// verify signature on dll, return true if signature is valid, false otherwise
static boolean check_dll(const char* dll_path)
{
	// see if the file exists
	if (_access(dll_path, 0)) {
		// file doesn't exist, return failure
		return false;
	}

	// Prepare to check Authenticode signature on dll to insure hackers haven't modified the file
	GUID action_guid = WINTRUST_ACTION_GENERIC_VERIFY_V2;
	DWORD dwPathLen = strlen(dll_path) + 1;
	WINTRUST_FILE_INFO file_info;
	memset(&file_info, 0, sizeof(file_info));
	file_info.cbStruct = sizeof(file_info);
	char* buffer = new char[2*dwPathLen];
	file_info.pcwszFilePath = (wchar_t*)buffer;
	if (!MultiByteToWideChar(CP_ACP, 0, dll_path, dwPathLen, (wchar_t*)buffer, dwPathLen)) {
		// could not convert path to Unicode, free memory and return failure
		delete [] buffer;
		return false;
	}

	WINTRUST_DATA trust_data;
	memset(&trust_data, 0, sizeof(trust_data));
	trust_data.cbStruct = sizeof(trust_data);
	trust_data.dwUIChoice = WTD_UI_NONE;
	trust_data.fdwRevocationChecks = WTD_REVOKE_NONE;
	trust_data.dwUnionChoice = WTD_CHOICE_FILE;
	trust_data.pFile = &file_info;

	HRESULT result;
	result = WinVerifyTrust((HWND)INVALID_HANDLE_VALUE, &action_guid, &trust_data);
	if (result != ERROR_SUCCESS) {
		// Authenticode check failed, free memory and return failure
		delete [] buffer;
		return false;
	}
	delete [] buffer;

	// Authenticode check succeeded, now verify the certificate used to sign the dll is ours and
	// not some imposter's
	// open dll
	HANDLE dll_handle = CreateFile(dll_path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL|FILE_FLAG_RANDOM_ACCESS, NULL);
	if (dll_handle == INVALID_HANDLE_VALUE) {
		return false;
	}
	DWORD cert_count;
	if (!ImageEnumerateCertificates(dll_handle, CERT_SECTION_TYPE_ANY, &cert_count, NULL, 0)) {
		CloseHandle(dll_handle);
		return false;
	}

	// there should only be one certificate associated with this dll, ours
	if (cert_count != 1) {
		CloseHandle(dll_handle);
		return false;
	}

	// first get the certificate header to get the proper size for certificate structure
	WIN_CERTIFICATE certHead;
	certHead.dwLength = 0;
	certHead.wRevision = WIN_CERT_REVISION_1_0;

	if (!ImageGetCertificateHeader(dll_handle, 0, &certHead)) {
		// problem getting certificate information, return failure
		CloseHandle(dll_handle);
		return false;
	}

	// allocate memory for certificate
	DWORD cert_len = certHead.dwLength;
	char* cert = new char[sizeof(WIN_CERTIFICATE) + cert_len];

	WIN_CERTIFICATE *cert_p = (WIN_CERTIFICATE*)cert;
	cert_p->dwLength = cert_len;
	cert_p->wRevision = WIN_CERT_REVISION_1_0;

	if (!ImageGetCertificateData(dll_handle, 0, cert_p, &cert_len)) {
		// problem getting certificate, return failure
		CloseHandle(dll_handle);
		delete [] cert;
		return false;
	}
	// done with file handle, close it
	CloseHandle(dll_handle);

	// extract the certificate used to sign the dll. Since the signature certificate is a PKCS#7 SignedData object,
	// we can verify the signature of the object which will have the side-effect of creating the certificate context
	// of the signing certificate.
	DWORD decodesize = 0;
	PCCERT_CONTEXT pCertContext;
	CRYPT_VERIFY_MESSAGE_PARA vPara;
	memset(&vPara, 0, sizeof(vPara));
	vPara.cbSize = sizeof(vPara);
	vPara.dwMsgAndCertEncodingType = X509_ASN_ENCODING | PKCS_7_ASN_ENCODING;
	if (!CryptVerifyMessageSignature(&vPara, 0, cert_p->bCertificate, cert_p->dwLength, NULL, &decodesize, &pCertContext)) {
		delete [] cert;
		return false;
	}

	// If the certificate that signed the dll matches ours, we're in business!
	// Compare the subject name, do not compare the entire certificate or the public key as these will change each
	// year as the certificates are renewed.
	DWORD subjectSize = CertGetNameString(pCertContext, CERT_NAME_SIMPLE_DISPLAY_TYPE, 0, NULL, NULL, 0);
	char *subjectName = new char[subjectSize];
	CertGetNameString(pCertContext, CERT_NAME_SIMPLE_DISPLAY_TYPE, 0, NULL, subjectName, subjectSize);
	if (strcmp(subjectName, "YoYoDyne Corporation")) {
		delete [] subjectName;
		delete [] cert;
		return false;
	}

	// clean up
	delete [] subjectName;
	delete [] cert;
	CertFreeCertificateContext(pCertContext);

	return true;
}