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Thursday, 30 January 2014

Using OpenAM as a REST based PDP

OpenAM has a powerful policy decision point functionality (PDP) that can be leveraged entirely over the REST endpoints provided out of the box.  These endpoints allow for nice decoupling between the PDP and authentication infrastructure and your app.  A few things to setup first...

Policies - policies map to the resource URL's that you want to protect, along with additional data such as the subjects (users) the policy will affect, as well as conditions such as IP address, time, authentication level requirements and so on.

Authentication Modules - an obvious component, but the modules can also be configured with an authentication level (an arbitrary numeric value) that provides an assurance level once a user has used a particular chain / module.  The auth level can then be leveraged via the policy.

Authentication

Authenticating the user over REST in v11 has changed slightly.  There is now the use of JSON based callbacks that allow for more flexible authentication scenarios.  For example, say the user is not authenticated but wants a session with an assurance level to be able to access app.example.com/examples.  The following could be called:

http://openam.example.com:8080/openam/json/authenticate?authIndexType=resource&authIndexValue=http%3A%2F%2Fapp.example.com%3A8081%2Fexamples

OpenAM will then return a JSON and associated JWT, either with a choice callback to choose a module that has the appropriate auth level, or the attributes and value placeholders for the module that matches. Sending the JSON back to OpenAM with the necessary username, password or other attributes filled in will result in a token and success URL for redirection:

{
      tokenId: "AQIC5wM2LY.......QtMzg2NzM4NzAwMjEwMDc2NzIyMQ..*"
      successUrl: "/openam/console"
}

Now comes the authorization part.  There are few avenues to take here.  Either taking the attribute and querying OpenAM for other attributes associated with it to help make an authorization decision natively, or performing policy queries.

Attribute Query

Taking the tokenID as the header cookie, a call is made to retrieve either the entire user object, or specific fields, by appending attributes to the URL:

http://openam.example.com:8080/openam/json/users/smof?_fields=uid,inetuserstatus,employeenumber

Returns:

{"uid":["smof"],"inetuserstatus":["Active"],"employeenumber":["123456"]}


Policy Decision

There are couple of endpoints for performing a URL access check.  The main one I use here is the ../entitlement/entitlement (note the two entitlements...) endpoint that is very flexible and also returns advice objects to assist with handling any deny messages.

By wanting to do a check against app.example.com:8081/examples, encode the URL and taking the subjects tokenID as the cookie, call:

http://openam.example.com:8080/openam/ws/1/entitlement/entitlement?action=GET&resource=http%3A%2F%2Fapp.example.com%3A8081%2Fexamples

A deny message (for example the user being authenticated to a module that didn't meet the 110 authlevel minimum...) would deliver:

{
  "statusMessage": "OK",
  "body": {
    "resourceName": "http://app.example.com:8081/examples",
    "advices": {
      "AuthLevelConditionAdvice": [
        "/:110"
      ]
    },
    "attributes": {},
    "actionsValues": {}
  },
  "statusCode": 200
}

A positive response would deliver:

{
  "statusMessage": "OK",
  "body": {
    "resourceName": "http://app.example.com:8081/examples",
    "advices": {},
    "attributes": {},
    "actionsValues": {
      "GET": true,
      "POST": true
    }
  },
  "statusCode": 200
}

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