In this report many relevant issues were presented, from a technical point of view. However, little has been done to motivate these studies. A more detailed investigation of applications, and a comparison with current techniques in steganography would have been interesting. For example, a thorough evaluation of the advantages natural language-based techniques can offer over image-based techniques could have offered valuable insights.
An important contribution of this project to natural language steganography is the linguistic sophistication of the model for word-substitution put forward. The lexical models employed in current substitution-based systems were often criticised and their inadequate behavior usually described with respect to language theory. These phenomena could have been demonstrated by example, showing texts and inadequate replacements carried out by current stegosystems. A more detailed analysis of how common these critical situations really are in typical text could have given clues for the construction of such systems, to decide whether the additional complexity introduced by statistical word-sense disambiguation is worth the effort.
Other linguistic models have been studied, in addition to the lexical ones, and put in relation to each other, and to their use for steganographic purposes. The steganographic aspects were then covered by information-theoretic models. However, little has been done to justify this choice. It might have been fruitful to present other characterizations of steganography and to compare their suitability to natural language steganography.
A central part of the problem motivating this report was that there are no models formalizing the design and analysis of natural language stegosystems. Although the present report somewhat improves the situation, by providing a systematic investigation of the topic, there is still no system to build upon for making formal claims about security or robustness in the natural language scenario. A more formal, perhaps axiomatic, treatment of the ideas and concepts that were used herein to evaluate current stegosystems could have done much to improve this situation.
Two approaches were presented herein, that are of significantly innovative nature. Unfortunately, both of them had to be presented in this report as position statements.
The first one was the secure and robust coding scheme. At the beginning of the project, a detailed formal analysis of the security and robustness this scheme can offer was anticipated. After dedicating much time to this analysis, it turned out to be too complex a topic for the scope of this project and time did not permit further investigation, for example carrying out a proof-of-concept implementation.
The second innovative contribution was the lexically more adequate coding technique. We kept emphasizing the importance of evaluating the property of steganograms to appear innocuous to humans empirically. A proof-of-concept implementation could provide important insights, and, most importantly, could be used to carry out such an empirical investigation.
Looking back on the project, I believe that I can be satisfied with its overall outcomes especially in the light of the usual scope of an undergraduate final-year project. The amount of work that went into this report was about twice as much as what is expected, considering the time- and word-count- criteria given in the project-guidelines. I did my best to maintain consistent quality of presentation, throughout this report, and a high level of commitment throughout the project. In particular, I tried very hard to put forward innovative material, and to present things in a new way. This is why I invested much time and effort to original research, directed towards a more formal treatment and the proof-of-concept implementation I had anticipated in the original project proposal. However, in the course of this research, I realized that a theoretical investigation had much more to offer, at the time being, which is also the reason why the project changed its face from a software engineering and systems research topic, to the theoretical investigation presented throughout this report. I am convinced that this decision was correct, since the methods used for the present analysis are much more suitable than the ones anticipated. A prototype would, due to the limited time-scope of this project, probably have required making many overly simplistic assumptions and using highly limited models. This would clearly not have lead to any contribution to the state of the art worth the effort.
If there is one thing that I can say for certain about this project, then that it was both demanding and rewarding. Deciding to dedicate significant time to original research, although this is not expected of an undergraduate, was very demanding because it was much work and required me to do a lot of background reading, which turned out to be rewarding because I discovered many exciting areas of research I would not otherwise have thought I could be interested in. Handing in a contribution to the 7th Information Security Conference coauthored by my supervisor was very demanding, since I had never contributed to an academic conference before, but, although the acceptance decision has not yet been made, it already proved to be highly rewarding, because I learned so much about technical writing, and the way academia works.
Most importantly, however, I hope that the impact of this project does not remain limited to the learning outcome I could claim to have gained for myself, but will turn out to be an important contribution to the state of the art in natural language steganography.