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Approov Token Binding Quickstart

This quickstart is for developers familiar with Python who are looking for a quick intro into how they can add Approov into an existing project. Therefore this will guide you through the necessary steps for adding Approov with token binding to an existing Python FastAPI server.

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Why?

To lock down your API server to your mobile app. Please read the brief summary in the Approov Overview at the root of this repo or visit our website for more details.

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How it works?

For more background, see the Approov Overview at the root of this repository.

The main functionality for the Approov token binding check is in the file src/approov-protected-server/token-binding-check/hello-server-protected.py. Take a look at the verifyApproovToken() and verifyApproovTokenBinding() functions to see the simple code for the checks.

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Requirements

To complete this quickstart you will need both the Python, FastAPI, and the Approov CLI tool installed.

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Approov Setup

To use Approov with the Python FastAPI server we need a small amount of configuration. First, Approov needs to know the API domain that will be protected. Second, the Python FastAPI server needs the Approov Base64 encoded secret that will be used to verify the tokens generated by the Approov cloud service.

Configure API Domain

Approov needs to know the domain name of the API for which it will issue tokens.

Add it with:

approov api -add your.api.domain.com

NOTE: By default a symmetric key (HS256) is used to sign the Approov token on a valid attestation of the mobile app for each API domain it's added with the Approov CLI, so that all APIs will share the same secret and the backend needs to take care to keep this secret secure.

A more secure alternative is to use asymmetric keys (RS256 or others) that allows for a different keyset to be used on each API domain and for the Approov token to be verified with a public key that can only verify, but not sign, Approov tokens.

To implement the asymmetric key you need to change from using the symmetric HS256 algorithm to an asymmetric algorithm, for example RS256, that requires you to first add a new key, and then specify it when adding each API domain. Please visit Managing Key Sets on the Approov documentation for more details.

Adding the API domain also configures the dynamic certificate pinning setup, out of the box.

NOTE: By default the pin is extracted from the public key of the leaf certificate served by the domain, as visible to the box issuing the Approov CLI command and the Approov servers.

Approov Secret

Approov tokens are signed with a symmetric secret. To verify tokens, we need to grab the secret using the Approov secret command and plug it into the Python FastAPI server environment to check the signatures of the Approov Tokens that it processes.

First, enable your Approov admin role with:

eval `approov role admin`

For the Windows powershell:

set APPROOV_ROLE=admin:___YOUR_APPROOV_ACCOUNT_NAME_HERE___

Next, retrieve the Approov secret with:

approov secret -get base64

Set the Approov Secret

Open the .env file and add the Approov secret to the var:

APPROOV_BASE64_SECRET=approov_base64_secret_here

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Approov Token Check

To check the Approov token we will use the jpadilla/pyjwt/ package, but you are free to use another one of your preference.

First, add to your requirements.txt file the JWT dependency:

PyJWT==1.7.1 # update the version to the latest one

Next, you need to install the dependency:

pip3 install -r requirements.txt

Now, add this code to your project, just before your first API endpoint:

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

# @link https://github.com/jpadilla/pyjwt/
import jwt
import base64
import hashlib

# @link https://github.com/theskumar/python-dotenv
from dotenv import load_dotenv, find_dotenv
load_dotenv(find_dotenv(), override=True)
from os import getenv

# Token secret value obtained with the Approov CLI tool:
#  - approov secret -get
approov_base64_secret = getenv('APPROOV_BASE64_SECRET')

if approov_base64_secret == None:
    raise ValueError("Missing the value for environment variable: APPROOV_BASE64_SECRET")

APPROOV_SECRET = base64.b64decode(approov_base64_secret)

app = FastAPI()

################################################################################
# ONLY ADD YOUR MIDDLEWARE BEFORE THIS LINE.
# - FastAPI seems to execute middleware in the reverse we declare it in the
#   code.
# - Approov middleware SHOULD be the first to be executed in the request life
#   cycle.
################################################################################

# @link https://approov.io/docs/latest/approov-usage-documentation/#token-binding
# @IMPORTANT FastAPI seems to execute middleware in the reverse order they
#            appear in the code, therefore this one must come right before the
#            verifyApproovToken() middleware.
@app.middleware("http")
async def verifyApproovTokenBinding(request: Request, call_next):
    if not 'pay' in request.state.approov_token_claims:
        # You may want to add some logging here.
        return JSONResponse({}, status_code = 401)

    # We use the Authorization token, but feel free to use another header in
    # the request. Beqar in mind that it needs to be the same header used in the
    # mobile app to qbind the request with the Approov token.
    token_binding_header = request.headers.get("Authorization")

    if not token_binding_header:
        # You may want to add some logging here.
        return JSONResponse({}, status_code = 401)

    # We need to hash and base64 encode the token binding header, because that's
    # how it was included in the Approov token on the mobile app.
    token_binding_header_hash = hashlib.sha256(token_binding_header.encode('utf-8')).digest()
    token_binding_header_encoded = base64.b64encode(token_binding_header_hash).decode('utf-8')

    if request.state.approov_token_claims['pay'] == token_binding_header_encoded:
        return await call_next(request)

    return JSONResponse({}, status_code = 401)

# @link https://approov.io/docs/latest/approov-usage-documentation/#backend-integration
# @IMPORTANT FastAPI seems to execute middleware in the reverse order they
#            appear in the code, therefore this one must come as the LAST of the
#            middleware's.
@app.middleware("http")
async def verifyApproovToken(request: Request, call_next):
    approov_token = request.headers.get("Approov-Token")

    # If we didn't find a token, then reject the request.
    if approov_token == "":
        # You may want to add some logging here.
        # return None
        return JSONResponse({}, status_code = 401)

    try:
        # Decode the Approov token explicitly with the HS256 algorithm to avoid
        # the algorithm None attack.
        request.state.approov_token_claims = jwt.decode(approov_token, APPROOV_SECRET, algorithms=['HS256'])
        return await call_next(request)
    except jwt.ExpiredSignatureError as e:
        # You may want to add some logging here.
        return JSONResponse({}, status_code = 401)
    except jwt.InvalidTokenError as e:
        # You may want to add some logging here.
        return JSONResponse({}, status_code = 401)

# @app.get("/")
# async def root():
#     return {"message": "Hello World"}

NOTE: When the Approov token validation fails we return a 401 with an empty body, because we don't want to give clues to an attacker about the reason the request failed, and you can go even further by returning a 400.

Using the middleware approach will ensure that all endpoints in your API will be protected by Approov.

A full working example for a simple Hello World server can be found at src/approov-protected-server/token-binding-check.

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Test your Approov Integration

The following examples below use cURL, but you can also use the Postman Collection to make the API requests. Just remember that you need to adjust the urls and tokens defined in the collection to match your deployment. Alternatively, the above README also contains instructions for using the preset dummy secret to test your Approov integration.

With Valid Approov Tokens

Generate a valid token example from the Approov Cloud service:

approov token -setDataHashInToken 'Bearer authorizationtoken' -genExample your.api.domain.com

Then make the request with the generated token:

curl -i --request GET 'https://your.api.domain.com/v1/shapes' \
  --header 'Authorization: Bearer authorizationtoken' \
  --header 'Approov-Token: APPROOV_TOKEN_EXAMPLE_HERE'

The request should be accepted. For example:

HTTP/1.1 200 OK

...

{"message": "Hello, World!"}

With Invalid Approov Tokens

No Authorization Token

Let's just remove the Authorization header from the request:

curl -i --request GET 'https://your.api.domain.com/v1/shapes' \
  --header 'Approov-Token: APPROOV_TOKEN_EXAMPLE_HERE'

The above request should fail with an Unauthorized error. For example:

HTTP/1.1 401 Unauthorized

...

{}
Same Approov Token with a Different Authorization Token

Make the request with the same generated token, but with another random authorization token:

curl -i --request GET 'https://your.api.domain.com/v1/shapes' \
  --header 'Authorization: Bearer anotherauthorizationtoken' \
  --header 'Approov-Token: APPROOV_TOKEN_EXAMPLE_HERE'

The above request should also fail with an Unauthorized error. For example:

HTTP/1.1 401 Unauthorized

...

{}

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Issues

If you find any issue while following our instructions then just report it here, with the steps to reproduce it, and we will sort it out and/or guide you to the correct path.

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Useful Links

If you wish to explore the Approov solution in more depth, then why not try one of the following links as a jumping off point:

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