceProvider(2048) { PersistKeyInCsp = false };
// RSAParameters rsaparam = RSAPrivKey.ExportParameters(true); // for testing
RSAPrivKey.ImportParameters(rsaparam);
I'm not allowed to use file system to store the certificate and as hardware device is not available sometimes, I like to store this certificate in Machine Certificate store (Certificates -> Local Computer -> personal)
As the X509Certificate2.PrivateKey document suggests, I should be able to set private key of the certificate and save it to store like following
X509Certificate2 Cert2 = new X509Certificate2();
Cert2.PrivateKey = RSAPrivKey;
X509Store cstore = new X509Store(StoreName.My, StoreLocation.LocalMachine);
cstore.Open(OpenFlags.MaxAllowed);
cstore.Add(Cert2);
cstore.Close();
However I get error m_safeCertContext is an invalid handle which according to this answer is happening because some properties of X509Certificate2 are not set, which is correct. I cannot assign validity date, issuer, ... to the certificate.
Question
Is it even possible to store RSACryptoServiceProvider in X509Certificate2? If yes, am I missing some steps?

How can I add a private key to X509Certificate2 after I have already added the public key?

My steps are:
Create X509Certificate2 with public key:
X509Certificate2 clientCertificate = new X509Certificate2("public key certificate blob as byte[]");
How do I want to load the private key blob to clientCertificate?

It looks like you can not import RSA private key with just .net framework tools.
Check out this thread How to read a PEM RSA private key from .NET.

Just in case anyone like me and up looking at this old post when searching for how to generate X509Certificate2 from pem fil/private key:
The .Net 5.0 framework has a very simplified approach to this:
var certPem = File.ReadAllText("cert.pem");
var eccPem = File.ReadAllText("ecc.pem");
var cert = X509Certificate2.CreateFromPem(certPem, eccPem);
(source: https://www.scottbrady91.com/C-Sharp/PEM-Loading-in-dotnet-core-and-dotnet)

Also see: How to import PKCS#8 RSA privateKey (created by OpenSSL) in C#
It includes a link to Mono's source code which can read PKCS#8 / PEM files and return an RSA instance from it.

How do you generate an x509Certificate based solely on a public key in RSA format?

I have an RSA public key in an XML format. I need to take this key and create an x.509 Certificate using that public key. I do not have access to the private key to complete the key pair. All examples that I have found involve either generating the key pair or having access to both the public and private keys.
Below is a small snippet from a test routine I have been working on to accomplish this.
RSACryptoServiceProvider provider = new RSACryptoServiceProvider();
provider.FromXmlString("<RSAKeyValue><Modulus>puEVvRbrLAz.......c1W5j/vqJSUrXo16k=</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>");
Org.BouncyCastle.Crypto.Parameters.RsaKeyParameters key = Org.BouncyCastle.Security.DotNetUtilities.GetRsaPublicKey(provider);
//<!*** Do some code to take RsaKeyParameters and create an x.509 Certificate ***>
var fOut = new System.IO.StreamWriter(#"C:\certificate.pem", false);
var pw = new Org.BouncyCastle.OpenSsl.PemWriter(fOut);
pw.WriteObject(key);

Just to clarify the issue of impossibility of generating a certificate using a public key only.
AFAIK, certificate must be signed. And it must be done with a private key - that's why you need one for generating a certificate. If certificate is signed with the paired private key of the open key you are going to issue a certificate for, than it is a self-signed certificate. Otherwise you need a private key of so called Certificate Authority for signing.

Get Apple Keychain to recognize Bouncy Castle .NET created PKCS12 (.p12) store

Our organization manages a stable of iOS applications for multiple clients, which means dealing with a lot of different developer identity certificates and push notification certificates.
I have had success with the Bouncy Castle C# Crypto API in simplifying management of the certificates and private keys for push notifications, essentially eliminating the need for the Keychain for all our push notification certificates.
I would like to extend this to the developer identity certificates. The goal would be to store all the private key and certificate information in the database for each developer identity. Then when a new developer or build machine needs to be provisioned, server side code could wrap all of the certificates and private keys into one p12 archive with one password that could be imported into the target Mac's Keychain.
Unfortunately, the Mac Keychain doesn't like the p12 files I'm generating. This is annoying since I can successfully import these files into the Windows certificate manager just fine.
The code I'm using (the important parts) looks like this:
private byte[] GetP12Bytes(List<DevIdentity> identities, string password)
{
Pkcs12Store store = new Pkcs12Store();
foreach(DevIdentity ident in identities)
{
// Easiest to create a Bouncy Castle cert by converting from .NET
var dotNetCert = new X509Certificate2(ident.CertificateBytes);
// This method (not shown) parses the CN= attribute out of the cert's distinguished name
string friendlyName = GetFriendlyName(dotNetCert.Subject);
// Now reconstitute the private key from saved value strings
BigInteger modulus = new BigInteger(ident.PrivateKey.Modulus);
BigInteger publicExponent = new BigInteger(ident.PrivateKey.PublicExponent);
BigInteger privateExponent = new BigInteger(ident.PrivateKey.Exponent);
BigInteger p = new BigInteger(ident.PrivateKey.P);
BigInteger q = new BigInteger(ident.PrivateKey.Q);
BigInteger dP = new BigInteger(ident.PrivateKey.DP);
BigInteger dQ = new BigInteger(ident.PrivateKey.DQ);
BigInteger qInv = new BigInteger(ident.PrivateKey.QInv);
RsaKeyParameters kp = new RsaPrivateCrtKeyParameters(modulus, publicExponent, privateExponent, p, q, dP, dQ, qInv);
AsymmetricKeyEntry privateKey = new AsymmetricKeyEntry(kp);
// Now let's convert to a Bouncy Castle cert and wrap it for packaging
Org.BouncyCastle.X509.X509Certificate cert = DotNetUtilities.FromX509Certificate(dotNetCert);
X509CertificateEntry certEntry = new X509CertificateEntry(cert);
// Set the private key and certificate into the store
store.SetCertificateEntry(friendlyName, certEntry);
store.SetKeyEntry(ident.PrivateKeyName, privateKey, new X509CertificateEntry[] { certEntry });
}
using (MemoryStream ms = new MemoryStream())
{
store.Save(ms, password.ToCharArray(), new SecureRandom());
ms.Flush();
byte[] p12Bytes = ms.ToArray();
return p12Bytes;
}
}
Like I said, this works great for import on Windows, but fails with a very generic error when importing into the Mac Keychain.
There is one major difference I can see when loading a Keychain-generated p12 and my own generated p12 file, but I do not know if this is the cause.
If I load the Mac Keychain generated p12 into a Bouncy Castle PKCS12Store, and then examine the keys, on the Keychain p12, both the certificate and the private key have an attribute with the key "1.2.840.113549.1.9.21" with equivalent values (a DerOctetString with value #af8a1d6891efeb32756c12b7bdd96b5ec673e11e).
If I do the same to my generated p12 file, the private key contains the "1.2.840.113549.1.9.21" attribute, but the Certificate does not.
If I Google "1.2.840.113549.1.9.21", I find out that this OID means PKCS_12_LOCAL_KEY_ID . My only theory is that the Keychain relies on this to match up the certificate and private key, and that my generated file does not have this, so it fails.
However, I've tried adding these values to a Hashtable and then using the CertificateEntry constructor that takes the attribute hashtable. If I do that, and then save the bytes, and then reload the bytes, that attribute is again missing.
So I'm flummoxed. Maybe this attribute is a glitch in the Bouncy Castle API? Maybe there's something I'm doing wrong. Maybe the Keychain has ridiculous non-standard requirements for incoming p12 files. In any case, any help that could be provided would be greatly appreciated.

BouncyCastle's Pkcs12Store takes care of setting both the Friendly Name and Local Key ID attributes for you (or at least it does so in the 1.7 release, circa April 2011). My guess is that you must have used an older version where this didn't work.
Here's how I'm saving an iPhone Developer identity to a Pkcs12Store instance (extra stuff and security omitted):
var store = new Pkcs12Store();
// pairs is IEnumerable<Tuple<X509Certificate, AsymmetricKeyParameter>>
foreach (var pair in pairs)
{
var cn = pair.Item1.SubjectDN
.GetValueList(X509Name.CN).OfType<string>().Single();
var certEntry = new X509CertificateEntry(pair.Item1);
store.SetCertificateEntry(cn, certEntry);
var keyEntry = new AsymmetricKeyEntry(pair.Item2);
store.SetKeyEntry("Developer Name", keyEntry, new[] { certEntry });
}
store.Save(stream, string.Empty.ToArray(), new SecureRandom());
Importing the store in Keychain Access.app on OS X 10.7 correctly places the certificate and private key in the keychain and places the certificate within the private key in the UI, as with a certificate and key generated by Keychain Access itself.
On a side note, it seems that Pkcs12Store uses the public key of the certificate to generate the value of the LocalKeyId attribute shared by the certificate and key entries.
You can see the relevant section of the Pkcs12Store source here.

Decrypt with PrivateKey X.509 Certificate

I have a problem to decrypt a message usgin X.509 Certificate.
I generate my certificate with makecert with this options:
makecert -r -pe -n "CN=MyCertificate" -ss CA -sr CurrentUser -a sha1 -sky signature -cy authority -sv CA.pvk CA.cer
And the PrivateKey was "mypassword".
My problem is when I want to decrypt a message encrypt with previous certificate in c#.
I found this class http://blog.shutupandcode.net/?p=660, but in the X509Decrypt method allways the PrivateKey is null.
public static byte[] X509Decrypt(byte[] data, string certificateFile, string password)
{
// load the certificate and decrypt the specified data
using (var ss = new System.Security.SecureString())
{
foreach (var keyChar in password.ToCharArray())
ss.AppendChar(keyChar);
// load the password protected certificate file
X509Certificate2 cert = new X509Certificate2(certificateFile, ss);
using (RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)cert.PrivateKey)
{
return rsa.Decrypt(data, true);
}
}
}
I tried passing the certificate file (.cer)
X509DecryptString(token, #"c:\CA.cer", "mypassword");
And passing the pvk file (.pvk)
X509DecryptString(token, #"c:\CA.pvk", "mypassword");
But allways have that the PrivateKey property is null.
Can anyone guide me to decrypt the message using the pvk file?
Thanks,
Jose

The certificate itself only contains the public key (+ some data), but not the private key. (It's very unlikely that the RSA private key is "mypassword". The password that protects your private key may be "mypassword", but the private key itself (more specifically the private exponent, in RSA) will be a rather long number.)
As a result (because CA.cer only contains the certificate), X509DecryptString(token, #"c:\CA.cer", "mypassword") will almost certainly not work.
X509DecryptString(token, #"c:\CA.pvk", "mypassword"); could work in principle, but you're creating a X509Certificate2 object from it, and it still needs the certificate and the private key. You should be able to load that from a PKCS#12 container (.p12/.pfx).
To create this container, you can use pvk2pfx:
pvk2pfx -spc CA.cer -pvk CA.pvk -pfx CA.pfx
(If you don't specify -pfx CA.pfx, it will launch the interactive interface, in which case you need to tick the box to export the private key.)
Then, try to decrypt using that pfx/p12 file instead.

I think you should be using "-sky exchange" to generate a public/private key pair.