1) SignedXml 클래스로는 signature에 sha256을 사용 할 수가 없습니다.
그런데 digest는 xmlenc#sha256으로 지정해도 256비트 결과가 나오고 오류가 없더군요.

signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";

rsa-sha256일 때 발생하는 예외
System.Security.Cryptography.CryptographicException: SignatureDescription could not be created for the signature algorithm supplied.
   at System.Security.Cryptography.Xml.SignedXml.ComputeSignature()


이걸 지원하지 않습니다. prefix 문제를 해결 하더라도 지원하지 않는 algorithm 때문에 에러가 날 것입니다.
관련 사이트
https://msdn.microsoft.com/en-us/library/windows/desktop/dd979768(v=vs.85).aspx

2) 서명 전에는 ds prefix가 없는 상태에서 진행합니다.
ancestor-or-self::ds:Signature => ancestor-or-self::Signature
ds:XPath => XPath

서명이 끝나면 SetPrefix()로 ds를 붙여줍니다.
제가 테스트해보니 이 함수를 그냥 쓰면 root node에는 ds가 붙지 않았고, namespace에도 붙지 않았습니다.
아래처럼 사용하세요.

            signedXml.ComputeSignature();

            XmlElement xmlDisigXml = signedXml.GetXml();
            var nameTable = new NameTable();
            XmlNamespaceManager xmlNmMgr = new XmlNamespaceManager(nameTable);
            xmlNmMgr.AddNamespace("ds", SignedXml.XmlDsigNamespaceUrl);
            xmlNmMgr.AddNamespace("xsi", "http://www.w3.org/2001/XMLSchema-instance");
            xmlNmMgr.AddNamespace("schemaLocation", "http://www.w3.org/2000/09/xmldsig# http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd");

            var xmldsigDoc = new XmlDocument(nameTable);
            xmldsigDoc.LoadXml(xmlDisigXml.OuterXml);
            SetPrefix(xmldsigDoc);


테스트 코드

        [Test]
        public void DoDigitalSign()
        {
            var priKey = GetSignPrivateKey("XmlSign.Test.Resources._2010.kmPri.key", "88888888");
            var signCert = new X509Certificate2(Datum.SIGNCERT);
            var transformXPath = "not(self::*[name()='TaxInvoice']" +
                                 "|ancestor-or-self::*[name()='ExchangedDocument']" +
                                 "|ancestor-or-self::Signature)";
            var transformXPath2 = "not(self::*[name()='TaxInvoice'] " +
                                 "|ancestor-or-self::*[name()='ExchangedDocument']" +
                                 "|ancestor-or-self::ds:Signature)";
            var signedFileName = "SignedTaxInvoice_Sampl.xml";
            var tbSignXml = GetResourceString("XmlSign.Test.Resources._2010.TaxInvoice_Sample.xml");

            SignXmlFile(
                tbSignXml,
                signedFileName,
                transformXPath, transformXPath2,
                priKey,
                signCert
                );
        }

        const string wszURI_XMLNS_DIGSIG_RSA_SHA256 = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
        const string wszURI_XMLNS_DIGSIG_SHA256 = "http://www.w3.org/2001/04/xmlenc#sha256";

        public void SignXmlFile(string tbSignXml, string signedFileName, string xpath, string xpath2, RSA signKey, X509Certificate2 signCert)
        {
            XmlDocument doc = new XmlDocument { PreserveWhitespace = true };
            doc.LoadXml(tbSignXml);

            var signedXml = new SignedXml(doc);
            //signedXml.SignedInfo.SignatureMethod = SignedXml.XmlDsigRSASHA1Url;
            signedXml.SignedInfo.SignatureMethod = wszURI_XMLNS_DIGSIG_RSA_SHA256;
            //"http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"; // see https://www.sysnet.pe.kr/Default.aspx?mode=3&sub=0&pageno=0&detail=1&wid=3653

            Reference reference = new Reference
            {
                Uri = "",
                //DigestMethod = SignedXml.XmlDsigSHA1Url
                DigestMethod = wszURI_XMLNS_DIGSIG_SHA256
            };
            reference.AddTransform(new XmlDsigC14NTransform(false));
            reference.AddTransform(CreateXPathTransform(xpath));
            signedXml.AddReference(reference);

            var keyInfo = new KeyInfo();
            keyInfo.AddClause(new KeyInfoX509Data(signCert));
            signedXml.KeyInfo = keyInfo;
            signedXml.SigningKey = signKey;

            signedXml.ComputeSignature();

            XmlElement xmlDisigXml = signedXml.GetXml();
            var nameTable = new NameTable();
            XmlNamespaceManager xmlNmMgr = new XmlNamespaceManager(nameTable);
            xmlNmMgr.AddNamespace("ds", SignedXml.XmlDsigNamespaceUrl);
            xmlNmMgr.AddNamespace("xsi", "http://www.w3.org/2001/XMLSchema-instance");
            xmlNmMgr.AddNamespace("schemaLocation", "http://www.w3.org/2000/09/xmldsig# http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd");

            var xmldsigDoc = new XmlDocument(nameTable);
            xmldsigDoc.LoadXml(xmlDisigXml.OuterXml);
            SetPrefix(xmldsigDoc);

            XmlElement root = doc.DocumentElement;
            root.InsertAfter(doc.ImportNode(xmldsigDoc.DocumentElement, true), root.FirstChild.NextSibling);
            doc.SelectSingleNode("descendant::ds:XPath", xmlNmMgr).InnerText = xpath2;
            
            using (XmlTextWriter writer = new XmlTextWriter(signedFileName, Encoding.UTF8))
            {
                doc.WriteTo(writer);
            }
        }

        #region functions

        public RSA GetSignPrivateKey(string uri, string passphrase)
        {
            PrivateKeyInfo privateKeyInfo =
                PrivateKeyInfoFactory.CreatePrivateKeyInfo(
                    passphrase.ToCharArray(),
                    EncryptedPrivateKeyInfo.GetInstance(
                        //Asn1Object.FromByteArray(GetResourceBytes(uri))));
                        Asn1Object.FromByteArray(Datum.SIGNPRIK)));
            return RsaUtils.GetRSAKey(privateKeyInfo);
        }

        private void SetPrefix(XmlNode node, string prefix = "ds")
        {
            foreach (XmlNode n in node.ChildNodes)
            {
                SetPrefix(n, prefix);
                n.Prefix = prefix;
            }
        }

        private static XmlDsigXPathTransform CreateXPathTransform(string xpath)
        {
            XmlDocument doc = new XmlDocument();
            // (note) 서명할 때는 ds prefix를 때고 해야 함
            XmlElement xPathElem = doc.CreateElement("XPath");
            xPathElem.InnerText = xpath;

            XmlDsigXPathTransform xForm = new XmlDsigXPathTransform();
            xForm.LoadInnerXml(xPathElem.SelectNodes("."));

            return xForm;
        }

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• [다음 글] ds:Signature 질문입니다.
• [이전 글] [답변]: ds:Signature 질문입니다.

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