(Ruby) Get GMail SMTP OAuth2 Access Token for Desktop App

Demonstrates how to get a GMail SMTP OAuth2 access token from a desktop application or script, which can be used with the XOAUTH2 authentication method.

This example requires a browser window to be displayed to allow the GMail account owner to authorize the access. The code for displaying the web browser is omitted. Suggestions for a few programming languages are provided in code comments. You'll need to find out how to display a web browser and navigate to the URL returned by the call to oauth2.StartAuth.

Chilkat Ruby Downloads install from rubygems.org gem install chilkat or download... Ruby Library for Windows, MacOS, Linux, Alpine Linux

require 'chilkat'

# This example requires the Chilkat API to have been previously unlocked.
# See Global Unlock Sample for sample code.

oauth2 = Chilkat::CkOAuth2.new()

# For Google OAuth2, set the listen port equal to the port used
# in the Authorized Redirect URL for the Client ID.
# For example, in this case the Authorized Redirect URL would be http://localhost:55568/
# Your app should choose a port not likely not used by any other application.
oauth2.put_ListenPort(55568)

oauth2.put_AuthorizationEndpoint("https://accounts.google.com/o/oauth2/v2/auth")
oauth2.put_TokenEndpoint("https://www.googleapis.com/oauth2/v4/token")

# Replace these with actual values.
oauth2.put_ClientId("GOOGLE-CLIENT-ID")
oauth2.put_ClientSecret("GOOGLE-CLIENT-SECRET")

oauth2.put_CodeChallenge(true)
oauth2.put_CodeChallengeMethod("S256")

# This is the full-permissions scope for GMail.
oauth2.put_Scope("https://mail.google.com/")

# -------------------------------------------------------------------------------------------------------
# Important: Google claims there is a send-only scope:  "https://www.googleapis.com/auth/gmail.send"
# However, it has been found to NOT work when sending email using the SMTP protocol.
# Perhaps it works for sending GMail using the GMail REST API, but it most certainly does not
# provide access/authentication for sending email by SMTP.  The full GMail scope is required.
# In any case, it may be much easier to avoid using OAuth2 altogether and instead use "App Passwords".
# See Send GMail with App Passwords.
# -------------------------------------------------------------------------------------------------------

# Begin the OAuth2 three-legged flow.  This returns a URL that should be loaded in a browser.
url = oauth2.startAuth()
if (oauth2.get_LastMethodSuccess() != true)
    print oauth2.lastErrorText() + "\n";
    exit
end

# At this point, your application should load the URL in a browser.
# For example, 
# in C#: System.Diagnostics.Process.Start(url);
#  in Java: Desktop.getDesktop().browse(new URI(url));
#  in VBScript: Set wsh=WScript.CreateObject("WScript.Shell")
#              wsh.Run url
# in Xojo: ShowURL(url)  (see http://docs.xojo.com/index.php/ShowURL)
# in Dataflex: Runprogram Background "c:\Program Files\Internet Explorer\iexplore.exe" sUrl        
# The Google account owner would interactively accept or deny the authorization request.

# Add the code to load the url in a web browser here...
# Add the code to load the url in a web browser here...
# Add the code to load the url in a web browser here...

# Now wait for the authorization.
# We'll wait for a max of 30 seconds.
numMsWaited = 0
while (numMsWaited < 30000) and (oauth2.get_AuthFlowState() < 3)
    oauth2.SleepMs(100)
    numMsWaited = numMsWaited + 100
end

# If there was no response from the browser within 30 seconds, then 
# the AuthFlowState will be equal to 1 or 2.
# 1: Waiting for Redirect. The OAuth2 background thread is waiting to receive the redirect HTTP request from the browser.
# 2: Waiting for Final Response. The OAuth2 background thread is waiting for the final access token response.
# In that case, cancel the background task started in the call to StartAuth.
if (oauth2.get_AuthFlowState() < 3)
    oauth2.Cancel()
    print "No response from the browser!" + "\n";
    exit
end

# Check the AuthFlowState to see if authorization was granted, denied, or if some error occurred
# The possible AuthFlowState values are:
# 3: Completed with Success. The OAuth2 flow has completed, the background thread exited, and the successful JSON response is available in AccessTokenResponse property.
# 4: Completed with Access Denied. The OAuth2 flow has completed, the background thread exited, and the error JSON is available in AccessTokenResponse property.
# 5: Failed Prior to Completion. The OAuth2 flow failed to complete, the background thread exited, and the error information is available in the FailureInfo property.
if (oauth2.get_AuthFlowState() == 5)
    print "OAuth2 failed to complete." + "\n";
    print oauth2.failureInfo() + "\n";
    exit
end

if (oauth2.get_AuthFlowState() == 4)
    print "OAuth2 authorization was denied." + "\n";
    print oauth2.accessTokenResponse() + "\n";
    exit
end

if (oauth2.get_AuthFlowState() != 3)
    print "Unexpected AuthFlowState:" + oauth2.get_AuthFlowState().to_s() + "\n";
    exit
end

# Save the full JSON access token response to a file.
sbJson = Chilkat::CkStringBuilder.new()
sbJson.Append(oauth2.accessTokenResponse())
sbJson.WriteFile("qa_data/tokens/_gmailSmtp.json","utf-8",false)

# The saved JSON response looks like this:

# 	{
# 	 "access_token": "ya39.Ci-XA_C5bGgRDC3UaD-h0_NeL-DVIQnI2gHtBBBHkZzrwlARkwX6R3O0PCDEzRlfaQ",
# 	 "token_type": "Bearer",
# 	 "expires_in": 3600,
# 	 "refresh_token": "1/r_2c_7jddspcdfesrrfKqfXtqo08D6Q-gUU0DsdfVMsx0c"
# 	}
# 
print "OAuth2 authorization granted!" + "\n";
print "Access Token = " + oauth2.accessToken() + "\n";