Get the "Key Usage" from Certificate - c++

Question

What API can I use to get the "Key Usage" from a certificate. I Specifically want to know if a given certificate has "Digital Signature" or not.
Below screenshot is the detail of a certificate in windows. I need the API that gives me the "Key Usage".
The code is for windows and I am writing my code in C++.
Thank you
Sam

Answer 1

Start with CertOpenStore, then call CertFindCertificateInStore in a loop until you find the certificate you are interested in.
The returned CERT_CONTEXT contains a pointer to a CERT_INFO struct. You will then want to walk the rgExtension member which is an array of CERT_EXTENSION objects. The one you care about has pszObjId set to szOID_KEY_USAGE_RESTRICTION, which will then give you this data: CERT_KEY_USAGE_RESTRICTION_INFO where the RestrictedKeyUsage member has the bit flags you are interested in.
You can also look at the szOID_KEY_USAGE extension, which will use the same bit flags, but the msdn documentation states that those fields are
advisory field[s], only, and does not imply that usage of the key is restricted
to the purpose indicated
Depending on what you need the information for, you could use either extension.

Answer 2

With the help of Josh Poley, I found the answer. Thank you Josh
bool CertHasDigitalSignature(PCCERT_CONTEXT pCert)
{
bool retVal(false);
CERT_EXTENSION* keyUsage;
keyUsage = CertFindExtension(szOID_KEY_USAGE, pCert->pCertInfo->cExtension, pCert->pCertInfo->rgExtension);
if(NULL != keyUsage)
{
DWORD strSz(0);
if(CryptFormatObject(X509_ASN_ENCODING, 0, 0, NULL, szOID_KEY_USAGE, 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, szOID_KEY_USAGE, keyUsage->Value.pbData ,keyUsage->Value.cbData, (void*)Buff.data(), &strSz))
{
if (std::wstring::npos != Buff.find(L"Digital Signature"))
retVal = true;
}
}
}
return retVal;
}