How to get new access token in OpenID Connect/OAuth2 Implicit Flow - oauth-2.0

Question

I am currently using OpenID Connect/Oauth2 Implicit Flow in a mobile app. I am bringing up a Web View for the user to login and obtaining the access token and expiry. However, when the access token expires, do I need to ask the user to log in again? Or is there a way to get a new access token silently using the current one, without bugging the user. I guess another option is to set the token expiry to be a really long time, but I have read that this is a bad idea.
Am I missing something here?

Answer 1

Since Implicit flow does not send a refresh token (as explained in section 9 of RFC6746), usage of refresh tokens is not possible. But as a workaround one can use client credential grant to obtain an access token.
A viable solution is to first follow the implicit flow and authenticate the client. Then client authentication grant can be used to do the required API calls.
Sample request (from RFC6749)
POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded
rant_type=client_credentials
Sample resposne (from RFC6749)
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"token_type":"example",
"expires_in":3600,
"example_parameter":"example_value"
}
P.S -
If you are using authorization code flow, you can use refresh_token to get a new access token. How the request should be formed can be obtained from OAuth2 documentation. Note that to do so, your authorization response should contain a `refresh_token.
A refresh token should be protected as valuable as a credential for a user. More can be read from keycloak documentation from here
Sample request and a response (from RFC6749)
Request
POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded
grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
Response
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache
{
"access_token": "TlBN45jURg",
"token_type": "Bearer",
"refresh_token": "9yNOxJtZa5",
"expires_in": 3600
}

Answer 2

The recommended way is to use an external browser and the Authorization Code Flow. Check the OAuth 2.0 for Native Apps RFC. For Android, there is also a supporting library AppAuth. With this flow, you can use a refresh token to get a new access token, but there is an issue with a client secret (usually needed for accessing /token endpoint), because you cannot keep it safe in a mobile app (it's described in the RFC).
If you decide to stick with the WebView and the Implicit Flow, which is not safe (your app can see the password) you could use the same technique as in JavaScript applications - request a new token with /auth?...&prompt=none URL which will return a new token without asking the user for credentials if he still has an open session there.