Visual C++ - Send email using Gmail/G Suite OAuth 2.0 in background service (service account)¶

#include "stdafx.h" // pre-compile header

#include "C:\Program Files (x86)\EASendMail\Include\tlh\EASendMailObj.tlh"
using namespace EASendMailObjLib;

#include "C:\Program Files (x86)\EASendMail\Include\tlh\msxml3.tlh"
using namespace MSXML2;

const int ConnectNormal = 0;
const int ConnectSSLAuto = 1;
const int ConnectSTARTTLS = 2;
const int ConnectDirectSSL = 3;
const int ConnectTryTLS = 4;

const int AuthAuto = -1;
const int AuthLogin = 0;
const int AuthNtlm = 1;
const int AuthCramMd5 = 2;
const int AuthPlain = 3;
const int AuthMsn = 4;
const int AuthXoauth2 = 5;

BOOL RequestAccessToken(const TCHAR* requestData, _bstr_t &accessToken)
{
    try
    {
        IServerXMLHTTPRequestPtr httpRequest = NULL;
        httpRequest.CreateInstance(__uuidof(MSXML2::ServerXMLHTTP));
        if (httpRequest == NULL)
        {
            printf("Failed to create XML HTTP Object, please make sure you install MSXML 3.0 on your machine.\r\n");
            return FALSE;
        }

        _bstr_t fullRequest = _bstr_t("grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=") + requestData;

        const char* postData = (const char*)fullRequest;
        LONG cdata = strlen(postData);
        LPSAFEARRAY psaHunk = ::SafeArrayCreateVectorEx(VT_UI1, 0, cdata, NULL);
        for (LONG k = 0; k < (int)cdata; k++)
        {
            BYTE ch = (BYTE)postData[k];
            ::SafeArrayPutElement(psaHunk, &k, &ch);
        }

        _variant_t requestBuffer;
        requestBuffer.vt = (VT_ARRAY | VT_UI1);
        requestBuffer.parray = psaHunk;

        _variant_t async(true);
        _bstr_t uri("https://oauth2.googleapis.com/token");

        httpRequest->setOption((MSXML2::SERVERXMLHTTP_OPTION)2, 13056);
        httpRequest->open(L"POST", uri, async, vtMissing, vtMissing);
        httpRequest->setRequestHeader(L"Content-Type", L"application/x-www-form-urlencoded");
        httpRequest->send(requestBuffer);

        while (httpRequest->readyState != 4) {
            httpRequest->waitForResponse(1);
        }

        long status = httpRequest->status;
        _bstr_t responseText = httpRequest->responseText;

        if (status < 200 || status >= 300)
        {
            printf("Failed to get access token from server: %d %s\r\n", status, (const char*)responseText);
            return FALSE;
        }

        IOAuthResponseParserPtr oauthParser = NULL;
        oauthParser.CreateInstance(__uuidof(EASendMailObjLib::OAuthResponseParser));
        oauthParser->Load(responseText);

        accessToken = oauthParser->AccessToken;
        if (accessToken.length() == 0)
        {
            printf("Failed to parse access token from server response: %d %s\r\n", status, (const char*)responseText);
            return FALSE;
        }

        return TRUE;
    }
    catch (_com_error &ep)
    {
        printf("Failed to get access token: %s", (const char*)ep.Description());
        return FALSE;
    }
}

BOOL GenerateRequestData(const char* gsuiteUser, _bstr_t &requestData)
{
    // service account email address
    // You should create your client id and client secret,
    const char* serviceAccount = "xxxx@xxxx.iam.gserviceaccount.com";
    const char* scope = "https://mail.google.com/";
    const char* aud = "https://oauth2.googleapis.com/token";

    ISimpleJsonParserPtr jwtPtr = NULL;
    jwtPtr.CreateInstance(__uuidof(EASendMailObjLib::SimpleJsonParser));

    _bstr_t header = jwtPtr->JwtBase64UrlEncode(_bstr_t("{\"alg\":\"RS256\",\"typ\":\"JWT\"}"));

    // token request timestamp
    long iat = jwtPtr->GetCurrentIAT();
    // token expiration time
    long exp = iat + 3600;

    char iatBuf[MAX_PATH + 1];
    char expBuf[MAX_PATH + 1];

    sprintf_s(iatBuf, MAX_PATH, "%d", iat);
    sprintf_s(expBuf, MAX_PATH, "%d", exp);

    _bstr_t playload = _bstr_t("{");
    playload += _bstr_t("\"iss\":\"") + _bstr_t(serviceAccount) + _bstr_t("\",");
    playload += _bstr_t("\"scope\":\"") + _bstr_t(scope) + _bstr_t("\",");
    playload += _bstr_t("\"aud\":\"") + _bstr_t(aud) + _bstr_t("\",");
    playload += _bstr_t("\"exp\":") + _bstr_t(expBuf) + _bstr_t(",");
    playload += _bstr_t("\"iat\":") + _bstr_t(iatBuf) + _bstr_t(",");
    playload += _bstr_t("\"sub\":\"") + _bstr_t(gsuiteUser) + _bstr_t("\"");
    playload += _bstr_t("}");

    playload = jwtPtr->JwtBase64UrlEncode(playload);

    ICertificatePtr cert = NULL;
    cert.CreateInstance(__uuidof(EASendMailObjLib::Certificate));

    // In web application, use CRYPT_MACHINE_KEYSET
    if (cert->LoadPFXFromFile(_bstr_t("D:\\MyData\\oauth-77dac4d192ec.p12"),
        _bstr_t("notasecret"), CRYPT_USER_KEYSET) == VARIANT_FALSE)
    {
        printf("Failed to load service account certificate!");
        return FALSE;
    }

    _bstr_t signature = jwtPtr->SignRs256(cert, header + _bstr_t(".") + playload);
    if (signature.length() == 0)
    {
        printf("Failed to sign request data!");
        return FALSE;
    }

    requestData = header + _bstr_t(".") + playload + _bstr_t(".") + signature;
    return TRUE;
}

void SendEmail()
{
    ::CoInitialize(NULL);

    // gsuiteUser is the full email address of the user in GSuite, for example: user@gsuitedomain
    const char* gsuiteUser = "user@mydomainingsuit.com";

    _bstr_t requestData, accessToken;
    if (!GenerateRequestData(gsuiteUser, requestData))
    {
        return;
    }

    // request access token from Google server by service account
    // withou user interaction
    if (!RequestAccessToken((const TCHAR*)requestData, accessToken))
    {
        return;
    }

    IMailPtr oSmtp = NULL;
    oSmtp.CreateInstance(__uuidof(EASendMailObjLib::Mail));
    oSmtp->LicenseCode = _bstr_t("TryIt");

    // Gmail SMTP server address
    oSmtp->ServerAddr = _bstr_t("smtp.gmail.com");
    oSmtp->ServerPort = 587;

    // Enable SSL/TLS connection
    oSmtp->ConnectType = ConnectSSLAuto;

    // Gmail OAUTH type
    oSmtp->AuthType = AuthXoauth2;
    oSmtp->UserName = _bstr_t(gsuiteUser);
    oSmtp->Password = accessToken;

    oSmtp->FromAddr = _bstr_t(gsuiteUser);

    // Please change recipient address to yours for test
    oSmtp->AddRecipient(_bstr_t("Support Team"),
        _bstr_t("support@emailarchitect.net"), 0);

    oSmtp->Subject = _bstr_t("Test email using G suite service account");
    oSmtp->BodyText = _bstr_t("Hello, this is a test....");

    if (oSmtp->SendMail() == 0)
        printf("Message delivered!");
    else
        printf((const char*)oSmtp->GetLastErrDescription());
}