authorization-endpoint
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An authorization endpoint is an HTTP endpoint that micropub and IndieAuth clients can use to identify a user or obtain an authorization code (which is then later exchanged for an access token) to be able to post to their website.
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Creating an Authorization Endpoint
An authorization endpoint must be able to both generate authorization codes as well as verify the authorization codes it issues.
Web Sign-In Form
The site contains a web sign-in form prompting the user to enter their personal web address to sign in. Upon submitting the form, the site begins the auth process by discovering the user's auth endpoint, e.g.:
See http://microformats.org/wiki/web-sign-in for more UI details / possibilities.
Identification header
Every endpoint MUST return a
IndieAuth: authorization_endpoint
HTTP header in all responses.
This is used by clients to verify that the server understands the IndieAuth protocol.
Discovery
The web application checks the user's website for a link with a rel-value of "authorization_endpoint":
<link rel="authorization_endpoint" href="http://indieauth.example.org/">
Use this URL as endpoint for processing.
Authorization Endpoint
To start the sign-in flow, the user's browser will be redirected to their authorization endpoint, with additional parameters in the query string.
Directing the user's browser to the auth endpoint is usually done via a HTTP Location header, but can optionally be an <a> tag with an href set to the authorization URL.
Request
Note: Values are shown with extra linebreaks and without URL encoding for readability.
https://auth.example.org/auth
?me=https://aaronparecki.com/
&client_id=https://webapp.example.org/
&redirect_uri=https://webapp.example.org/auth/callback
&state=1234567890
&response_type=id
Parameters:
- me
- Full URI of the user's homepage
- client_id
- Full URI of the application's/website's home page. Used to identify the application. An authorization endpoint may show the application's icon and title to the user during the auth process.
- redirect_uri
- Full URI to redirect back to when the login process is finished
- state
- Web application-internal state variable; can contain anything
- Optional. Auth endpoints MUST support them, though.
- response_type
- id (identification only) or code (identification + authorization)
- Optional. Defaults to id.
- scope
- Not used and omitted in identification mode (response_type=id)
- For authorization, the scope contains a space-separated list of scopes that the web application requests permission for, e.g. "create". Multiple values are supported, e.g. create delete
Redirect URI verification
- why is redirect_uri separate from state?
- the callback URL shouldn't be dynamic per request so that callback URLs can be registered. "state" is allowed to vary per request
- why should callback urls be registered?
- without registration it's easier to perform a redirect attack. more background here: http://tools.ietf.org/html/rfc6749#section-3.1.2.2
- how does the client website register the callback at the server?
- haven't written this part up yet, but the idea is for the client to publish its registered redirect URIs on its web page with a <link> tag
- since client IDs are always URLs, it's all discoverable that way
- for client_id https://example.com/ a server can find its valid redirect URIs by looking for
- <link rel="redirect_uri" href="https://example.com/callback">
- at example.com
Verify the user
The authorization server should present an interface describing the request being made. It must indicate:
- The client_id making the request
- Optionally the server can include the name and logo if an h-card is found on the client_id URL.
- The scope if authorization is requested.
TODO: Example. See h-x-app.
Redirect to web application
The authorization server presents the request information to the user, and if they approve, generates an authorization code and redirects the user to the redirect URI specified.
HTTP/1.1 302 Found
Location: https://webapp.example.org/auth/callback
?code=xxxxxxxx
&state=1234567890
&me=https://aaronparecki.com/
Parameters:
- code
- Authorization code
- state
- State variable sent by the web application
- me
- Full URI of the user's homepage
- This may be different from the me parameter that the user originally entered, but MUST be on the same domain.
Auth code verification
The web application finally queries the authorization endpoint to verify the auth code present in the query string. It makes a POST request to the authorization server with the following values:
POST https://auth.example.org/auth Content-type: application/x-www-form-urlencoded code=xxxxxxxx &redirect_uri=https://webapp.example.org/auth/callback &client_id=https://webapp.example.org/ &state=1234567890
After the authorization server verifies that redirect_uri, client_id and state match the code given, the response will include the "me" value indicating the URL of the user who signed in.
HTTP/1.1 200 OK Content-Type: application/x-www-form-urlencoded me=https://aaronparecki.com/
Parameters:
- me
- Full URI of the user's homepage
- This may be different from the me parameter that the user originally entered, but MUST be on the same domain.
- This MUST be the same as the me parameter that "redirect to web application" returned.
Using an Authorization Service
You can use an existing authorization service such as indieauth.com if you don't want to build your own authorization service.
FAQ
Why are auth codes verified with a POST instead of a GET
If auth codes are sent as a GET request in the query string, they may leak to log files or the HTTP "referer". The decision was made by the OAuth 2.0 working group to use POST requests and the HTTP Authorization header for sending these sensitive tokens and auth codes.
Can the authorization codes be used more than once
No, the authorization code must not be used more than once. If the code is used more than once, the authorization server must deny the request.[1] A maximum lifetime of 10 minutes is recommended for the authorization codes, although many implementations have a lifetime of 30-60 seconds.
Can I add additional parameters for the authorization request
No, but you can use the "state" parameter to encode or reference additional application-specific parameters. The state parameter will be passed around and was designed for this purpose as well as to prevent CSRF attacks.
Does the auth server have to support the state parameter
Yes, the state parameter can be used by the client to maintain state between the request and the callback, so the auth server must support it. It is also used to prevent CSRF attacks.
