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The AuthPortal Flow is the mechanism with which Apps obtain a Firebase User from AuthPortal. The easiest way to implement AuthPortal Flow in your client application is by using the SDK. Follow the Getting Started guide for instructions. You do not need to understand this document in order to integrate AuthPortal in your project. The following is a visualization of the AuthPortal Flow:

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Step 1: Generate code_verifier and code_challenge

Generate code_verifier and code_challenge as follows: 
code_verifier = BASE64URL-ENCODE(random 32 bytes)
code_challenge = BASE64URL-ENCODE(SHA256(ascii(code_verifier)))
Do not reuse the same code_verifier and code_challenge, a new pair must be generated for every authentication attempt.

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Step 2: Redirect Users To Authorization Endpoint

The Authorization Endpoint is as follows: 
GET https://<domain>/authorize
  ?response_type=code
  &client_id=<client_id>
  &code_challenge=<code_challenge>
  &code_challenge_method=S256
  &scope=firebase_auth
  &redirect_uri=<redirect_uri>
You obtain your <domain> as well as <client_id> after registering with AuthPortal. All <redirect_uri>s must be identical to those previously registered with AuthPortal. You may optionally pass an optional state parameter, which will be returned to the redirect_uri. Redirect your users to the authorization endpoint, where they will be prompted to log in.

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Step 3: Handle Redirect & Call Token Endpoint

After successful login, AuthPortal redirect users back to <redirect_uri>, and two query parameters will be set: code and iss. If state was provided in the query above, it will also be set. You must verify that the iss matches https://<domain>. To obtain the firebase user object, a final background API call to the Token Endpoint is necessary. Obtain the stored code_challenge and make the following HTTP POST API call: 
POST https://<domain>/oauth/token
  ?grant_type=authorization_code
  &client_id=<client_id>
  &redirect_uri=<redirect_uri>
  &code=<code>
  &code_verifier=<code_verifier>
The result will look like this: 
{
  "access_token": "random token, can be ignored",
  "token_type": "Bearer",
  "firebase_user": { /* <firebase user serialized as JSON> */ }
}
You can use the firebase-js-sdk to initialize firebase authentication with the data provided in the serialized firebase user.

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OAuth 2.0 Compatibility

AuthPortal Flow is based on OAuth 2.0 Authorization Code Flow (as per RFC6749). It extends OAuth 2.0 by requiring a scope=firebase_user parameter, which returns a firebase_user object in the token response after successful authorization. PKCE (as per RFC7636) is mandatory. iss (as per RFC9207) is always returned. response_mode=web_message for sign-in via popup is supported.