// Открываем хранилище сертификатов
if (!(hCertStore = CertOpenStore(
CERT_STORE_PROV_SYSTEM,
0,
NULL,
CERT_SYSTEM_STORE_CURRENT_USER,
CERT_STORE_NAME)))
{
printf("You can not open the vault MY.");
}
else {
printf("open the vault MY.\n");
}

// Получаем указатель на наш сертификат
if (!(pCertContext = CryptUIDlgSelectCertificateFromStore(
hCertStore,
NULL,
NULL,
NULL,
CRYPTUI_SELECT_LOCATION_COLUMN,
0,
NULL)))
{
printf("no change cert. \n");
}
else
{
printf("ok change cert. \n");
}


HCRYPTPROV hCryptProv = NULL;
DWORD dwKeySpecSender;
if (CryptAcquireCertificatePrivateKey(
pCertContext,
0,
NULL,
&hCryptProv,
&dwKeySpecSender,
NULL))
{
printf("dwKeySpecSender - %d.\n", dwKeySpecSender);
printf("The CSP handle has been acquired. \n");
}
else
{
printf("Error during CryptAcquireCertificatePrivateKey.");
}

HCRYPTKEY key = NULL;
if (CryptGetUserKey(hCryptProv, dwKeySpecSender, &key))
{
printf("The CSP handle has been key. \n");
}
else
{
printf("not CSP handle has been key. \n");
}

DWORD cbSize = sizeof(DWORD);
DWORD algId;
DWORD dwKeyId = 0;

CryptGetKeyParam(key, KP_ALGID, (BYTE*)&dwKeyId, &cbSize, 0);

X509Certificate2 ^ certX509 = gcnew X509Certificate2((BYTE*)dwKeyId, cbSize);

// Create a new XML document.
XmlDocument^ doc = gcnew XmlDocument;

// Load the passed XML file using its name.
doc->Load(gcnew XmlTextReader("FileName"));

// Create a SignedXml object.
SignedXml^ signedXml = gcnew SignedXml(doc);

// Add the key to the SignedXml document.
//signedXml->SigningKey = Key;
signedXml->SigningKey = certX509->PrivateKey;

Console::WriteLine("SignatureMethod: {0}", signedXml->SignatureMethod);

// Create a reference to be signed.
Reference^ reference = gcnew Reference;
reference->Uri = "";
//reference->DigestMethod = "http://www.w3.org/2001/04/xmldsig-more#gostr3411";
//reference->DigestMethod->Insert(0, "http://www.w3.org/2001/04/xmldsig-more#gostr3411");

// Add an enveloped transformation to the reference.
XmlDsigEnvelopedSignatureTransform^ env = gcnew XmlDsigEnvelopedSignatureTransform;
reference->AddTransform(env);

// Add the reference to the SignedXml object.
signedXml->AddReference(reference);

// Compute the signature.
signedXml->ComputeSignature();

можно еще более заморочиться и использовать апи
- CryptAcquireContext(ref pProv, Cnf.ContName, "", 75, 0)
- CryptGetUserKey(pProv, WinCryptH.AT_KEYEXCHANGE, ref pUserKey)
- CryptGetKeyParam(pUserKey, WinCryptH.KP_CERTIFICATE, pCert, ref dt_len, 0)

и далее new X509Certificate(pCert, dt_len)

signedXml.SigningKey = Certificate.PrivateKey;

Алгоритм ключа сертификата не поддерживается