Oauth2 Proxy is used to provide the redirection / cookie check in order to enforce JWT presence even via web browser

General concept:
https://medium.com/@senthilrch/api-authentication-using-istio-ingress-gateway-oauth2-proxy-and-keycloak-part-2-of-2-dbb3fb9cd0d0
https://discuss.istio.io/t/how-to-implement-istio-authorization-using-oauth2-and-keycloak/13707

Add Oauth2-Proxy client to Keycloak-Realm:

https://oauth2-proxy.github.io/oauth2-proxy/docs/configuration/oauth_provider#keycloak-oidc-auth-provider

https://discuss.istio.io/t/how-to-implement-istio-authorization-based-on-keycloak-user-role/13716

https://doc.primekey.com/ejbca/ejbca-operations/ejbca-operations-guide/ca-operations-guide/oauth-provider-management/configuring-audience-claims

General Architecture

Authentication Message Flow

from: https://medium.com/@senthilrch/api-authentication-using-istio-ingress-gateway-oauth2-proxy-and-keycloak-a980c996c259

1. User enters the hostname of the server in the browser. The request is sent to the Istio Ingress Gateway.
2. Ingress Gateway forwards the request to OAuth2-Proxy for authentication.
3. The OAuth2-Proxy determines the request is not authenticated yet. It creates state in its Redis Key value store.
4. OAuth2-Proxy redirects the browser to /auth endpoint of Keycloak.
5. The browser sends a request to the /auth endpoint which is sent to the Ingress Gateway.
6. Ingress Gateway forwards the request to Keycloak
7. Keycloak serves the login screen containing fields for Username and Password.
8. User enters Username and Password and submits the form. The browser sends a request to the /authenticate endpoint of Keycloak. The request is sent to the Ingress Gateway.
9. Ingress Gateway forwards the request to Keycloak, which verifies the credentials of the user and generates an Authorization code.
10. Keycloak then redirects the browser to the /callback endpoint of OAuth2-Proxy with Authorization code as query parameter in the URL.
11. The browser sends a request to the /callback endpoint which is sent to the Ingress Gateway.
12. Ingress Gateway forwards the request to OAuth2-Proxy
13. OAuth2-Proxy invokes the /token endpoint of Keycloak using the Authorization code.
14. Keycloak validates the Authorization code and responds back with access token, id token and refresh token
15. OAuth2-Proxy persists the tokens in its Redis Key value store.
16. OAuth2-Proxy redirects the browser to the original url requested by the user i.e. hostname. It instructs the browser to store a cookie with a specific key configured in OAuth2-Proxy
17. The browser sends a request to hostname, this time the request will contain the cookie header containing cookie that was sent by OAuth2-Proxy. The request is sent to the Ingress Gateway.
18. Ingress Gateway forwards the request to OAuth2-Proxy to verify if the request is authenticated properly
19. OAuth2-Proxy responds back with a HTTP 200 OK response that contains the access token in “X-Auth-Request-Access-Token” header
20. Ingress Gateway perform API Access control checks. It then forwards the request to upstream service with “X-Auth-Request-Access-Token” header in it.
21. The upstream service serves the request.