const string IssuerIdentifierOid = "2.5.29.35";
                    var issuerKeyId = cert
                      .Extensions
                      .Cast<X509Extension>()
                      .Where(e => IssuerIdentifierOid.Equals(e.Oid.Value, StringComparison.OrdinalIgnoreCase))
                      .Single()
                      .Format(false);

WINCRYPT32API
PCERT_RDN_ATTR
WINAPI
CertFindRDNAttr(
    _In_ LPCSTR pszObjId,
    _In_ PCERT_NAME_INFO pName
    );

WINCRYPT32API
PCERT_EXTENSION
WINAPI
CertFindExtension(
    _In_ LPCSTR pszObjId,
    _In_ DWORD cExtensions,
    _In_reads_(cExtensions) CERT_EXTENSION rgExtensions[]
    );

std::string oid = "2.5.29.37";
CERT_EXTENSION* keyUsage;
keyUsage = CertFindExtension(oid.c_str(), pCertContext->pCertInfo->cExtension, pCertContext->pCertInfo->rgExtension);
if (NULL != keyUsage)
{
DWORD strSz(0);
if (CryptFormatObject(X509_ASN_ENCODING, 0, 0, NULL, oid.c_str(), keyUsage->Value.pbData, keyUsage->Value.cbData, NULL, &strSz))
{
std::wstring Buff;
Buff.resize((strSz / sizeof(wchar_t)) + 1);
if (CryptFormatObject(X509_ASN_ENCODING, 0, 0, NULL, oid.c_str(), keyUsage->Value.pbData, keyUsage->Value.cbData, (void*)Buff.data(), &strSz))
{
//if (std::wstring::npos != Buff.find(L"Digital Signature"))
}
}
}