11. The product according to claim 8 , wherein the instructions cause the computer to chain together pairs of the documents between which the respective compression distances are below the specified threshold.
8. A computer program product comprising a computer-readable storage medium in which computer readable program code is stored, which program code, when read by a computer, causes the computer to: calculate a compression distance between a pair of different documents that do not contain duplicated content, by measuring how much a respective compression of each of the documents is improved by using information included in the other of the documents, wherein said measuring comprises: compressing each of the documents to create first and second compressed files; concatenating the documents to generate a concatenated document and compressing the concatenated document to create a third compressed file; finding respective first and second differences in size between the first and second compressed files and the third compressed file; and computing a product of the first and second differences; and identify the pair of documents as having a common author responsively to the compression distance, wherein the instructions cause the computer to compute distances between multiple documents of unknown authorship, and to group at least two of the documents between which the compression distance is below a specified threshold as belonging to the common author.