Abstract: A directed graph of data includes nodes 10, properties 22 attached to nodes and resources 34. The nodes 10, properties 22 and resources 34 are classified as indexable or non indexable. At least one property 30 links a pair of nodes, one node being a parent and one being a child. An indexing algorithm indexes each indexable property pointing to a resource as related to any respective indexable nodes linked to the node to which the indexable property is attached through a chain of indexable links from child to parent through intermediate nodes, all of which are non-indexable.

Abstract: A hierarchical digital signature method is provided by which different levels of data elements of a data entry are processed in turn to derive function values (for example hash function values) for data elements in the level which are to form part of the digital signature. All function values for one level are combined in a further function to provide an additional function value, and the additional function value is used in the processing of data elements in the next level nearer to the root. This provides a digital signature which can use only selected data elements of a data entry, and which is not sensitive to the ordering of the data entry structure.