Get public key from certificate, and Convert the key to the RSAParameter
byte[] pk = cer.GetPublicKey();
byte[] m = new byte[pk.Length - 3];
Buffer.BlockCopy(pk, 0, m, 0, m.Length);
byte[] e = new byte[3];
Buffer.BlockCopy(pk, m.Length, e, 0, 3);
String key = "<RSAKeyValue><Modulus>" + Convert.ToBase64String(m)+"</Modulus><Exponent>"+Convert.ToBase64String(e)+"</Exponent></RSAKeyValue>";
Console.WriteLine(key);
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.FromXmlString(key);
Console.WriteLine(rsa.ToXmlString(false));