Anonymous authoring includes writing reviews, comments and blogs, using pseudonyms with the general assumption that using these pseudonyms will protect the real identity of authors and allows them to freely express their views. It has been shown, however, that writing style may be used to trace authors across multiple Websites. This is a serious threat to privacy and may even result in revealing the authors’s identities. In obfuscating authors’ writing style, an authored document is modified to hide the writing characteristics of the author. In this paper we first show that existing obfuscation systems are insecure and propose a general approach for constructing obfuscation algorithms, and then instantiate the framework to give an algorithm that semi-automatically modifies an author’s document. We provide a secure obfuscation scheme that is able to hide an author’s document securely among other authors’ documents in a corpus. As part of our obfuscation algorithm we present a new algorithm for identifying an author’s unique words that would be of independent interest.We present a security model and use it to analyze our scheme and also the previous schemes. We implement our scheme and give its performances through experiments.We show that our algorithm can be used to obfuscate documents securely and effectively.