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Use oauth2 service account to authenticate to google API in python

I followed the directions from https://developers.google.com/accounts/docs/OAuth2ServiceAccount to use a service account to authenticate with the Google Cloud Storage API. I tried to send JWT to google authentication server in python but got error:

urllib2.HTTPError: HTTP Error 400: Bad Request

It looks like there is something wrong with the way I do, sign or submit the JWT? The error was not specific, so it could be part of the process. Does anyone have any idea?

import Crypto.PublicKey.RSA as RSA
import Crypto.Hash.SHA as SHA
import Crypto.Signature.PKCS1_v1_5 as PKCS1_v1_5
import base64
import json
import time
import urllib2
import urllib

# Settings
json_key_file = 'GooglePM-9f75ad112f87-service.json'

# Load the private key associated with the Google service account
with open(json_key_file) as json_file:
    json_data = json.load(json_file)
    key = RSA.importKey(json_data['private_key'])

# Create an PKCS1_v1_5 object
signer = PKCS1_v1_5.new(key)

# Encode the JWT header
header_b64 = base64.urlsafe_b64encode(json.dumps({'alg':'RS256','typ':'JWT'}))

# JWT claims
jwt = {
    'iss': json_data['client_email'],
    'scope': 'https://www.googleapis.com/auth/devstorage.read_write',
    'aud': 'https://accounts.google.com/o/oauth2/token',
    'exp': int(time.time())+3600,
    'iat': int(time.time())
    }
jwt_json = json.dumps(jwt)

# Encode the JWT claims
jwt_json_b64 = base64.urlsafe_b64encode(jwt_json)

# Sign the JWT header and claims
msg_hash = SHA.new(header_b64 + "." + jwt_json_b64)
signature_b64 = base64.urlsafe_b64encode(signer.sign(msg_hash))

# Make the complete message
jwt_complete = header_b64 + "." + jwt_json_b64 + "." + signature_b64

data = {'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer', 
    'assertion': jwt_complete}

f = urllib2.urlopen("https://accounts.google.com/o/oauth2/token", urllib.urlencode(data))

print f.read()

If I try to use curl to send to the server, I get an invalid grants error:

(venv)$ curl -d 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=eyJhbGciOiAiUlMyNTYiLCAidHlwIjogIkpXVCJ9.eyJpc3MiOiAiMTM1MDY3NjIyMTk4LWVhbWUwZnFqdTNvamRoZ29zdDg2dnBpdTBsYW91NnZlQGRldmVsb3Blci5nc2VydmljZWFjY291bnQuY29tIiwgInNjb3BlIjogImh0dHBzOi8vd3d3Lmdvb2dsZWFwaXMuY29tL2F1dGgvZGV2c3RvcmFnZS5yZWFkX3dyaXRlIiwgImF1ZCI6ICJodHRwczovL2FjY291bnRzLmdvb2dsZS5jb20vby9vYXV0aDIvdG9rZW4iLCAiZXhwIjogMTQwODY1MTU2OCwgImlhdCI6IDE0MDg2NDg1NTh9.HWC7h3QiOy7QsSuta4leq_Gjwmy9IdF-MUwflPhiohzAJ-Amykd56Ye4Y_Saf_sAc5STzOCmrSPzOTYvGXr6X_T_AmSTxXK2AJ2SpAiEUs2_Wp5h18xTUY3Y_hkKvSZLh5bRzeJ_0xRcmRIPE6tua0FHFwUDdnCIGdh4DGg6i4E%3D' https://accounts.google.com/o/oauth2/token
{
  "error" : "invalid_grant"
}
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4 answers


Ok, here's the best way to do it! Google already has a python client API that handles some complexities. The following code works after installing the google python API: https://developers.google.com/api-client-library/python/guide/aaa_oauth



from oauth2client.client import SignedJwtAssertionCredentials
import json
import urllib
import urllib2

# Settings
json_key_file = 'GooglePM-9f75ad112f87-service.json'

# Load the private key associated with the Google service account
with open(json_key_file) as json_file:
    json_data = json.load(json_file)

# Get and sign JWT
credential = SignedJwtAssertionCredentials(json_data['client_email'], json_data['private_key'], 'https://www.googleapis.com/auth/devstorage.read_write')
jwt_complete = credential._generate_assertion()

# Get token from server
data = {'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer', 
    'assertion': jwt_complete}
f = urllib2.urlopen("https://accounts.google.com/o/oauth2/token", urllib.urlencode(data))

print f.read()
+8


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maybe a little simpler:



import oauth2client.service_account

jsonfile = 'GooglePM-9f7sdf342f87-service.json'
# use static method .from_json_keyfile_name(filename)
credentials = oauth2client.service_account.ServiceAccountCredentials.from_json_keyfile_name(jsonfile)
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Your first algorithm is completely fine and works, but you need SHA256, not SHA. Thank you for the code.

+1


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The source code also works using Python 3.6. You should use bytes instead of str only in base64 case:

import requests
import json as js
import Crypto.PublicKey.RSA as RSA
import Crypto.Hash.SHA256 as SHA
import Crypto.Signature.PKCS1_v1_5 as PKCS1_v1_5
import base64
import time

# Settings
json_key_file = 'google-api.json'

# Load the private key associated with the Google service account
with open(json_key_file) as json_file:
    json_data = js.load(json_file)
    key = RSA.importKey(json_data['private_key'])

# Create an PKCS1_v1_5 object
signer = PKCS1_v1_5.new(key)

header = js.dumps({'alg':'RS256','typ':'JWT'})

# Encode the JWT header
header_b64 = base64.urlsafe_b64encode(header.encode("UTF-8"))

# JWT claims
jwt = {
    'iss': json_data['client_email'],
    'scope': 'https://www.googleapis.com/auth/analytics.readonly',
    'aud': 'https://accounts.google.com/o/oauth2/token',
    'exp': int(time.time())+3600,
    'iat': int(time.time())
    }
jwt_json = js.dumps(jwt)

# Encode the JWT claims
jwt_json_b64 = base64.urlsafe_b64encode(jwt_json.encode("UTF-8"))

# Sign the JWT header and claims
msg_hash = SHA.new((header_b64.decode("UTF-8") + "." + jwt_json_b64.decode("UTF-8")).encode("UTF-8"))
signature_b64 = base64.urlsafe_b64encode(signer.sign(msg_hash))

# Make the complete message
jwt_complete = (header_b64.decode("UTF-8") + "." + jwt_json_b64.decode("UTF-8") + "." + signature_b64.decode("UTF-8")).encode("UTF-8")

data = {'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer', 
'assertion': jwt_complete}

requests.post(url="https://accounts.google.com/o/oauth2/token",data=data).text
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