A computer-implemented method of permitting authentication of data added to an initial plurality of unique data items stored on a computer, all of the data items being converted via a one-way function to respective equal length unique digests (or hashes); and the hashes being stored as a linked dataset in which the value of each successive hash is dependent on the value of the previous hash combined with a new hash resulting from the hashing of the most recently added data. The most recent hash in the linked dataset (hash chain) is the cumulative result of all previous hashes in their original sequence, each hash in the dataset being unique to the most recently added data. Each cumulative hash is constructed by incorporating one or more pre-existing cumulative hashes into the source data, before the new hash is calculated.