Create RSACryptoServiceProvider object using RSA private key file in C#

Question

How can I properly create the RSACryptoServiceProvider object using an RSA private key file? I generated an RSA private key and exported it to a p12 file on OSX using the methods:

openssl genrsa -out rsakey.pem 2048
openssl pkcs12 -export -out rsakey.p12 -inkey rsakey.pem -nocerts

In my C# code, which runs on a Windows laptop, I am getting an error trying to instantiate the X509Certificate2 object so that I can grab the private key as an RSACryptoServiceProvider object out of it:

var privateKey = new X509Certificate2(p12_file, "pass", X509KeyStorageFlags.Exportable | X509KeyStorageFlags.MachineKeySet).PrivateKey as RSACryptoServiceProvider;

CryptographicException: The parameter is incorrect.

Below is the entire C# code for reference. Note that I am just trying to get the private key as an RSACryptoServiceProvider object by following the docs for the X509Certificate2 class.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Net;
using System.Security;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Globalization;
using Jose;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string p12_file = @"C:\Root\rsakey.p12";

            DateTime issued = DateTime.Now;
            DateTime expire = DateTime.Now.AddHours(10);

            var payload = new Dictionary<string, object>()
            {
                { "iss", "auth0_user"},
                { "exp", ToUnixTime(expire).ToString() }
            };

            var privateKey = new X509Certificate2(p12_file, "pass", X509KeyStorageFlags.Exportable | X509KeyStorageFlags.MachineKeySet).PrivateKey as RSACryptoServiceProvider;

            string token = Jose.JWT.Encode(payload, privateKey, JwsAlgorithm.RS256);

            Console.WriteLine(token);

        }

        static long ToUnixTime(DateTime dateTime)
        {
            return (int)(dateTime.ToUniversalTime().Subtract(new DateTime(1970, 1, 1))).TotalSeconds;
        }

    }
}

UPDATE: I tried to create my self-signed cert using

openssl req -key rsakey.pem -new -out domain.crt

and then wrapping it into a p12:

openssl pkcs12 -export -out rsakey.p12 -in rsakey.pem -certfile domain.crt

but the command just hangs forever until I kill it.

Answer 1

.NET has no built-in functionality for reading private keys (or public keys, for that matter) which are not associated with certificates.

Your easiest fix is to wrap the new RSA key in a self-signed cert, and then put both the cert and the private key into the .p12 file.

The alternatives are to manually parse the private key file/blob and build an RSAParameters structure; or to use a 3rd party library which can do that for you.