1. A method for processing information, comprising: calculating, using a computer, a compression distance between a pair of different documents that do not contain duplicated content, comprising 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 responsively to the compression distance, identifying the pair of documents as having a common author, wherein identifying the pair comprises grouping at least two of the documents between which the compression distance is below a specified threshold as belonging to the common author.
5. The method according to claim 1 , wherein the documents have respective uniform resource locators (URLs), and wherein grouping the set of the documents comprises identifying a cluster of the documents that share a common feature in the respective URLs, and selecting the documents in the cluster for which the compression distances are below the specified threshold.