(PowerShell) 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 .NET Downloads Chilkat .NET Assemblies

Add-Type -Path "C:\chilkat\ChilkatDotNet47-9.5.0-x64\ChilkatDotNet47.dll"

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

$oauth2 = New-Object Chilkat.OAuth2

# 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.ListenPort = 55568

$oauth2.AuthorizationEndpoint = "https://accounts.google.com/o/oauth2/v2/auth"
$oauth2.TokenEndpoint = "https://www.googleapis.com/oauth2/v4/token"

# Replace these with actual values.
$oauth2.ClientId = "GOOGLE-CLIENT-ID"
$oauth2.ClientSecret = "GOOGLE-CLIENT-SECRET"

$oauth2.CodeChallenge = $true
$oauth2.CodeChallengeMethod = "S256"

# This is the full-permissions scope for GMail.
$oauth2.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.LastMethodSuccess -ne $true) {
    $($oauth2.LastErrorText)
    exit
}

# 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 -lt 30000) -and ($oauth2.AuthFlowState -lt 3)) {
    $oauth2.SleepMs(100)
    $numMsWaited = $numMsWaited + 100
}

# 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.AuthFlowState -lt 3) {
    $oauth2.Cancel()
    $("No response from the browser!")
    exit
}

# 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.AuthFlowState -eq 5) {
    $("OAuth2 failed to complete.")
    $($oauth2.FailureInfo)
    exit
}

if ($oauth2.AuthFlowState -eq 4) {
    $("OAuth2 authorization was denied.")
    $($oauth2.AccessTokenResponse)
    exit
}

if ($oauth2.AuthFlowState -ne 3) {
    $("Unexpected AuthFlowState:" + $oauth2.AuthFlowState)
    exit
}

# Save the full JSON access token response to a file.
$sbJson = New-Object Chilkat.StringBuilder
$sbJson.Append($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"
# 	}
# 
$("OAuth2 authorization granted!")
$("Access Token = " + $oauth2.AccessToken)