SingedXML With rsa-sha512

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C#

XML

cryptography

the bold line make an exeption of:
"SignatureDescription could not be created for the signature algorithm supplied."
i run it from win 7 SP1 32 bit,
i tried to install KB2973337 & KB2973351.
nothing helps!!!! its urgent!!! please help!!!
(without the bold line all work, but i need rsa-sha512 especially!!)
the certificate key check twice and its ok!!
also i tries to add this sugnature:
CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA512SignatureDescription), XmlDsigRsaSha512);

and derive:
public class RSAPKCS1SHA512SignatureDescription : SignatureDescription {....}
nothing work!!! ==> get the exeption: "Invalid algorithm specified."

someone told me its something on the Operation System, this why i installed to KB's
... but still nothing.

This is my code:
---------------

XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);

SignedXml signedXml = new SignedXml(doc);

signedXml.SignedInfo.CanonicalizationMethod = "http://www.w3.org/2001/10/xml-exc-c14n#";//SignedXml.XmlDsigExcC14NTransformUrl
signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha512";
signedXml.SigningKey = cert.PrivateKey;

Reference reference = new Reference("");
reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());
reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha512"; signedXml.AddReference(reference);

KeyInfo keyInfo = new KeyInfo();
keyInfo.AddClause(new KeyInfoX509Data(cert));
signedXml.KeyInfo = keyInfo;

signedXml.ComputeSignature();