(Unicode C++) OAuth2 for a GMail using a JSON Service Account Key

This example shows how to obtain an OAuth2 access token for Gmail using a Google Service Account and a JSON private key. Once acquired, the access token can be used to send emails. Remember, upon token expiration, this process needs to be repeated to obtain a new token. Note: This procedure is specific to OAuth2 with Google Service Account keys.

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#include <CkFileAccessW.h>
#include <CkAuthGoogleW.h>
#include <CkSocketW.h>
#include <CkMailManW.h>
#include <CkEmailW.h>

void ChilkatSample(void)
    {
    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // --------------------------------------------------------------------------------
    // For a step-by-step guide for setting up your Google Workspace service account,
    // see Setup Google Workspace Account for Sending SMTP GMail from a Service Account
    // --------------------------------------------------------------------------------

    // First load the JSON key into a string.
    CkFileAccessW fac;
    const wchar_t *jsonKey = fac.readEntireTextFile(L"qa_data/googleApi/chilkat25-b4214220e565.json",L"utf-8");
    if (fac.get_LastMethodSuccess() != true) {
        wprintf(L"%s\n",fac.lastErrorText());
        return;
    }

    // A Google service account JSON private key looks like this:

    // {
    //   "type": "service_account",
    //   "project_id": "chilkat25",
    //   "private_key_id": "b4214220f565881e19eeb97c2699bf5a0d1e3e0b",
    //   "private_key": "-----BEGIN PRIVATE KEY-----\nMIIEvQ...NXcM=\n-----END PRIVATE KEY-----\n",
    //   "client_email": "chilkatsvc@chilkat25.iam.gserviceaccount.com",
    //   "client_id": "109122032928932715958",
    //   "auth_uri": "https://accounts.google.com/o/oauth2/auth",
    //   "token_uri": "https://oauth2.googleapis.com/token",
    //   "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
    //   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/chilkatsvc%40chilkat25.iam.gserviceaccount.com",
    //   "universe_domain": "googleapis.com"
    // }

    CkAuthGoogleW gAuth;
    gAuth.put_JsonKey(jsonKey);

    // Specify a scope.
    gAuth.put_Scope(L"https://mail.google.com/");

    // Request an access token that is valid for this many seconds.
    gAuth.put_ExpireNumSeconds(3600);

    // When using a Google Workspace account with Gmail APIs, a service account can impersonate a user 
    // via a process called domain-wide delegation � and the "sub" claim in the JWT is what enables this.
    // Domain-wide delegation allows a Google Workspace administrator to authorize a service account to 
    // act on behalf of any user in the domain, without user interaction.

    // This is required for server-to-server access to user data � such as reading/sending Gmail from a background service.
    // This is your company email address.
    gAuth.put_SubEmailAddress(L"info@chilkat.xyz");

    // Connect to www.googleapis.com using TLS
    CkSocketW tlsSock;
    bool success = tlsSock.Connect(L"www.googleapis.com",443,true,5000);
    if (success != true) {
        wprintf(L"%s\n",tlsSock.lastErrorText());
        return;
    }

    // Send the request to obtain the access token.
    success = gAuth.ObtainAccessToken(tlsSock);
    if (success != true) {
        wprintf(L"%s\n",gAuth.lastErrorText());
        return;
    }

    // Examine the access token:
    const wchar_t *accessToken = gAuth.accessToken();
    wprintf(L"Access Token: %s\n",accessToken);

    // Sample output:
    // ya29.a0AW4XtxjGTD67Z8 .... IRw0218

    // The access token allows us to send unlimited emails while it's valid. Once it expires, we must obtain and use a new one.

    // -----------------------------------------------------------------------
    CkMailManW mailman;

    // Set the properties for the GMail SMTP server:
    mailman.put_SmtpHost(L"smtp.gmail.com");
    mailman.put_SmtpPort(587);
    mailman.put_StartTLS(true);

    mailman.put_SmtpUsername(L"info@chilkat.xyz");
    mailman.put_OAuth2AccessToken(accessToken);

    // Create a new email object
    CkEmailW email;

    email.put_Subject(L"This is a test");
    email.put_Body(L"This is a test");
    email.put_From(L"Chilkat Test <info@chilkat.xyz>");
    success = email.AddTo(L"Chilkat Software",L"info@chilkatsoft.com");
    // To add more recipients, call AddTo, AddCC, or AddBcc once per recipient.

    success = mailman.SendEmail(email);
    if (success != true) {
        wprintf(L"%s\n",mailman.lastErrorText());
        return;
    }

    success = mailman.CloseSmtpConnection();
    if (success != true) {
        wprintf(L"Connection to SMTP server not closed cleanly.\n");
    }

    wprintf(L"Successfully sent email using Gmail with a service account key.\n");
    }