DotNetOpenAuth：消息签名不正确

csharp
using DotNetOpenAuth.Messaging;
using DotNetOpenAuth.OAuth;
using System;
public class MessageSigner
{
    public string SignMessage(string message, string privateKey)
    {
        var signer = new HmacSha1SigningBindingElement();
        var messageFactory = new DefaultMessageFactory();
        var request = messageFactory.GetNewRequestMessage();
        request.SetDirectResponse(true);
        request.Message = message;
        signer.Sign(request, privateKey);
        return request.GetDirectResponse();
    }
    public bool VerifyMessage(string message, string signature, string publicKey)
    {
        var signer = new HmacSha1SigningBindingElement();
        var messageFactory = new DefaultMessageFactory();
        var request = messageFactory.GetNewRequestMessage();
        request.SetDirectResponse(true);
        request.Message = message;
        return signer.Verify(request, signature, publicKey);
    }
}
public class Program
{
    public static void Main()
    {
        var signer = new MessageSigner();
        var message = "Hello, world!";
        var privateKey = "myPrivateKey";
        var publicKey = "myPublicKey";
        var signature = signer.SignMessage(message, privateKey);
        Console.WriteLine("Signature: " + signature);
        var isValid = signer.VerifyMessage(message, signature, publicKey);
        Console.WriteLine("Is Valid: " + isValid);
    }
}