How do I create an Access Token from Service Account Credentials using REST API?

NOTE: As noted in the comments, really this not a solution for the question because it uses SDK. Anyway, as the answers seems useful for other users, I've not deleted it

There is a simpler way to generate a token from a service account, using Google libraries

from google.auth.transport import requests
from google.oauth2 import service_account

CREDENTIAL_SCOPES = ["https://www.googleapis.com/auth/cloud-platform"]
CREDENTIALS_KEY_PATH = '/PATH/TO/SERVICE_ACCOUNT.json'

def get_service_account_token():
  credentials = service_account.Credentials.from_service_account_file(
          CREDENTIALS_KEY_PATH, scopes=CREDENTIAL_SCOPES)
  credentials.refresh(requests.Request())
  return credentials.token

Or if you want to use the default authentication

import google
from google.auth.transport import requests

CREDENTIAL_SCOPES = ["https://www.googleapis.com/auth/cloud-platform"] 

def get_default_token():
  credentials, project_id = google.auth.default(scopes=CREDENTIAL_SCOPES)
  credentials.refresh(requests.Request())
  return credentials.token

When credentials object is created, the token is empty, but after refreshing credentials, it contains the access token that can be used as header in the API requests

The following example shows you several important steps to call Google Cloud APIs without using an SDK in Python. Similar code works in just about any language (c#, java, php, nodejs).

Change the source code with the filename of your service account Json file, your Google Zone and your Project ID.

This example will list the instances in one zone for the specified project. From this example you will know the framework to call an API to create GCE instances.

This code will show you how to:

1. How to load service account credentials from a Json file.
2. How to extract the Private Key used to sign requests.
3. How to create a JWT (Json Web Token) for Google Oauth 2.0.
4. How to set the Google Scopes (permissions).
5. How to sign a JWT to create a Signed-JWT (JWS).
6. How to exchange the Signed-JWT for a Google OAuth 2.0 Access Token.
7. How to set the expiration time. This program defaults to 3600 seconds (1 Hour).
8. How to call a Google API and set the Authorization Header.
9. How to process the returned Json results and display the name of each instance.

Example program in Python 3.x:

'''
This program lists lists the Google Compute Engine Instances in one zone
'''
# Author: John Hanley
# https://www.jhanley.com

import time
import json
import jwt
import requests
import httplib2

# Project ID for this request.
project = 'development-123456'

# The name of the zone for this request.
zone = 'us-west1-a'

# Service Account Credentials, Json format
json_filename = 'service-account.json'

# Permissions to request for Access Token
scopes = "https://www.googleapis.com/auth/cloud-platform"

# Set how long this token will be valid in seconds
expires_in = 3600   # Expires in 1 hour

def load_json_credentials(filename):
    ''' Load the Google Service Account Credentials from Json file '''

    with open(filename, 'r') as f:
        data = f.read()

    return json.loads(data)

def load_private_key(json_cred):
    ''' Return the private key from the json credentials '''

    return json_cred['private_key']

def create_signed_jwt(pkey, pkey_id, email, scope):
    ''' Create a Signed JWT from a service account Json credentials file
    This Signed JWT will later be exchanged for an Access Token '''

    # Google Endpoint for creating OAuth 2.0 Access Tokens from Signed-JWT
    auth_url = "https://www.googleapis.com/oauth2/v4/token"

    issued = int(time.time())
    expires = issued + expires_in   # expires_in is in seconds

    # Note: this token expires and cannot be refreshed. The token must be recreated

    # JWT Headers
    additional_headers = {
            'kid': pkey_id,
            "alg": "RS256",
            "typ": "JWT"    # Google uses SHA256withRSA
    }

    # JWT Payload
    payload = {
        "iss": email,       # Issuer claim
        "sub": email,       # Issuer claim
        "aud": auth_url,    # Audience claim
        "iat": issued,      # Issued At claim
        "exp": expires,     # Expire time
        "scope": scope      # Permissions
    }

    # Encode the headers and payload and sign creating a Signed JWT (JWS)
    sig = jwt.encode(payload, pkey, algorithm="RS256", headers=additional_headers)

    return sig

def exchangeJwtForAccessToken(signed_jwt):
    '''
    This function takes a Signed JWT and exchanges it for a Google OAuth Access Token
    '''

    auth_url = "https://www.googleapis.com/oauth2/v4/token"

    params = {
        "grant_type": "urn:ietf:params:oauth:grant-type:jwt-bearer",
        "assertion": signed_jwt
    }

    r = requests.post(auth_url, data=params)

    if r.ok:
        return(r.json()['access_token'], '')

    return None, r.text

def gce_list_instances(accessToken):
    '''
    This functions lists the Google Compute Engine Instances in one zone
    '''

    # Endpoint that we will call
    url = "https://www.googleapis.com/compute/v1/projects/" + project + "/zones/" + zone + "/instances"

    # One of the headers is "Authorization: Bearer $TOKEN"
    headers = {
        "Host": "www.googleapis.com",
        "Authorization": "Bearer " + accessToken,
        "Content-Type": "application/json"
    }

    h = httplib2.Http()

    resp, content = h.request(uri=url, method="GET", headers=headers)

    status = int(resp.status)

    if status < 200 or status >= 300:
        print('Error: HTTP Request failed')
        return

    j = json.loads(content.decode('utf-8').replace('\n', ''))

    print('Compute instances in zone', zone)
    print('------------------------------------------------------------')
    for item in j['items']:
        print(item['name'])

if __name__ == '__main__':
    cred = load_json_credentials(json_filename)

    private_key = load_private_key(cred)

    s_jwt = create_signed_jwt(
            private_key,
            cred['private_key_id'],
            cred['client_email'],
            scopes)

    token, err = exchangeJwtForAccessToken(s_jwt)

    if token is None:
        print('Error:', err)
        exit(1)

    gce_list_instances(token)

For more information visit my blog. I write articles like this and publish the source code to help others understand how to write software for the cloud.

www.jhanley.com