Some embodiments are directed to a hash tree computation device. The hash tree computation device computes a top hash of a hash tree. A hash preimage of a leaf node of the hash tree comprises a type of the leaf node. A hash preimage of an internal node of the hash tree comprises a type count comprising a number of descendants of the internal node having a given type. The hash tree computation device computes the top hash by computing hashes of a current node and of its ancestors, where a hash of an ancestor is computed based on its type count, the type count being computed from types or type counts of its descendants.