Selective and Authentic Third Party distribution of XML Documents Yashaswini Harsha Kumar Netaji Mandava Oct 16th 2006 Contents Terminology Security Properties XML Overview Merkle Hash function Access Control Model Architecture Actor Interactions References Terminology Owner Producer of information Publisher Manages the owner information and answers subject queries Subject A person who produces queries for documents Merkle Hash Trees A Merkle hash tree is a tree of hashes in which the leaves are hashes of data blocks in for instance a file or set of files Nodes further up in the tree are the hashes of their respective children For example in the picture to the right hash 0 is the result of hashing hash 0 0 and then hash 0 1 That is hash 0 hash hash 0 0 hash 0 1 Security properties Document Source Authenticity The subject receiving a document is assured that the contents of the document originated at the claimed source Document Contents Authenticity The integrity of the document received by a subject with respect to the original document is respected Completeness of Response A subject must be able to verify that he or she has received all the document s or portion s of document s that is entitled to access according to the stated access control policies XML revisited 1 Nested and tagged XML elements Attributes Label Set of element tags and attribute names Value A set of attribute element values XML revisited 2 Is an XML document a tuple XML revisited 3 XML revisited 4 Edges element attribute element sub element Link Edges Links between elements IDREF Merkle Hash function Ensures Authenticity of the document as well as the schema Associate a hash value with each node in the graph representation of the XML document The hash value of a node is obtained by applying a hash function over the concatenation of its children The hash values are computed using the Merkle Hash Function Merkle Hash Function Mh MhXd Collision Resistant Hash function CRHF hash is a CRHF if it is hard to find two different messages m1 and m2 such that hash m1 hash m2 Codomain of MhXd the codomain of a function f X Y is the set Y Hash is a mapping from a domain usually called keys into a codomain usually called values Keypoints If a subject knows the correct Merkle hash value of a node the Publisher cannot forge the value of the descendant children The Publisher returns the Merkle Hash value of the of the root of an XML document d along with the query result The Publisher receives the hash value of the root from the Owner The Merkle hash value of the root of the document is critical therefore it is signed by the Owner known as as Merkle Signature and is verified by the subject Please refer to the paper for details on generating the Merkle Signature Access Control Model for XML Documents 1 Subjects are qualified by means of credentials A credential is a set of properties concerning a subject that are relevant for security purposes for example age position within an organization Credentials are encoded using an XML based language called X Sec Access control policies specify conditions on the credentials and properties of the credentials using an XPath compliant language Access Control Model 2 The access control model provides varying access granularity levels and can express policies that apply to 1 all the instances of a DTD XML Schema 2 collections of documents not necessarily instances of the same DTD XMLSchema 3 selected portions within a document s or a link or a set of links Access Control Model 3 Access control policies are encoded using X Sec Policy Base The XML file encoding the access control policies of the Owner System Architecture 1 Architecture 2 Relies on the distinction between the Owner and the Publisher The Owner specifies the access control policies sends the Publisher the documents he is entitled to manage along with information on which subjects can access which portions of the document according to the access control policies it has specified sends the Publisher the Merkle Signature of the document optional The above additional information is encoded in XML and attached to the original document forming the so called security enhanced XML document The structure of the original XML document is also supplied by the Owner in the form of an XML document called secure structure Architecture 3 The Subject Registers with the Owner during the mandatory subscription phase Receives the subject policy configuration from the Owner in a secure manner Submits queries to the Publisher along with its policy configuration Receives a view of the requested document s Verifies authenticity of the answer for authenticity and completeness Architecture 4 The Publisher Receives documents access control policies secure structure and optional Merkle Signature from the Owner On receiving a query from a Subject the Publisher computes a view of the requested document s which contains all and only those portions of the requested document s for which the subject has an authorization according to the access control policies in place at the Owner site Responds to the Subjects query with a Reply Document view hash values computed over parts of the document not contained in the view Subject Owner Interaction 1 Subject Owner Interactions 2 Policies P1 and P2 apply to Alice Brown Only policy P6 applies to Tom Moore Thus the policy configuration of Alice Brown and Tom Moore are respectively the signatures of the identifiers 1 2 and 6 with the Owner s private key Owner Publisher Interaction 1 Security Enhanced XML Document Which access control policies apply to the corresponding document Policy information is specified at the element level Policy Configuration PC is an additional attribute of the corresponding element whose value is the set of policies that apply to the element It is represented as a string of hexadecimal values PC should not be confused with Subject Policy Configuration Each policy in the set is identified by a policy identifier unique to the Policy base The element Policy in the security enhanced XML document contains the identifiers of all policies Refer to the paper for the formal definition of the Security Enhanced XML Document Owner Publisher Interaction 2 Secure Structure The secure structure of the XML document is generated by hashing with a standard hash function each tag and attribute name Subject Publisher Interaction 1 Subject s submits a query his her policy specification to a Publisher Publisher