// читаем расширения сертификата
	PCERT_EXTENSION pExtension = NULL;
	pExtension = CertFindExtension(szOID_KEY_USAGE,
		pUserCert->pCertInfo->cExtension,
		pUserCert->pCertInfo->rgExtension);

	PCRYPT_DATA_BLOB usages = NULL;
	DWORD dwSize= 0;
	BYTE bUsage;

	if(pExtension)
	{
		CryptDecodeObject(ENCODING, X509_KEY_USAGE,
			pExtension->Value.pbData, pExtension->Value.cbData,
			0, 0, &dwSize);

		usages = (CRYPT_DATA_BLOB*)malloc(dwSize);	
		usages->cbData = dwSize;

		CryptDecodeObject(ENCODING, X509_KEY_USAGE,
			pExtension->Value.pbData, pExtension->Value.cbData,										
			0, (LPVOID)usages, &dwSize);			

		bUsage = (BYTE)usages->pbData;
		if( (bUsage & CERT_DIGITAL_SIGNATURE_KEY_USAGE) > 0)
			MessageBox("такой тип использования есть");
				
		if(usages) free(usages);
	}