public class PrefixedSignedXml : SignedXml
{
  public PrefixedSignedXml(XmlDocument document)
    : base(document){}
  public PrefixedSignedXml(XmlElement element)
    : base(element) { }

  public string SignaturePrefix { get; private set; }

  public void ComputeSignature(string signaturePrefix)
  {
    SignaturePrefix = signaturePrefix;
    BuildDigestedReferences();
    var key = SigningKey;
    if (key == null)
      throw new CryptographicException(GetSecurityResource("Cryptography_Xml_LoadKeyFailed"));
    if (SignedInfo.SignatureMethod == null)
    {
      if (key is DSA)
        SignedInfo.SignatureMethod = XmlDsigDSAUrl;
      else if (key is RSA)
      {
        if (SignedInfo.SignatureMethod == null)
          SignedInfo.SignatureMethod = XmlDsigRSASHA1Url;
      }
      else
        throw new CryptographicException(GetSecurityResource("Cryptography_Xml_CreatedKeyFailed"));
    }
    var signatureDescription = CryptoConfig.CreateFromName(SignedInfo.SignatureMethod) as SignatureDescription;
    if (signatureDescription == null)
      throw new CryptographicException(GetSecurityResource("Cryptography_Xml_SignatureDescriptionNotCreated"));
    var hashAlg = signatureDescription.CreateDigest();
    if (hashAlg == null)
      throw new CryptographicException(GetSecurityResource("Cryptography_Xml_CreateHashAlgorithmFailed"));
    GetC14NDigest(hashAlg);
    var asymmetricSignatureFormatter = signatureDescription.CreateFormatter(key);
    m_signature.SignatureValue = asymmetricSignatureFormatter.CreateSignature(hashAlg);
  }

  public new XmlElement GetXml()
  {
    var e = base.GetXml();
    SetPrefix(e);
    return e;
  }

  private void BuildDigestedReferences()
  {
    typeof(SignedXml).GetMethod("BuildDigestedReferences",
      BindingFlags.NonPublic | BindingFlags.Instance)
      .Invoke(this, new object[] { });
  }

  private byte[] GetC14NDigest(HashAlgorithm hash)
  {
    var document = new XmlDocument {PreserveWhitespace = true};
    var e = SignedInfo.GetXml();
    document.AppendChild(document.ImportNode(e, true));
    var canonicalizationMethodObject = SignedInfo.CanonicalizationMethodObject;
    SetPrefix(document.DocumentElement);
    canonicalizationMethodObject.LoadInput(document);
    return canonicalizationMethodObject.GetDigestedOutput(hash);
  }

  void SetPrefix(XmlNode node)
  {
    var nodes = node.SelectNodes(@"//descendant-or-self::*");
    if (nodes==null)
      throw new InvalidOperationException("Signature is empty.");
    nodes.Cast<XmlElement>()
      .ToList().ForEach(n => n.Prefix = SignaturePrefix);
  }

  private static readonly MethodInfo getSecurityResourcesMethod = typeof (SignedXml)
    .Assembly
    .GetType("System.Security.SecurityResources")
    .GetMethod("GetResourceString", BindingFlags.Static | BindingFlags.NonPublic);
  static string GetSecurityResource(string key)
  {
    return (string) getSecurityResourcesMethod.Invoke(null, new object[] {key});
  }
}