OIDC has several methods, or flows, that clients or applications can use to authenticate users and receive _identity_ and _access_ tokens. The method depends on the type of application or client requesting access.
[[_oidc-auth-flows-authorization]]
===== Authorization Code Flow
The Authorization Code Flow is a browser-based protocol and suits authenticating and authorizing browser-based applications. It uses browser redirects to obtain _identity_ and _access_ tokens.
. A user connects to an application using a browser. The application detects the user is not logged into the application.
. The application redirects the browser to {project_name} for authentication.
. The application passes a callback URL as a query parameter in the browser redirect. {project_name} uses the parameter upon successful authentication.
. {project_name} authenticates the user and creates a one-time, short lived, temporary code.
. {project_name} redirects to the application using the callback URL and adds the temporary code as a query parameter in the callback URL.
. The application extracts the temporary code and makes a background REST invocation to {project_name}
to exchange the code for an _identity_ and _access_ and _refresh_ token. To prevent replay attacks, the temporary code cannot be used more than once.
[NOTE]
====
A system is vulnerable to a stolen token for the lifetime of that token. For security and scalability reasons, access_ tokens are generally set to expire quickly so subsequent token requests fail. If a token expires, an application can obtain a new access token using the additional _refresh_ token sent by the login protocol.
====
[[_confidential-clients]]
_Confidential_ clients provide client secrets when they exchange the temporary codes for tokens. _Public_ clients are not required to provide client secrets.
_Public_ clients are secure when HTTPS is strictly enforced and redirect URIs registered for the client are strictly controlled. HTML5/JavaScript clients have to be _public_ clients because there is no way to securely transmit the client secret to HTML5/JavaScript clients. For more details, see the xref:assembly-managing-clients_{context}[Managing Clients] chapter.
{project_name} also supports the https://tools.ietf.org/html/rfc7636[Proof Key for Code Exchange] specification.
[[_oidc-auth-flows-implicit]]
===== Implicit Flow
The Implicit Flow is a browser-based protocol. It is similar to the Authorization Code Flow but with fewer requests and no refresh tokens.
[NOTE]
====
The possibility exists of _access_ tokens leaking in the browser history when tokens are transmitted via redirect URIs (see below).
Also, this flow does not provide clients with refresh tokens. Therefore, access tokens have to be long-lived or users have to re-authenticate when they expire.
We do not advise using this flow. This flow is supported because it is in the OIDC and OAuth 2.0 specification.
====
The protocol works as follows:
. A user connects to an application using a browser. The application detects the user is not logged into the application.
. The application redirects the browser to {project_name} for authentication.
. The application passes a callback URL as a query parameter in the browser redirect. {project_name} uses the query parameter upon successful authentication.
. {project_name} authenticates the user and creates an _identity_ and _access_ token. {project_name}
redirects to the application using the callback URL and additionally adds the _identity_ and _access_ tokens as a query parameter in the callback URL.
. The application extracts the _identity_ and _access_ tokens from the callback URL.
[[_oidc-auth-flows-direct]]
===== Resource Owner Password Credentials Grant (Direct Access Grants)
_Direct Access Grants_ are used by REST clients to obtain tokens on behalf of users. It is a HTTP POST request that contains:
* The credentials of the user. The credentials are sent within form parameters.
* The id of the client.
* The clients secret (if it is a confidential client).
The HTTP response contains the _identity_, _access_, and _refresh_ tokens.
[[_client_credentials_grant]]
===== Client Credentials Grant
The _Client Credentials Grant_ creates a token based on the metadata and permissions of a service account associated with the client instead of obtaining a token that works on behalf of an external user. _Client Credentials Grants_ are used by REST clients.
See the <<_service_accounts,Service Accounts>> chapter for more information.
