Google Cloud Endpoints (GCE) is an API management system providing features to help you create, maintain, and secure your APIs. GCE uses OpenAPI to define your API’s endpoints, input and output, errors, and security description. For more information on the OpenAPI spec, see the OpenAPI Specification repository on GitHub. This tutorial will cover how to secure Google Cloud Endpoints with Auth0.

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Prerequisites

Before you begin you’ll need a deployed GCE API. If you haven’t already created an API, complete the Cloud Endpoints Quickstart located in Google documentation. The quickstart will walk you through creating a simple GCE API with a single endpoint, /airportName, that returns the name of an airport from its three-letter IATA code. 
curl --request GET \
  --url 'https://%7ByourGceProject%7D.appspot.com/airportName?iataCode=SFO'

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Define the API in Auth0

Go to Auth0 Dashboard > Applications > APIs, and create a new API.
Dashboard - Create APIs - Integrations -Google Endpoints
Make note of the API identifier (http://google_api in the screenshot above) to use in the following step.

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Update the API Configuration

Next, we’ll update the OpenAPI configuration file for the GCE API. For the sample API created during the quickstart this file is openapi.yaml.

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Add Security Definitions

Open the configuration file and add a new securityDefinitions section. In this section, add a new definition (auth0_jwt) with the following fields:
FieldDescription
authorizationUrlThe authorization URL, should be set to “https:///authorize”
flowThe flow used by the OAuth2 security scheme. Valid values are “implicit”, “password”, “application” or “accessCode”.
typeThe type of the security scheme. Valid values are “basic”, “apiKey” or “oauth2”
x-google-issuerThe issuer of a credential, should be set to “https:///“
x-google-jwks_uriThe URI of the public key set to validate the signature. Set this to "https://\{yourDomain}/.well-known/jwks.json"
x-google-audiencesThe API’s identifier, make sure this value matches what you defined on the Auth0 dashboard for the API.
securityDefinitions:
  auth0_jwt:
    authorizationUrl: "https://{yourDomain}/authorize"
    flow: "implicit"
    type: "oauth2"
    x-google-issuer: "https://{yourDomain}/"
    x-google-jwks_uri: "https://{yourDomain}/.well-known/jwks.json"
    x-google-audiences: "{yourApiIdentifier}"

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Update the Endpoint

Now, update the endpoint by adding a security field with the securityDefinition we created in the previous step. 
paths:
  "/airportName":
    get:
      description: "Get the airport name for a given IATA code."
      operationId: "airportName"
      parameters:
        -
          name: iataCode
          in: query
          required: true
          type: string
      responses:
        200:
          description: "Success."
          schema:
            type: string
        400:
          description: "The IATA code is invalid or missing."
      security:
       - auth0_jwt: []
In the above example, the security field tells the GCE proxy that our /airportName path expects to be secured with the auth0-jwt definition. After updating the OpenAPI configuration, it should look something like this: 
---
swagger: "2.0"
info:
  title: "Airport Codes"
  description: "Get the name of an airport from its three-letter IATA code."
  version: "1.0.0"
host: "{yourGceProject}.appspot.com"
schemes:
  - "https"
paths:
  "/airportName":
    get:
      description: "Get the airport name for a given IATA code."
      operationId: "airportName"
      parameters:
        -
          name: iataCode
          in: query
          required: true
          type: string
      responses:
        200:
          description: "Success."
          schema:
            type: string
        400:
          description: "The IATA code is invalid or missing."
      security:
       - auth0_jwt: []
securityDefinitions:
  auth0_jwt:
    authorizationUrl: "https://{yourDomain}/authorize"
    flow: "implicit"
    type: "oauth2"
    x-google-issuer: "https://{yourDomain}/"
    x-google-jwks_uri: "https://{yourDomain}/.well-known/jwks.json"
    x-google-audiences: "{yourApiIdentifier}"

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Redeploy the API

Next, redeploy your GCE API to apply the configuration changes. If you followed along with the Cloud Endpoints Quickstart you can redeploy by entering the following in Google’s Cloud Shell: 
cd endpoints-quickstart/scripts
./deploy_api.sh

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Test the API

Once you’ve redeployed, call the API again with no security. 
curl --request GET \
  --url 'https://%7ByourGceProject%7D.appspot.com/airportName?iataCode=SFO'
You’ll get the following response: 
{
 "code": 16,
 "message": "JWT validation failed: Missing or invalid credentials",
 "details": [
  {
   "@type": "type.googleapis.com/google.rpc.DebugInfo",
   "stackEntries": [],
   "detail": "auth"
  }
 ]
}
Which is exactly what we want! Now go to the Test page of your Google Endpoints API definition on the Auth0 Dashboard, and copy the under the Response: Perform a GET request to your API with an Authorization Header of Bearer {ACCESS_TOKEN} to obtain authorized access: 
curl --request GET \
  --url 'https://%7ByourGceProject%7D.appspot.com/airportName?iataCode=SFO' \
  --header 'authorization: Bearer {accessToken}'
And that’s it!