I resolved the problem, The point is that a P12 file (as a PFX) contains more then 1 certificate, so it must be loaded in this way:
X509Certificate2Collection certificates = new X509Certificate2Collection();
certificates.Import(certName, password, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet);
and added to a HttpWebRequest in this way: request.ClientCertificates = certificates;
Thanks everybody for support.
COMPLETE SAMPLE CODE
string host = @"https://localhost/";
string certName = @"C:\temp\cert.pfx";
string password = @"password";
try
{
X509Certificate2Collection certificates = new X509Certificate2Collection();
certificates.Import(certName, password, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet);
ServicePointManager.ServerCertificateValidationCallback = (a, b, c, d) => true;
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(host);
req.AllowAutoRedirect = true;
req.ClientCertificates = certificates;
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
string postData = "login-form-type=cert";
byte[] postBytes = Encoding.UTF8.GetBytes(postData);
req.ContentLength = postBytes.Length;
Stream postStream = req.GetRequestStream();
postStream.Write(postBytes, 0, postBytes.Length);
postStream.Flush();
postStream.Close();
WebResponse resp = req.GetResponse();
Stream stream = resp.GetResponseStream();
using (StreamReader reader = new StreamReader(stream))
{
string line = reader.ReadLine();
while (line != null)
{
Console.WriteLine(line);
line = reader.ReadLine();
}
}
stream.Close();
}
catch(Exception e)
{
Console.WriteLine(e);
}