(Ruby) 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.

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.

# --------------------------------------------------------------------------------
# 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.
fac = Chilkat::CkFileAccess.new()
jsonKey = fac.readEntireTextFile("qa_data/googleApi/chilkat25-b4214220e565.json","utf-8")
if (fac.get_LastMethodSuccess() != true)
    print fac.lastErrorText() + "\n";
    exit
end

# 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"
# }

gAuth = Chilkat::CkAuthGoogle.new()
gAuth.put_JsonKey(jsonKey)

# Specify a scope.
gAuth.put_Scope("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("info@chilkat.xyz")

# Connect to www.googleapis.com using TLS
tlsSock = Chilkat::CkSocket.new()
success = tlsSock.Connect("www.googleapis.com",443,true,5000)
if (success != true)
    print tlsSock.lastErrorText() + "\n";
    exit
end

# Send the request to obtain the access token.
success = gAuth.ObtainAccessToken(tlsSock)
if (success != true)
    print gAuth.lastErrorText() + "\n";
    exit
end

# Examine the access token:
accessToken = gAuth.accessToken()
print "Access Token: " + accessToken + "\n";

# 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.

# -----------------------------------------------------------------------
mailman = Chilkat::CkMailMan.new()

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

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

# Create a new email object
email = Chilkat::CkEmail.new()

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

success = mailman.SendEmail(email)
if (success != true)
    print mailman.lastErrorText() + "\n";
    exit
end

success = mailman.CloseSmtpConnection()
if (success != true)
    print "Connection to SMTP server not closed cleanly." + "\n";
end

print "Successfully sent email using Gmail with a service account key." + "\n";