The Abbreviated Language for Authorization (ALFA) is a domain-specific language used in the formulation of access-control policies.

History

Origin

XACML, the eXtensible Access Control Markup Language, uses XML as its main encoding language. Writing XACML policies directly in XACML leads to bloated, human-unfriendly text, therefore a new, more lightweight, notation was necessary. Axiomatics researcher, Pablo Giambiagi, therefore designed ALFA, the Axiomatics Language for Authorization.

ALFA maps directly into XACML. ALFA contains the same structural elements as XACML i.e. PolicySet, Policy, and Rule.

Axiomatics donates ALFA to OASIS

In March 2014, Axiomatics announced it was donating ALFA to the OASIS XACML Technical Committee in order to advance its standardization.

ALFA was consequently renamed Abbreviated Language for Authorization and filed for standardization.

Sample use cases

  • Medical use case: doctors can view the medical records of patients they have a relationship with.
  • Financial use case: employees in Singapore can view the customer accounts of employees based in Singapore.
  • Insurance use case: an insurance agent can approve the claim of a user if the claim is in the same region as the agent and if the claim amount is less than the agent's approval amount.

The words doctor, view, medical record, Singapore... are all examples of attribute values. Attributes make up the building blocks of policies in ABAC and consequently in ALFA.

Structure

Just like XACML, ALFA has three structural elements:

  • PolicySet
  • Policy
  • Rule

Like in XACML, a PolicySet can contain PolicySet and Policy elements. A Policy can contain Rule elements. A Rule contains a decision (either Permit or Deny). In addition, in ALFA, it's possible to add Rule elements to PolicySet and Policy elements. PolicySet, Policy, and Rule elements can be nested or referenced to.

In order to resolve conflicts between siblings, ALFA (as does XACML) uses combining algorithms. There are several combining algorithms that may be used.

Data types

ALFA supports all the data types that are defined in the OASIS XACML Core Specification. Some datatypes e.g. numerical (integer, double) and boolean map directly from ALFA to XACML. Others need to be converted such as date or time attributes. To convert an attribute into the relevant data type, use the "value":datatype notation. See below for examples

Native attribute values mapped directly from ALFA to XACML

String, integer, double, and boolean all map directly from ALFA to XACML. They do not need a conversion

ALFA policy using boolean attributes

Attribute values which need an explicit conversion

The following attribute datatypes need an explicit conversion:

Example: ALFA policy using anyURI

This policy, converts a String value to anyURI.

Sample policies

A simple policy & rule with a condition

The following ALFA example represents a XACML policy which contains a single rule. The policy and rule both have a target. The rule also has a condition which is used to compare 2 attributes together to implement a relationship check (user ID must be equal to owner). Whenever one needs to check 2 attributes together, they must use a condition.

Using time in a XACML policy written in ALFA

Policy references

ALFA can use policy (set) references. They are in fact used implicitly when doing the following.

Obligations and advice

Obligations and advice are statements in XACML that can be returned from the PDP to the PEP alongside the decision (Permit, Deny...). Obligations and advice are triggered on either Permit or Deny.

Break the glass authorization scenario

Start by defining the attributes and obligations:

The policy can now be defined with 3 rules:

  • the first rule is for normal access (doctors can view records of patients they are assigned to.
  • the second rule is for special access because the glass has been broken.
  • the third rule is the rule that triggers the obligation telling the user how to break the glass.

Time-based fine-grained authorization policy

The following is an example of an ABAC policy implemented using ALFA. It uses time as attributes. It uses a XACML condition to compare the currentTime attribute to the value representing 5pm (expressed in 24-hour time). Note the use of :time to convert the String value to the right data type.

HL7 policies

Use cases

HL7 defines a series of medical access control which can be easily defined in ALFA.

Sample ALFA policies for HL7

Access control based on category of action

Implementations

VS Code extension

A free extension for the VS Code editor that supports code completion, syntax highlighting, refactoring, and go-to-definition navigation. It can also compile ALFA into XACML 3.0.

Plugin for Eclipse

The ALFA Plugin for Eclipse is a tool that converts your Eclipse programming IDE to a dedicated editor of authorization policies using ALFA syntax. ALFA policies can then easily be converted into XACML 3.0 policies and loaded into your XACML policy management tool.

External References