签名失败的核心验证错误

我设计了一个向供应商发送SOAP请求的WCF.net客户端。为了满足供应商WS安全性需求，我必须创建一个自定义SOAP报头，并将带有自定义报头的请求发送到供应商端的web服务。因此，我通过实现从MessageHeader派生的新类(见下文)创建了一个自定义头(header)

public class SignOnlyMessageHeader : MessageHeader
{
    private const string PREFIX_CP = "wsse";
    public string m_Username { get; set; }
    public string m_Envelope { get; set; }
    public SignOnlyMessageHeader(string Username, string Envelope)
    {
        m_Username = Username;
        m_Envelope = Envelope;
    }
    public override string Name
    {
        get { return "wsse:Security"; }
    }
    public override string Namespace
    {
        get { return null; }
    }
    public override bool MustUnderstand
    {
        get
        {
            return false;
        }
    }
    protected override void OnWriteStartHeader(XmlDictionaryWriter writer, MessageVersion messageVersion)
    {
        base.OnWriteStartHeader(writer, messageVersion);
        writer.WriteXmlnsAttribute(PREFIX_CP, "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd");
    }
    protected override void OnWriteHeaderContents(XmlDictionaryWriter writer, MessageVersion messageVersion)
    {
        writer.WriteStartElement(PREFIX_CP, "UsernameToken", null);
        writer.WriteAttributeString("wsu:Id", "UsernameToken-20");
        writer.WriteXmlnsAttribute("wsu", "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd");
        writer.WriteElementString(PREFIX_CP, "Username", null, m_Username);
        writer.WriteEndElement();
        SignXmlFile(writer);
    }
    public void SignXmlFile(XmlDictionaryWriter writer)
    {
        string certificatePath = "C:''Users''22428-cert.p12";
        System.Security.Cryptography.X509Certificates.X509Certificate2 cert = new X509Certificate2(certificatePath, "changeit");
        // Create a new XML document.
        XmlDocument doc = new XmlDocument();
        // Format the document to ignore white spaces.
        doc.PreserveWhitespace = false;
        doc.LoadXml(m_Envelope);
        // Create a SignedXml object.
        SignedXml signedXml = new SignedXml(doc);
        // Add the key to the SignedXml document. 
        //signedXml.SigningKey = Key;
        signedXml.SigningKey = cert.PrivateKey;
        // Create a new KeyInfo object.
        KeyInfo keyInfo = new KeyInfo();
        keyInfo.Id = "";
        // Load the certificate into a KeyInfoX509Data object
        // and add it to the KeyInfo object.
        KeyInfoX509Data keyInfoData = new KeyInfoX509Data();
        keyInfoData.AddCertificate(cert);
        keyInfo.AddClause(keyInfoData);
        // Add the KeyInfo object to the SignedXml object.
        signedXml.KeyInfo = keyInfo;
        signedXml.SignedInfo.CanonicalizationMethod = "http://www.w3.org/2001/10/xml-exc-c14n#";
        // Create a reference to be signed.
        Reference reference = new Reference();
        reference.Uri = "";
        // Add an enveloped transformation to the reference.
        XmlDsigEnvelopedSignatureTransform env = new XmlDsigEnvelopedSignatureTransform();
        reference.AddTransform(env);
        reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256";
        // Add the reference to the SignedXml object.
        signedXml.AddReference(reference);
        signedXml.Signature.Id = "";
        // Compute the signature.
        signedXml.ComputeSignature();
        // Get the XML representation of the signature and save
        // it to an XmlElement object.
        XmlElement xmlDigitalSignature = signedXml.GetXml();
        // Check the signature and return the result.
        if (!signedXml.CheckSignature(new X509Certificate2(certificatePath, "changeit"), true))
        {
            Console.WriteLine("invalid signature");
        }
        xmlDigitalSignature.WriteTo(writer);
    }

所以在创建自定义头类之后，我覆盖了IClientMessageInspector。beforeendrequest方法拦截传出请求，并将自定义报头添加到soap请求中。见下面的代码，

    object IClientMessageInspector.BeforeSendRequest(ref System.ServiceModel.Channels.Message request, IClientChannel channel)
    {
        request.Headers.RemoveAt(0);
        SignOnlyMessageHeader header = new SignOnlyMessageHeader("x509user", env);
        request.Headers.Add(header);
        return null;
    }

最终结果是我拦截了SOAP请求，并正确地将当前报头替换为自定义报头。在发出请求之前，我检查了更新的SOAP请求(放置了一个断点)，结构与供应商请求的完全匹配。但是在供应商端处理请求后，我收到了一个错误。它只说"签名核心验证失败"。我认为我在"SignXmlFile"方法中正确地签署了整个信封。我甚至检查了方法(if (!signedXml)中的有效性。CheckSignature(new X509Certificate2(certificatePath， "changeit")， true))，语句返回false，表示签名有效。

我做错了什么