I am very new to XACML. And I am using XACML to express policy. But I can't find any good examples except a few from the OASIS XACML Technical Committee.
Ok, here is my question:
I want to express policy using XACML. Users can access to the resources only if they satisfy the policy. The policy is an logical expression. For example:
(not A1) and (A2 OR A3) and (2 of (A4, A5,A6))
2 of (A4,A5,A6) refers that it is true only if 2 or more of A4,A5,A6 is true.
"AllOf" and "AnyOf" can be used to express "AND" and "OR", but I don't know how to express "2 of (A4,A5,A6)" and "not A1".
Thank you!
Based on the requirement you have, you need to use a XACML condition. Conditions live within rules only so this means you'll have to put your logic inside the rule.
This is because you will need a function not allowed in XACML targets: n-of.
This is also because a XACML Target cannot have negative expressions. The only way you can express Not(A1) is via a condition.
The reason for that is that XACML deals with attribute bags. So when you write in a target:
role==manager
What you are in fact saying is: if the user has at least one role equal to manager...
So what would the opposite of that be?
With respect to your 2 of (a,b,c), you can use the XACML function called n-of (urn:oasis:names:tc:xacml:1.0:function:n-of defined in A.3.5 Logical functions)
The outcome is in ALFA
/**
* (not A1) and (A2 OR A3) and (2 of (A4, A5,A6))
*/
policy stackoverflow{
apply firstApplicable
rule so{
condition not(A1=="some value") && (A2=="" || A3=="") && nOf(2, stringOneAndOnly(A4)=="value", stringOneAndOnly(A5)=="value", stringOneAndOnly(A6)=="value")
permit
}
}
In the example above I made A1 through A6 string attributes instead of boolean to show how you would compare with values. Note that I have to use the stringOneAndOnly function to make sure there is a single value for each of the attributes used in the nOf function.
The XACML output is the following:
<?xml version="1.0" encoding="UTF-8"?>
<!--This file was generated by the ALFA Plugin for Eclipse from Axiomatics AB (http://www.axiomatics.com).
Any modification to this file will be lost upon recompilation of the source ALFA file-->
<xacml3:Policy xmlns:xacml3="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17"
PolicyId="http://axiomatics.com/alfa/identifier/policing.principles.stackoverflow"
RuleCombiningAlgId="urn:oasis:names:tc:xacml:1.0:rule-combining-algorithm:first-applicable"
Version="1.0">
<xacml3:Description>(not A1) and (A2 OR A3) and (2 of (A4, A5,A6))</xacml3:Description>
<xacml3:PolicyDefaults>
<xacml3:XPathVersion>http://www.w3.org/TR/1999/REC-xpath-19991116</xacml3:XPathVersion>
</xacml3:PolicyDefaults>
<xacml3:Target />
<xacml3:Rule
Effect="Permit"
RuleId="http://axiomatics.com/alfa/identifier/policing.principles.stackoverflow.so">
<xacml3:Description />
<xacml3:Target />
<xacml3:Condition>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:not" >
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of">
<xacml3:Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal"/>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">some value</xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="A1"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
</xacml3:Apply>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:or">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of">
<xacml3:Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal"/>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string"></xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="A2"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of">
<xacml3:Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal"/>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string"></xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="A3"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
</xacml3:Apply>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:n-of" >
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#integer">2</xacml3:AttributeValue>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-one-and-only" >
<xacml3:AttributeDesignator
AttributeId="A4"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">value</xacml3:AttributeValue>
</xacml3:Apply>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-one-and-only" >
<xacml3:AttributeDesignator
AttributeId="A5"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">value</xacml3:AttributeValue>
</xacml3:Apply>
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:string-one-and-only" >
<xacml3:AttributeDesignator
AttributeId="A6"
DataType="http://www.w3.org/2001/XMLSchema#string"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource"
MustBePresent="false"
/>
</xacml3:Apply>
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">value</xacml3:AttributeValue>
</xacml3:Apply>
</xacml3:Apply>
</xacml3:Apply>
</xacml3:Apply>
</xacml3:Condition>
</xacml3:Rule>
</xacml3:Policy>
--- EDIT ---
To express negative conditions e.g. not(gender==male), you have two options:
- either the set of possible values is finite e.g. true/false, male/female, hot/warm/cold and you are happy building a policy or rule per case.
- or the set of possible values is too long or infinite e.g. a numerical value or a list of citizenships (180+ of those).
In the former case you can write the following:
policy checkGender{
apply firstApplicable
rule male{
target clause gender=="male"
permit
}
rule female{
target clause gender=="female"
permit
}
/**
* Optionally add a catch all case
*/
rule other{
target clause ... // Here you'd have to define other checks you are interested in
}
}
In the latter case, you need to write a negative condition. To do that you need to use a XACML condition. Since XACML conditions only live inside rules, you need to go down to the XACML Rule level.
policy checkGender{
apply firstApplicable
rule notMale{
condition not(gender=="male")
permit
}
}
来源：https://stackoverflow.com/questions/23646066/using-xacml-to-express-policy-which-is-a-logical-expression