CODEWITHSUNDEEP
pythonazure-active-directoryjwtdecodepublic-key-encryption
Esben Eickhardt
Esben Eickhardt

Reputation: 3842

How to verify JWT produced by Azure Ad?

Problem

When I receive a JWK from Azure AD in Python, I would like to validate and decode it. I, however, keep getting the error "Signature verification failed".

My Setup

I have the following setup:

  1. Azure Setup
    In Azure I have created an app registration with the setting "Personal Microsoft accounts only".
  2. Python Setup
    In Python I use the MSAL package for receiving tokens. And I use a public key from Azure to verify the token.

Code

Using the credentials from the Azure Portal I set up a client for getting tokens.

import msal
ad_auth_client = msal.ConfidentialClientApplication(
    client_id = client_id,
    client_credential = client_secret,
    authority = "https://login.microsoftonline.com/consumers"
)
my_token = ad_auth_client.acquire_token_for_client(scopes=['https://graph.microsoft.com/.default'])

If I throw the token into a site like https://jwt.io/ everything looks good. Next I need public keys from Azure for verifying the token.

import requests
response = requests.get("https://login.microsoftonline.com/common/discovery/keys")
keys = response.json()['keys']

To match up the public key to the token, I use the 'kid' in the token header. I also get which algorithm was used for encryption.

import jwt
token_headers = jwt.get_unverified_header(my_token['access_token'])
token_alg = token_headers['alg']
token_kid = token_headers['kid']
public_key = None
for key in keys:
    if key['kid'] == token_kid:
        public_key = key

Now I have the correct public key from Azure to verify my token, but the problem is that it is a JWT key. Before I can use it for decoding, I need to convert it to a RSA PEM key.

from cryptography.hazmat.primitives import serialization
rsa_pem_key = jwt.algorithms.RSAAlgorithm.from_jwk(json.dumps(public_key))
rsa_pem_key_bytes = rsa_pem_key.public_bytes(
    encoding=serialization.Encoding.PEM, 
    format=serialization.PublicFormat.SubjectPublicKeyInfo
)

This is what the Azure Public Key looks like:

b'-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAyr3v1uETrFfT17zvOiy0\n1w8nO+1t67cmiZLZxq2ISDdte9dw+IxCR7lPV2wezczIRgcWmYgFnsk2j6m10H4t\nKzcqZM0JJ/NigY29pFimxlL7/qXMB1PorFJdlAKvp5SgjSTwLrXjkr1AqWwbpzG2\nyZUNN3GE8GvmTeo4yweQbNCd+yO/Zpozx0J34wHBEMuaw+ZfCUk7mdKKsg+EcE4Z\nv0Xgl9wP2MpKPx0V8gLazxe6UQ9ShzNuruSOncpLYJN/oQ4aKf5ptOp1rsfDY2IK\n9frtmRTKOdQ+MEmSdjGL/88IQcvCs7jqVz53XKoXRlXB8tMIGOcg+ICer6yxe2it\nIQIDAQAB\n-----END PUBLIC KEY-----\n'

The last thing I need to do is to verify the token using the public key.

decoded_token = jwt.decode(
    my_token['access_token'], 
    key=rsa_pem_key_bytes,
    verify=True,
    algorithms=[token_alg],
    audience=[client_id],
    issuer="https://login.microsoftonline.com/consumers"
)

The result I get is:

jwt.exceptions.InvalidSignatureError: Signature verification failed

What I also tried

I also tried to follow this popular guide: How to verify JWT id_token produced by MS Azure AD? Placing the x5c into the certificate pre- and postfixes just generated errors of invalid formatting.

What is next?

Can you guys see any obvious errors? My main guess is that there is something wrong with the audience or the issuer, but I cannot pin down what it is, and Microsoft's documentation is horrible as always. Also, there is a secret key in the app registration in Azure, but it does not seem to work either.

Update

So it turns out that my verification code was correct, but that I was trying to verify the wrong token. After creating slight modifications I now receive an id_token, which can be decoded and verified.

Upvotes: 11

Views: 11292

Answers (4)

MaFF
MaFF

Reputation: 10076

There are at least 2 options to decode Microsoft Azure AD ID tokens:

Option 1: Using jwt

The code provided by OP gives me the exception InvalidIssuerError. Even replacing the issuer argument by https://login.microsoftonline.com/{your-tenant-id} did not work for me. However omitting this argument all together allowed me to decode the ID token. Here is the full code:

# Get the public keys from Microsoft
import requests
response = requests.get("https://login.microsoftonline.com/common/discovery/keys")
keys = response.json()['keys']

# Format keys as PEM
from cryptography.hazmat.primitives import serialization
rsa_pem_key = jwt.algorithms.RSAAlgorithm.from_jwk(json.dumps(public_key))
rsa_pem_key_bytes = rsa_pem_key.public_bytes(
    encoding=serialization.Encoding.PEM, 
    format=serialization.PublicFormat.SubjectPublicKeyInfo
)

# Get algorithm from token header
alg = jwt.get_unverified_header(your-id-token)['alg']

# Decode token
jwt.decode(
    your-id-token,
    key=rsa_pem_key_bytes,
    algorithms=[alg],
    verify=True,
    audience=[your-client-id],
    options={"verify_signature": True}
)

Option 2: use msal's decode_id_token

Microsoft's package msal provides a function to decode the id token. The code simply becomes:

from msal.oauth2cli.oidc import decode_id_token 
decode_id_token(id_token=your-token-id, client_id=your-client-id)

Upvotes: 2

ws_geek
ws_geek

Reputation: 1

The issue looks to be related to that the JWT you generated was for MS Graph API, which the results JWT would contain a "nonce" in JWT Header, and is not meant to be validated.

The tweak is to generate JWT with scope != https://graph.microsoft.com/.default, but instead in form = "scope": [ "api://client-id/.default" ].

After which you can verify JWT header no longer has "nonce" in JWT Header, and normal validation like code above would work.

Upvotes: 0

Ishan Juneja
Ishan Juneja

Reputation: 49

Try changing your

scopes=['https://graph.microsoft.com/.default']

To

scopes:['[YOUR_CLIENT_ID]/.default']

Upvotes: 1

juunas
juunas

Reputation: 58723

You acquired the token for the scope https://graph.microsoft.com/.default, which means the access token is for MS Graph API. You should not be validating that, it is the job of the API it is meant for. Graph API is also special and uses a different method of signing than other APIs.

Upvotes: 2

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