APPROOV INTEGRATION IN A PYTHON FLASK API

Monday 04 February 2019 By Paulo Renato

Topics: Mobile App Authentication, Integration

Screenshot from 2019-02-01 11-58-11

This walk-though will show us how simple it is to integrate Approov in a current API server using Python and the Flask framework.

We will see the requirements, dependencies and a step by step walk-through over the code necessary to implement Approov in a Python Flask API.

Before we tackle the integration of Approov we need first to know how Approov validation is processed in the server and how to setup the environment to follow this walk-through.

Note that this article assumes a basic understanding of the Approov mechanics. If you need an overview of that, please read first the Approov in Detail page.

 

APPROOV TOKEN VALIDATION PROCESS

We will understand at a high level how the Approov token itself is validated and also how to validate the optional custom payload claim.

The Approov Token

API calls protected by Approov will typically include a header holding an Approov JWT token. This token must be checked to ensure it has not expired and that it is properly signed with the secret shared between the back-end and the Approov cloud service.

We will use a Python package to help us in the validation of the Approov JWT token. Just to be sure that we are on the same page, a JWT token has 3 parts which are separated by dots and represented as a string in the format of header.payload.signature. Read more about JWT tokens here.

The Custom Payload Claim

A custom payload claim in an Approov token is a base64 encoded sha256 hash of some unique identifier we may want to tie with the Approov token to enhance the security of that request, such as an OAUTH2 token, but you are free to use whatsoever you may want.

Dummy example for the JWT token middle part, the payload:
{
    "exp": 123456789, # required - the timestamp for when the token expires.
    "pay":"f3U2fniBJVE04Tdecj0d6orV9qT9t52TjfHxdUqDBgY=" # optional - a sha256 hash of the claim, encoded with base64.
}

The custom payload claim in an Approov token is the one in the pay key:

"pay":"f3U2fniBJVE04Tdecj0d6orV9qT9t52TjfHxdUqDBgY="

ALERT:

Please bear in mind that the custom payload claim is not meant to pass application data to the API server, but you can pass a base64 encoded sha256 hash of the data you want to send from the mobile app into the API server and have it hash the data that was sent as usual on the request and compare it with the custom payload claim base64 sha256 hash to confirm that the integrity of the data was not compromised.

SYSTEM CLOCK

In order to correctly check for the expiration times of the Approov tokens it is very important that the Python Flask server is automatically synchronizing the system clock over the network with an authoritative time source. In Linux this is usually done with a NTP server.

REQUIREMENTS

The example code in this walk-through is based on the Approov Shapes Demo server that uses Python 3 and Flask. The full source code can be found in this Github repository.

Docker is required only if you want to use the docker environment provided by the stack bash script, which is a wrapper around docker commands.

Postman is the tool we recommend to be used when simulating the queries against the API, but feel free to use any other tool of your choice.

THE DOCKER STACK

We recommend the use of the included Docker stack to play with this Approov integration.

For details in how to use it you need to follow the setup instructions in the Approov Shapes Demo Server walk-through, but feel free to use your local environment to play with this Approov integration.

THE POSTMAN COLLECTION

Import this Postman collection which contains all the API endpoints for the Approov Shapes Demo Server and we strongly recommend you follow this demo walk-through after finishing the Approov integration that we are about to start.

The Approov tokens used in the headers of this Postman collection were generated by this helper script and they cover all necessary scenarios, but feel free to use the script to generate some more valid and invalid tokens, with different expiry times and custom payload claims. Some examples of using it can be found here.

INSTALL DEPENDENCIES

If not already using the packages pyjwt and python-dotenv in your Python Flask API project, please add them:

pip3 install pyjwt python-dotenv

ORIGINAL SERVER

Let’s use the original-server.py as an example for a current server where we want to add Approov to protect some or all the endpoints and we will add to it only the necessary code to integrate Approov. The end result can be seen in the approov-protected-server.py.

 Learn More about Mobile API Security! 

HOW TO INTEGRATE

We will learn how to go from the original-server.py to the approov-protected-server.py and how to configure the server.

In order to be able to check the Approov token the PyJWT library needs to know the secret used by the Approov cloud service to sign it. A secure way to do this is by passing it as an environment variable, as it can be seen here.

Next we need to define two core methods to be used during the Approov token check process. The method _decodeApproovToken() is to decode and simultaneously check the token with the library PyJWT and _isValidPayloadInApproovToken() is to check the custom payload claim in the Approov Token. We also define some other methods to help with the Approov integration and these are probably the ones you may want to customize for your use case.

The custom claim is optional in the Approov Token payload, but needs to be a base64 encoded string from a hash of some value you want to tie up with the Approov token. A good example is to tie up the user authentication token with the Approov token, but your needs and requirements may be different.

Let’s breakdown the implementation of the approov-protected-server.py to make it easier to adapt to your current project.

Import Dependencies

We require the dependencies we installed before, plus some more system dependencies:

Setup Environment

If you don’t have already an .env file, then you need to create one in the root of your project by using this .env.example as your starting point.

The .env file must contain these four variables:

APPROOV_BASE64_SECRET=YXBwcm9vdi1iYXNlNjQtZW5jb2RlZC1zZWNyZXQ=
APPROOV_LOGGING_ENABLED=true
APPROOV_ABORT_REQUEST_ON_INVALID_TOKEN=true
APPROOV_ABORT_REQUEST_ON_INVALID_CUSTOM_PAYLOAD_CLAIM=true

Now we can read them from our code, as is done here:

Methods

Let’s start by adding this method to enable logging for Approov specific occurrences;

Next we need to add this method to decode the Approov token;

Now we need to add this method to get the Approov token and validate it at each endpoint we want to protect;

We also need to add this method to handle requests with an invalid Approov token;

Then we need this method to check the custom payload claim in the Approov token;

Finally we need this method to handle the validation of the custom payload claim in the Approov token.

Endpoints

To protect specific endpoints in a current server we only need to add the Approov token check for each endpoint we want to protect, as we have done in the shapes endpoint;

or if using the custom payload claim in the Approov token, as we have done in the forms endpoint.

Approov in Action

Let's see how to query the Python Flask API from Postman with the collection we told you to install in the requirements section.

NOTE:

For your convenience the Postman collection includes a token that only expires in the very distant future for this call "Approov Token with valid signature and expire time". For the call "Expired Approov Token with valid signature" an expired token is also included.

 

Postman view with an Approov token correctly signed and not expired:

postman-shapes-valid-token

 

Postman view with token correctly signed but this time it is expired:

postman-shapes-expired-token

 

Shell view with the logs for the above requests:

shell-shapes-valid-and-expired-token

 

Request Overview:

We used an helper script to generate an Approov Token that was valid for 1 minute.

In Postman we performed 2 requests with the same token and the first one was successful, but the second request, performed 2 minutes later, failed with a 400 response because the token has already expired as we can see by the log messages in the shell view.

Play Time for the Approov Shapes Demo Server

If you have not done it already, now is the time to follow the Approov Shapes Demo Server walk-through to get a feel for how all this works.

The Approov Shapes Demo Server contains endpoints with and without the Approov protection. The protected endpoints differ in the sense that one uses the optional custom payload claim in the Approov token.

We will demonstrate how to call each API endpoint with screen-shots from Postman and from the shell. Postman is used here as an easy way to demonstrate how you can play with the Approov integration in the API server, but to see a real demo of how Approov would work in production you need to request a demo here.

The Code Difference

If we compare the original-server.py with the approov-protected-server.py we will see this file difference:

As we can see the Approov integration in a current server is simple, easy and is done with just a few lines of code.

PRODUCTION

In order to protect the communication between your mobile app and the API server is important to only communicate over a secure communication channel, also known as https.

Please bear in mind that https on its own is not enough, certificate pinning must be also used to pin the connection between the mobile app and the API server in order to prevent Man in the Middle Attacks.

We do not use https and certificate pinning in this Approov integration example because we want to be able to run the Approov Shapes Demo Server in localhost.

However in production it should be mandatory to implement at least static pinning or dynamic pinning.

Test Drive Approov!