While there are sophisticated and efficient steganographic schemes with images a

Question

While there are sophisticated and efficient steganographic schemes with images as cover available, I am yet ignorant of the existence of any fairly efficient and secure schemes with texts as cover. This state of affairs is IMHO unsatisfying, since the communication channel may under circumstances only be able to transfer printable natural language texts, and that in rather small volume, due to constraints posed by the warden.

To be concrete on the efficiency issue, I would desire that the ratio of stego bits to covertext bits be no less than, say, 1/50. For references to existing schemes, if any, or else novel basic ideas that could lead to the development of such a scheme (if possible user-friendly and not too hard to implement), I should be very grateful.

Explanation / Answer

The most famous text based steganographic scheme is the acrostic: using the first letters of words / sentences. If the mean sentence length is 15-20 words and mean word length is 5 letters, then efficiency is ~1%. You could use shorter than average sentences and/or words to increase the efficiency to within your bounds of >2%.

Obviously this is a specific example of a class of functions that uses the nth letter of the nth word of each sentence as the hidden message.

This scheme certainly meets the user friendly and not too hard to implement category (provided you have some imagination / a good thesaurus), but won't stand up to much cryptanalysis!

Quick thought - base 26 encode the output from a strong authenticated encryption function, then construct sentences with the first letter of the first word as above?

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