How can verify without private key (NBitcoin)

Question

I am using NBitcoin nuget package. I try to create private and pub key(address). And after that , I try to sign to some message and then verify the this signature with pub key. But NBitcoin , for the verify using the BitcoinSecret object which is the has private key object. So , why for the verify NBitcoin using this object? And How can I verify signature without private key , just using address(pubKey),signature and message ? Thanks

     static void Main(string[] args)
            {

                Key Key = new Key(); //Create private key
                //We can take private key
                var privateKey = Key.GetBitcoinSecret(Network.Main);

               //Get the public key, and derive the address on the Main network
                BitcoinAddress address = privateKey.PubKey.GetAddress(Network.Main); 


                For the sign to data , create secret object.
                BitcoinSecret secret = new BitcoinSecret(Key, Network.Main);

                 string message = $"I am Nicolas";
                Console.WriteLine("Message:" + message + "\n");
                sign message with private key.
                string signature = secret.PrivateKey.SignMessage(message);
                Console.WriteLine("Signature:" + signature + "\n");






   /*      Now I dont understand this code. For the verify , I know that we need 
to signature message , message and pub or adres value.\n But in this code using
 again private secret object which has got private key.  How we can verify 
signature messaga with pub or address and message (dont include private key)*/
                if (secret.PubKey.VerifyMessage(message, signature))
                {
                    Console.WriteLine("thats okey");
                }


                Console.Read();

           }

Answer 1

The public key can not exist without the private key, as the public key is derived from the private key by utilizing some kind of one-way-function. If you want to use the public key without the private key, then generate it from the private key, like you did

var pubKey = privateKey.PubKey;

store the public key to some location the verifyer has access to

File.WriteAllBytes("some/public/location/MyPubKey.key", pubKey.ToBytes());

let the verifyer read the public key without ever knowing of the private key

var pubKeyForVerification = File.ReadAllBytes("some/public/location/MyPubKey.key");

This is all there is to it. It is safe to store the public key anywhere you want, because it's practically impossible to learn the private key from it.