Keycloak

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=== Client initiated account linking
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Some applications want to integrate with social providers like Facebook, but do not want to provide an option to login via
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these social providers.  {project_name} offers a browser-based API that applications can use to link an existing
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user account to a specific external IDP.  This is called client-initiated account linking. Account linking can only be initiated by OIDC applications.
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The way it works is that the application forwards the user's browser to a URL on the {project_name} server requesting
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that it wants to link the user's account to a specific external provider (i.e. Facebook).  The server
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initiates a login with the external provider.  The browser logs in at the external provider and is redirected
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back to the server.  The server establishes the link and redirects back to the application with a confirmation.
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There are some preconditions that must be met by the client application before it can initiate this protocol:
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* The desired identity provider must be configured and enabled for the user's realm in the admin console.
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* The user account must already be logged in as an existing user via the OIDC protocol
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* The user must have an `account.manage-account` or `account.manage-account-links` role mapping.
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* The application must be granted the scope for those roles within its access token
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* The application must have access to its access token as it needs information within it to generate the redirect URL.
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To initiate the login, the application must fabricate a URL and redirect the user's browser to this URL.  The URL looks like this:
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[source,subs="attributes+"]
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----
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/{auth-server-root}{kc_realms_path}/{realm}/broker/{provider}/link?client_id={id}&redirect_uri={uri}&nonce={nonce}&hash={hash}
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----
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Here's a description of each path and query param:
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provider::
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  This is the provider alias of the external IDP that you defined in the `Identity Provider` section of the admin console.
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client_id::
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  This is the OIDC client id of your application.  When you registered the application as a client in the admin console,
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  you had to specify this client id.
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redirect_uri::
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  This is the application callback URL you want to redirect to after the account link is established.  It must be a valid
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  client redirect URI pattern.  In other words, it must match one of the valid URL patterns you defined when you registered
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  the client in the admin console.
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nonce::
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  This is a random string that your application must generate
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hash::
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  This is a Base64 URL encoded hash.  This hash is generated by Base64 URL encoding a SHA_256 hash of `nonce` + `token.getSessionState()` + `token.getIssuedFor()` + `provider`.
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  The token variable are obtained from the OIDC access token.  Basically you are hashing the random nonce, the user session id, the client id, and the identity
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  provider alias you want to access.
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Here's an example of Java Servlet code that generates the URL to establish the account link.
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[source,java,subs="attributes+"]
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----
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   KeycloakSecurityContext session = (KeycloakSecurityContext) httpServletRequest.getAttribute(KeycloakSecurityContext.class.getName());
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   AccessToken token = session.getToken();
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   String clientId = token.getIssuedFor();
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   String nonce = UUID.randomUUID().toString();
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   MessageDigest md = null;
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   try {
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      md = MessageDigest.getInstance("SHA-256");
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   } catch (NoSuchAlgorithmException e) {
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      throw new RuntimeException(e);
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   }
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   String input = nonce + token.getSessionState() + clientId + provider;
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   byte[] check = md.digest(input.getBytes(StandardCharsets.UTF_8));
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   String hash = Base64Url.encode(check);
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   request.getSession().setAttribute("hash", hash);
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   String redirectUri = ...;
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   String accountLinkUrl = KeycloakUriBuilder.fromUri(authServerRootUrl)
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                    .path("{kc_realms_path}/{realm}/broker/{provider}/link")
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                    .queryParam("nonce", nonce)
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                    .queryParam("hash", hash)
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                    .queryParam("client_id", clientId)
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                    .queryParam("redirect_uri", redirectUri).build(realm, provider).toString();
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----
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Why is this hash included?  We do this so that the auth server is guaranteed to know that the client application initiated the request and no other rogue app
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just randomly asked for a user account to be linked to a specific provider.  The auth server will first check to see if the user is logged in by checking the SSO
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cookie set at login.  It will then try to regenerate the hash based on the current login and match it up to the hash sent by the application.
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After the account has been linked, the auth server will redirect back to the `redirect_uri`.  If there is a problem servicing the link request,
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the auth server may or may not redirect back to the `redirect_uri`.  The browser may just end up at an error page instead of being redirected back
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to the application.  If there is an error condition and the auth server deems it safe enough to redirect back to the client app, an additional
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`error` query parameter will be appended to the `redirect_uri`.
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[WARNING]
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   While this API guarantees that the application initiated the request, it does not completely prevent CSRF attacks for this operation.  The application
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   is still responsible for guarding against CSRF attacks target at itself.
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==== Refreshing external tokens
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If you are using the external token generated by logging into the provider (i.e. a Facebook or GitHub token), you can refresh this token by re-initiating the account linking API.
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