Xacml policy test occurence of string in string bag - authorization

Question

I'm trying to come up with a rule that says the string must begin with ABC but not be ABC123, ABC456, ABC789.
I'm trying write this to evaluate against a string bag, any pointers much appreciated.

Answer 1

You'll need two things:
A check that matches your attribute (e.g. a) with the value (ABC)
A condition that checks the same attribute is not either of ABC123, ABC456, ABC789.
There are (at least) two ways you could do this:
You could write a policy that checks the attribute starts with ABC and then have a rule that denies access if the value is one of the 3 forbidden ones or
You could write a policy that checks the attribute starts with ABC and only returns permit if the the value is not one of the 3 forbidden ones.
ALFA Example
ALFA, the abbreviated language for AuthZ, is a lightweight syntax for XACML policies. (Source: alfa, Wikipedia)
namespace com.axiomatics{
attribute a{
category = subjectCat
id = "com.axiomatics.a"
type = string
}
/**
* This policy allows access if the string starts with ABC but is not ABC123, ABC456, or ABC789
*/
policy example{
apply firstApplicable
/**
* Deny specific values
*/
rule denySpecificValues{
target clause a == "ABC123" or a == "ABC456" or a == "ABC789"
deny
}
/**
* Allow if the string starts with ABC
*/
rule startsWithABC{
target clause stringStartsWith("ABC", a)
permit
}
}
/**
* This policy allows access if the string starts with ABC but is not ABC123, ABC456, or ABC789
*/
policy anotherExample{
apply firstApplicable
/**
* Allow if the string with ABC
*/
rule startsWithABC{
target clause stringStartsWith("ABC", a)
permit
condition not (a == "ABC123") && not(a == "ABC456") || not(a == "ABC789")
}
}
}
XACML XML Equivalent
<?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/com.axiomatics.example"
RuleCombiningAlgId="urn:oasis:names:tc:xacml:1.0:rule-combining-algorithm:first-applicable"
Version="1.0">
<xacml3:Description>This policy allows access if the string starts with
ABC but is not ABC123, ABC456, or ABC789</xacml3:Description>
<xacml3:PolicyDefaults>
<xacml3:XPathVersion>http://www.w3.org/TR/1999/REC-xpath-19991116
</xacml3:XPathVersion>
</xacml3:PolicyDefaults>
<xacml3:Target />
<xacml3:Rule Effect="Deny"
RuleId="com.axiomatics.example.denySpecificValues">
<xacml3:Description>Deny specific values</xacml3:Description>
<xacml3:Target>
<xacml3:AnyOf>
<xacml3:AllOf>
<xacml3:Match
MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">ABC123</xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="com.axiomatics.a"
Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
DataType="http://www.w3.org/2001/XMLSchema#string"
MustBePresent="false" />
</xacml3:Match>
</xacml3:AllOf>
<xacml3:AllOf>
<xacml3:Match
MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">ABC456</xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="com.axiomatics.a"
Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
DataType="http://www.w3.org/2001/XMLSchema#string"
MustBePresent="false" />
</xacml3:Match>
</xacml3:AllOf>
<xacml3:AllOf>
<xacml3:Match
MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">ABC789</xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="com.axiomatics.a"
Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
DataType="http://www.w3.org/2001/XMLSchema#string"
MustBePresent="false" />
</xacml3:Match>
</xacml3:AllOf>
</xacml3:AnyOf>
</xacml3:Target>
</xacml3:Rule>
<xacml3:Rule Effect="Permit"
RuleId="com.axiomatics.example.startsWithABC">
<xacml3:Description>Allow if the string starts with ABC
</xacml3:Description>
<xacml3:Target>
<xacml3:AnyOf>
<xacml3:AllOf>
<xacml3:Match
MatchId="urn:oasis:names:tc:xacml:3.0:function:string-starts-with">
<xacml3:AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">ABC</xacml3:AttributeValue>
<xacml3:AttributeDesignator
AttributeId="com.axiomatics.a"
Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
DataType="http://www.w3.org/2001/XMLSchema#string"
MustBePresent="false" />
</xacml3:Match>
</xacml3:AllOf>
</xacml3:AnyOf>
</xacml3:Target>
</xacml3:Rule>
</xacml3:Policy>