ASP.NET MVC4 C# application needs to verify message signature as described in Digitally sign in PHP using private key, verify in C#

I tried to create public key for this using BouncyCastle CreateKey

string base64pubkey = @"-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
";
            RsaKeyParameters pubKey = PublicKeyFactory.CreateKey(
                Convert.FromBase64String(base64pubkey)) as RsaKeyParameters;

but this returns exception since FromBase64String accepts only pure base64 string.

How to fix this ? Probably I need to extract key from base64pubkey in binary form to pass to CreateKey

Here is a simple example showing one possible solution.

using System;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

namespace ReadKeyFromCert
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            string base64X509Cert = @"-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
";
            base64X509Cert = base64X509Cert.Replace ("-----BEGIN CERTIFICATE-----", "");
            base64X509Cert = base64X509Cert.Replace ("-----END CERTIFICATE-----", "");
            var derCert = Convert.FromBase64String (base64X509Cert);
            var x509 = new X509Certificate2 (derCert);
            RSACryptoServiceProvider rsa = (RSACryptoServiceProvider) x509.PublicKey.Key;
            Console.WriteLine (rsa.ToXmlString(false));
        }
    }
}