A method of protecting an electronic document, the method comprising dividing an original document into data blocks; generating a corresponding hash value for each data block; generating a corresponding hash value for a group of data blocks (GOB); generating a hash tree structure based on the corresponding hash values for the data blocks and for the GOB; calculating a hash value of a root node of the hash tree structure, the root node having no parent in the hash tree structure; signing the hash value of the root node; and generating auxiliary authentication data; wherein said auxiliary authentication data enables authentication of portions of the electronic document based on a reconstruction of the hash tree structure.