Empirical Paraphrasing of Modern Greek Text in Two Phases: An Application to Steganography

This paper describes the application of paraphrasing to steganography, using Modern Greek text as the cover medium. Paraphrases are learned in two phases: a set of shallow empirical rules are applied to every input sentence, leading to an initial pool of paraphrases. The pool is then filtered through supervised learning techniques. The syntactic transformations are shallow and require minimal linguistic resources, allowing the methodology to be easily portable to other inflectional languages. A secret key shared between two communicating parties helps them agree on one chosen paraphrase, the presence of which (or not) represents a binary bit of hidden information. The ability to simultaneously apply more than one rules, and each rule more than one times, to an input sentence increases the paraphrase pool size, ensuring thereby steganographic security.