A Policy enables developers to configure fine-grained access control for Things and other entities in an easy way.

Authorization concept

A specific policy provides someone (called subject), permission to read and/or write a given resource.

Please note, that in most cases it makes sense to grant read permission in addition to a write permission, because write does not imply read.

Who can be addressed?

A Subject ID must conform to one of the following rules:

  • The ID of a User defined in the nginx reverse proxy prefixed with nginx.
  • Different JWT providers with their JWT “iss” fields - the currently supported are listed in the table below.
Prefix Type Description
google jwt A JWT issued by Google

Which Resources can be controlled?

A Policy can contain access control definitions for several resources:

  • Policy: Someone who was granted write permission at the policy root resource (i.e. policy:/) is allowed to manage the policy itself.
  • Thing: The resource can be defined as fine-grained as necessary for the respective use case: e.g. thing:/ as top-level resource or on sub-resources such as thing:/features. At runtime, the permissions are propagated down to all Thing sub-entities.
    • In case you grant read permission on top-level and revoke it at a sub-entity, the subject can read the upper part only.
    • In case you omit a subject at top-level but grant permission at a sub-entity, the subject can access the lower part only (and the Thing ID).

Policy

The Policy resource (addressable as policy:/) defines the access control for the Policy itself.

Resource Addressed data, description
policy:/ The Policy itself (top-level)
Applies to the Policy and all of its sub-resources.
policy:/policyId The Policy’s ID.
However, such a reference is not recommended because write is not supported anyway, and read on the ID only, does not provide any benefit.
policy:/entries Applies to all entries of the Policy.
policy:/entries/X Applies to all subjects and resources of the specific entry X.
policy:/entries/X/subjects Applies to all subjects of the specific entry X.
policy:/entries/X/subjects/Y Applies to subject Y of the specific entry X.
policy:/entries/X/resources Applies to all resources of the specific entry X.
policy:/entries/X/resources/Y Applies to resource Y of the specific entry X.

The Things example at the end of the page also defines access control on the policy resource.

Thing

The Thing resource (addressable as thing:/) defines the access control for Things.

The access control definitions defined in a Policy’s Thing resource will be applied to all Things referencing this Policy.

Resource Addressed data, description
thing:/ The Thing itself (top-level).
Applies to the Thing and all of its sub-resources.
thing:/thingId The Thing’s ID.
Not recommended, because write is not supported anyway and read on the ID only does not provide any benefit.
thing:/policyId Applies to the Policy ID of the Thing, which implicitly defines its access control.
Please double-check write permissions on this resource.
thing:/attributes Applies to all attributes of the Thing.
thing:/attributes/X Applies to the specific attribute X and its sub-paths.
X may be a nested path such as tire/pressure.

Find a Things example at the end of the page.

Feature

Resource Addressed data, description
thing:/features Applies to all Features of the Thing.
thing:/features/X Applies to the Feature with ID X and all its sub-paths.
thing:/features/X/properties Applies to all properties of the Feature X.
thing:/features/X/properties/Y Applies to the property with path Y (and its sub-paths) of the Feature with ID X.
Y may be a nested path such as tire/pressure.

Find a Things example at the end of the page.

Message

The Message resource (addressable as message:/) defines the access control for Messages.

The access control definitions defined in a Policy’s Message resource will be applied to all Messages sent to or from Things referencing this Policy.

  • For sending messages to a Thing or its Features write permission is required
  • For receiving messages from a Thing or its Features read permission is required.

Such permissions can be defined at resources of different granularity.

Resource Addressed data, description
message:/ All messages (top-level)
Applies to all messages sent to or from Things referencing this Policy and all messages sent to or from features of these Things.
message:/inbox Applies to all messages sent to a specific Thing (or multiple things referencing this Policy)
message:/inbox/messages/X Applies to all messages on message-subject X, sent to the Things referencing this Policy
message:/outbox Applies to all messages sent from the Things referencing this Policy
message:/outbox/messages/X Applies to all messages on message-subject X, sent from the Things referencing this Policy
message:/features Messages for all Features
Applies to all messages sent to or from all Features of Things referencing this Policy
message:/features/Y Applies to all messages sent to or from Feature Y of the Things referencing this Policy
message:/features/Y/inbox Applies to all messages sent to Feature Y of the Things referencing this Policy
message:/features/Y/inbox/messages/X Applies to all messages on message-subject X sent to Feature Y of the Things referencing this Policy
message:/features/Y/outbox Applies to all messages sent from Feature Y of the Things referencing this Policy
message:/features/Y/outbox/messages/X Applies to all messages on message-subject X sent from Feature Y of the Things referencing this Policy

The Things example at the end of the page also defines access control on messages.

Grant and Revoke some Permission

Change Permission Description
grant READ All subjects named in the section are granted read permission on the resources specified in the path, and all subsequent paths, except they are revoked at a subsequent policy label.
grant WRITE All subjects named in the section are granted write permission on the resources specified in the path, and all subsequent paths, except they are revoked at a subsequent policy label.
revoke READ All subjects named in the section are prohibited to read on the resources specified in the path, and all subsequent paths, except they are granted again such permission at a subsequent policy label.
revoke WRITE All subjects named in the section are prohibited to write on the resources specified in the path, and all subsequent paths, except they are granted again such permission at a subsequent policy label.

Tools for editing a Policy

The Policy can be edited with a text editor of your choice. Just make sure it is in valid JSON representation, and that at least one valid subject is granted write permission at the root resources.

In case of fine-grained access on Things, keep an eye on your actual Thing structure to make sure that all paths will be granted or revoked the permissions your use case is supposed to support.

Example

Given you need to support the following scenario:

  • Owner: The Thing my.namespace:thing-0123 is owned by a user. Thus, she needs full access and admin rights for the complete Thing. In our example her ID is ditto
  • Observer of changes at featureX and featureY:
    • Another application needs to be informed on each change at those features. In our example its ID is observer-client.
    • There is a group of users who are allowed to read both features. In our example the group ID is some-users.
  • Privacy: The value of the “city” property at “featureY” is confidential and needs to be “hidden” from the group of users.
Policy Example
Example Thing with link to a Policy ID

Your Policy then might look like the following:

Policy Example 2
Example Policy

The correct Policy JSON object notation would be as shown in the following code block.

  {
    "policyId": "my.namespace:policy-a",
    "entries": {
      "owner": {
        "subjects": {
          "nginx:ditto": {
            "type": "nginx basic auth user"
          }
        },
        "resources": {
          "thing:/": {
            "grant": ["READ", "WRITE"],
            "revoke": []
          },
          "policy:/": {
            "grant": ["READ", "WRITE"],
            "revoke": []
          },
          "message:/": {
            "grant": ["READ", "WRITE"],
            "revoke": []
          }
        }
      },
      "observer": {
        "subjects": {
          "nginx:observer-client": {
            "type": "technical client"
          },
          "nginx:some-users": {
            "type": "a group of users"
          }
        },
        "resources": {
          "thing:/features/featureX": {
            "grant": ["READ"],
            "revoke": []
          },
          "thing:/features/featureY": {
            "grant": ["READ"],
            "revoke": []
          }
        }
      },
      "private": {
        "subjects": {
          "nginx:some-users": {
            "type": "a group of users"
          },
          "resources": {
            "thing:/features/featureX/properties/location/city": {
              "grant": [],
              "revoke": ["READ"]
            }
          }
        }
      }
    }
  }

The Policy can be found:

  • Via GET request at /api/2/policies/<policyId>, and
  • Via GET request at /api/2/things/{thingId}/policyId
  • At any Thing itself in its JSON representation. It is however not included by default, but can be retrieved by specifying the /api/2/things/<thingId>?fields=_policy query parameter.
Tags: model