Does XACML distinguish between "attribute value is null" and "attribute is missing"

Question

I am reading latest XACML specification and as far as I can see, there is no notion of null for attribute values. Is this true?

So, am I right saying that AttributeDesignator with MustBePresent=false should return empty bag if requested attribute's value is null? And vice versa, AttributeDesignator with MustBePresent=true should return Indeterminate if requested attribute's value is null?

So, is it true that XACML is not able to distinguish between "attribute value is null" and "attribute is missing" situations?

Or is that just out of the scope of the standard and is implementation-dependent?

Answer 1

In XACML an AttributeDesignator conceptually evaluates to a bag of attribute values. That bag may be empty.

As far as I know none of the XACML datatypes can have null values. i.e. there is no concept of null attribute value in XACML.

However, do note though that an empty string is a valid string, although it's not null.