Azure B2C Access Token NULL Spring OAuth 2.0 - spring

Question

I am trying to get a custom web application to work with Azure B2C OAuth and the Spring OAuth2.0 framework.
The authentication leg comes back fine and I receive a JWT token. When the request for a token occurs afterwards I get the following error:
java.lang.IllegalStateException: Access token provider returned a null access token, which is illegal according to the contract.
at org.springframework.security.oauth2.client.OAuth2RestTemplate.acquireAccessToken(OAuth2RestTemplate.java:223) ~[spring-security-oauth2-2.0.8.RELEASE.jar:na]
at org.springframework.security.oauth2.client.OAuth2RestTemplate.getAccessToken(OAuth2RestTemplate.java:173) ~[spring-security-oauth2-2.0.8.RELEASE.jar:na]
...
From some debugging of the spring code I can see the token is expected to be called access_token as seen in the OAuth2AccessToken class. From looking at the B2C tutorials their token is called token_id. Furthermore the applications.yml config I have for my spring application has a field called tokenName. Surely this should be used to pick up the token name field instead of the hardcoded static variable as above.
Am I missing something and is there a solution to my problem. Can I override the token name field used by the spring OAuth framework?

Answer 1

I'm going to go ahead and post this as the answer.
I started with the Spring tutorial, and made some modifications to it. For a working example, see the public github repo: https://github.com/Pytry/azure-b2c-oauth2.git
To properly parse the token received you will need:
A custom implementation of an AccessToken that will parse the JWT, pulling and setting variables as required by spring security. I extended DefaultOAuth2AccessToken and added this parsing to a private method called by the constructor.
If you are going to verify the RSA signature using the public keys, you will need a custom JWT object so you can access the header information. I chose to extend springs JWT, and add some parsing on creation to access the header. It may also be useful to have some custom Pojos for parsing the returned meta data and rsakey information into.
An extension of the JwtAccessTokenConverter, with an overridden "decode" method. Azure does not give a "user_name" nor a "client_id" in the returned id_token, so you need to add those. I also included some logic in the super class that I found suefull (such as converting strings to int/long when appropriate).
A custom UserDetailsManger to override the default in memory one. This can either retrioeve user information from the GraphAPI, or it can load it from your user repository. I actually did not create either of these, and instead used the default in memory service, but injected it into the token converter; then whenever a user was properly authenticated, I would add them to the managers store, or update them if they already existed.
There are a few things I have not done yet.
RSA verification is not being done. Any help on this is appreciated.