I am trying to perform a Diffie-Hellman key exchange using 2 ECDSA x509 certificates.
Here is the method where I extract the keys from the certificates for computation of the derived key.
private byte[] GetDerivedKey(X509Certificate2 publicCertificate, X509Certificate2 privateCertificate)
{
byte[] derivedKey;
using (var privateKey = privateCertificate.GetECDsaPrivateKey())
using (var publicKey = publicCertificate.GetECDsaPublicKey())
{
var privateParams = privateKey.ExportParameters(true); //This line is failing
var publicParams = publicKey.ExportParameters(false);
using (var privateCng = ECDiffieHellmanCng.Create(privateParams))
using (var publicCng = ECDiffieHellmanCng.Create(publicParams))
{
derivedKey = privateCng.DeriveKeyMaterial(publicCng.PublicKey);
}
}
return derivedKey;
}
I've commented on the line that is failing privateKey.ExportParameters(true) with the error:
System.Security.Cryptography.CryptographicException : The requested operation is not supported.
at System.Security.Cryptography.NCryptNative.ExportKey(SafeNCryptKeyHandle key, String format)
at System.Security.Cryptography.CngKey.Export(CngKeyBlobFormat format)
at System.Security.Cryptography.ECCng.ExportParameters(CngKey key, Boolean includePrivateParameters, ECParameters& ecparams)
at System.Security.Cryptography.ECDsaCng.ExportParameters(Boolean includePrivateParameters)
Because this is a self signed certificate that I am generating, I assume I am doing something wrong.
I first create a root CA certificate and pass in the private key to sign my certificate.
private X509Certificate2 CreateECSDACertificate(string certificateName,
string issuerCertificateName,
TimeSpan lifetime,
AsymmetricKeyParameter issuerPrivateKey,
string certificateFriendlyName = null)
{
// Generating Random Numbers
var randomGenerator = new CryptoApiRandomGenerator();
var random = new SecureRandom(randomGenerator);
var signatureFactory = new Asn1SignatureFactory("SHA256WithECDSA", issuerPrivateKey, random);
// The Certificate Generator
var certificateGenerator = new X509V3CertificateGenerator();
// Serial Number
var serialNumber = BigIntegers.CreateRandomInRange(BigInteger.One, BigInteger.ValueOf(Int64.MaxValue), random);
certificateGenerator.SetSerialNumber(serialNumber);
// Issuer and Subject Name
var subjectDistinguishedName = new X509Name($"CN={certificateName}");
var issuerDistinguishedName = new X509Name($"CN={issuerCertificateName}");
certificateGenerator.SetSubjectDN(subjectDistinguishedName);
certificateGenerator.SetIssuerDN(issuerDistinguishedName);
// Valid For
var notBefore = DateTime.UtcNow.Date;
var notAfter = notBefore.Add(lifetime);
certificateGenerator.SetNotBefore(notBefore);
certificateGenerator.SetNotAfter(notAfter);
//key generation
var keyGenerationParameters = new KeyGenerationParameters(random, _keyStrength);
var keyPairGenerator = new ECKeyPairGenerator();
keyPairGenerator.Init(keyGenerationParameters);
var subjectKeyPair = keyPairGenerator.GenerateKeyPair();
certificateGenerator.SetPublicKey(subjectKeyPair.Public);
var certificate = certificateGenerator.Generate(signatureFactory);
var store = new Pkcs12Store();
var certificateEntry = new X509CertificateEntry(certificate);
store.SetCertificateEntry(certificateName, certificateEntry);
store.SetKeyEntry(certificateName, new AsymmetricKeyEntry(subjectKeyPair.Private), new[] { certificateEntry });
X509Certificate2 x509;
using (var pfxStream = new MemoryStream())
{
store.Save(pfxStream, null, new SecureRandom());
pfxStream.Seek(0, SeekOrigin.Begin);
x509 = new X509Certificate2(pfxStream.ToArray());
}
x509.FriendlyName = certificateFriendlyName;
return x509;
}
The .HasPrivateKey() method returns true, which I've read can return a false positive.
When I add my certificates to the store, I can verify the cert chain.
[Test]
public void CreateSelfSignedCertificate_AfterAddingToStore_CanBuildChain()
{
var result = _target.CreateSelfSignedCertificate(_subject, _issuer, TimeSpan.FromDays(356), _certificateFriendlyName, _issuerFriendlyName);
_store.TryAddCertificateToStore(result.CertificateAuthority, _caStoreName, _location);
_store.TryAddCertificateToStore(result.Certificate, _certStoreName, _location);
var chain = new X509Chain
{
ChainPolicy =
{
RevocationMode = X509RevocationMode.NoCheck
}
};
var chainBuilt = chain.Build(result.Certificate);
if (!chainBuilt)
{
foreach (var status in chain.ChainStatus)
{
Assert.Warn(string.Format("Chain error: {0} {1}", status.Status, status.StatusInformation));
}
}
Assert.IsTrue(chainBuilt, "Chain");
}
I thought at first that maybe the private cert had to come from the cert store, so I imported it and then pulled it back out, but I get the same error, which is another reason I believe I'm not doing something quite right.
EDIT:
I have another class generating RSA x509's using the same code for putting the private key into the certificate. It allows me to export the RSA private key.
The variable _keyStrength is 384 and my signature factory is using "SHA256withECDSA". I have also tried using "SHA384withECDSA" but I get the same error.
OK. It's a blind shot but after looking at your code I noticed two things:
X509Certificate2 class you are using wrong constructor. You should use one with a password parameter and give a null into itX509Certificate2 class you do not specify, if the private key should be exportable. I think that this is the reason why privateKey.ExportParameters(true) gives you an exception. You should use this constructor and specify null as passwordI thought it was a bug. It's possible that it is. We clearly stated in X509Constructor that the private key should be exportable. I used X509KeyStorageFlags.EphemeralKeySet | X509KeyStorageFlags.Exportable flags too. But when I looked at the CngKey it had ExportPolicy set to AllowExport but not AllowPlaintextExport.
It was exportable in some way. privateKey.Key.Export(CngKeyBlobFormat.OpaqueTransportBlob) worked. But privateKey.ExportParameters(true) did not.
I've searched for a solution how to change ExportPolicy of CngKey. I found this SO question that helped me to change it. After that the ExportParameters worked.
The fixed version of your GetDerivedKey method is
private byte[] GetDerivedKey(X509Certificate2 publicCertificate, X509Certificate2 privateCertificate)
{
byte[] derivedKey;
using (var privateKey = privateCertificate.GetECDsaPrivateKey())
using (var publicKey = privateCertificate.GetECDsaPublicKey())
{
var myPrivateKeyToMessWith = privateKey as ECDsaCng;
// start - taken from https://stackoverflow.com/q/48542233/3245057
// make private key exportable:
byte[] bytes = BitConverter.GetBytes((int)(CngExportPolicies.AllowExport | CngExportPolicies.AllowPlaintextExport));
CngProperty pty = new CngProperty(NCryptExportPolicyProperty, bytes, CngPropertyOptions.Persist);
myPrivateKeyToMessWith.Key.SetProperty(pty);
// end - taken from https://stackoverflow.com/q/48542233/3245057
var privateParams = myPrivateKeyToMessWith.ExportParameters(true); //This line is NOT failing anymore
var publicParams = publicKey.ExportParameters(false);
using (var privateCng = ECDiffieHellmanCng.Create(privateParams))
using (var publicCng = ECDiffieHellmanCng.Create(publicParams))
{
derivedKey = privateCng.DeriveKeyMaterial(publicCng.PublicKey);
}
}
return derivedKey;
}