Military Communications and Information Technology: A Trusted ...

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Figure 4. The CBIS XML schema structure
Filtering means the process of stripping the document clean from those
elements to which the user’s clearance does not entitle her. This is performed on
the element-granularity level, and based on the cbis:elementClassification
element (and derivatives). If a cbis:element does not pass the Filter-component,
it is first checked if the cbis:element lies on a direct path from the document
root to an allowed element or is a sibling of such a cbis:element.
In such a case, the cbis:element is included, but it is stripped of all optional
fields and mandatory fields are set to empty, NULL or default values. The content-
Signature-element of the type UNSIGNED CONTENT HASH, is however included.
This signature type is assumed to contain the compound hash of the element and its
subelements in the form of Merkle hash tree nodes (see [5] for details).
According to [5], the compound hashes can then be used on the documentlevel
to calculate the original signature, and compare that to the cbis:docume
ntIntegritySignature of type CONTENT INTEGRITY. This way the User
Agent can first verify that the content in general is complete and valid, and after
that check individual elements’ validity.
Writing changes to a partial document and integrating them back to the original
is partially versioning a document, but mainly requires a reasonable amount of book-