C# side

 static void Main()
        {

            //string publicKeyFile = File.ReadAllText("E:\\publickey.rsa");
            string privateKeyFile = File.ReadAllText("E:\\privatekey.rsa");
            string cipherFile = File.ReadAllText("E:\\cipher.rsa");

            //Convert base64 to DER
            byte[] privateKeyArr = Convert.FromBase64String(privateKeyFile);

            Console.WriteLine(cipherFile.Length);

            //Dump the bytes to .der file
            File.WriteAllBytes("private-key.der", privateKeyArr);

            byte[] cipherArr = Convert.FromBase64String(cipherFile);

            //Load the der file
            AsnKeyParser keyParser = new AsnKeyParser("private-key.der");
            RSAParameters privateKey = keyParser.ParseRSAPrivateKey();

            CspParameters csp = new CspParameters();
            csp.KeyContainerName = "RSA Test (OK to Delete)";
            csp.ProviderType = 1;
            csp.KeyNumber = 1;

            RSACryptoServiceProvider rsp = new RSACryptoServiceProvider(csp);
            rsp.PersistKeyInCsp = false;
            rsp.ImportParameters(privateKey);

            //Causes exception here..
            var data = rsp.Decrypt(cipherArr, true);

            string text = Encoding.Default.GetString(data);

            Console.WriteLine(text);
        }