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Are there any steganography algorithms which are capable of hiding a (optionally encrypted) text message within another innocuous text message?

The scenario I envisage is that I would like to carry on an email conversation, which to a man in the middle looks completely innocent, even upon close scrutiny; but which infact contains the true message well hidden within the visible text.

I am aware of this technique being used to hide text within media files, however this sounds to me like it would be both cumbersome and suspicion-arousing to the man-in-the-middle if every message and its response has a media file attachment.

I don't currently have anything worth hiding and I am not doing anything illegal, but I value my privacy and I am very interested in cryptography.

An example would be:

Sender apparent message: Hi there Bob. How was your weekend? Mine was ... more text ... ciao

Sender real message: Did you find the password I requested?

Receiver apparent message: Pretty good. I caught up with ... more text ... ciao

Receiver real message: Yes, it was "password"

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You may find "chaffing and winnowing" to be of interest, though it isn't precisely steganography. –  Brian Sep 21 '12 at 17:31
@Brian thanks for the lead. i read the wikipedia article but it doesn't look quite like what i'm after. there was a link to the "null cypher" though which may be closer to what i am after... –  mulllhausen Sep 24 '12 at 1:59
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4 Answers

up vote 9 down vote accepted

Yes, there exists algorithms that hide messages inside messages that can look quite innocent. Take for instance spammimic. It gives the possibility to hide your message inside a typical looking spam message.

A google search for "Steganography hiding text in text" gives you more research and examples around this.

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+1 for spammimic. It rocks :) –  Polynomial Sep 20 '12 at 8:02
Yeah it's a nice service, but the message is really long for such a short text to hide. –  george_h Sep 22 '12 at 9:21
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My personal (maybe biased though) opinion is that spammimic isn't very "natural". A humble attempt of mine is to use the number of words in a line of emails or similar text documents, e.g. HTML source files, where one normally doesn't care too much about the ruggedness of the line ends, to transmit one stego bit. (A Python code to help do that formatting is available under the name EMAILSTEGANO in http://s13.zetaboards.com/Crypto/index/) Its bit rate is of course unfortunately very low. On the other hand occasionally very short stego messages could be sufficient for one's purposes. Note that for hand-written texts, the said problem of more or less unsatisfactory ruggedness of the line ends may even completely disappear, if corresponding care is taken in writing.

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i reckon it would be a fun project to extend your idea into a more efficient form. for example you could use the number of words, the number of capitals, the number of "accidental" double spaces, etc to encode far more than one bit per line. –  mulllhausen Sep 24 '12 at 2:11
leetspeak may also provide a steganography approach of this type –  mulllhausen Sep 24 '12 at 4:56
@mulllhausen: Simulating typos, I guess, may not be very simple to automate and, anyway, if not used very sparely, could raise the suspicion of the warden. –  Mok-Kong Shen Sep 24 '12 at 8:19
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I have a brilliant example for you! I've recently seen ONE application of steganography being used to hide a text message within a text document.

There is a National Geographic video on YouTube regarding the Aryan Brotherhood and how they use to communicate while in prison, across the nation. The gang was created inside a maximum security prison in California, and managed from other super max prisons. They are the most violent gang in prison and while only making up 1/10 of 1% of the population are responsible for more than 20% of the murders that take place within the prisons.

The steganographic technology that they employed was a bi-literal cipher developed 400 years ago by Sir Francis Bacon and was broken by a multi-jurisdictional federal organization including experts at the FBI, NSA and other orgs. Naturally, you cannot use this technology since it has been broken, but some of the logic behind it is still solid.

You really need to see the video if you do not understand what I am stating here. As stated, the texts are meshed together. In this technology, one "alphabet" is written in plain block letters, and the other "alphabet" is written in cursive. The plain block letters become As, and the cursive letters become Bs. Then the letters are arranged in groups of five, and they must then be deciphered using a key.

I know this sounds easy to break should it be posted on the Internet but there are some very close fonts that may make this a capable technology unless every document is poured over, and any document may contain dozens of fonts. One way to hide the font changes would be to place the different fonts in a PDF document, or image, and a special technology would be necessary to extract the different fonts, something which is not common with most OCR software.

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it sounds like a good technique. unicode equivalent letters might also work in a similar fashion as the block vs cursive in their cipher. –  mulllhausen Nov 26 '13 at 0:25
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Steganographical methods of the kind mentioned in my previous answer are commonly termed syntactic ones. I like to mention another syntactic method, due to Rhinedahl, which can be explained as follows:

Let the stego message be a string where each character is coded as 5 bits. One attempts to write for each set of 5 bits a sentence for the cover text according to a rule e.g.:

1st bit = number of noun phrases in the sentence modulo 2.

2nd bit = number of adjectives modulo 2.

3rd bit = number of adverbs modulo 2.

4th bit = number of clauses modulo 2.

5th bit = was the main verb transitive (=1) or intransitive (=0)?

This obviously has a much higher stego bit rate than my humble scheme EMAILSTEGANO. The method is in fact not too difficult for manual work. However, it is not feasible to fully automate it. The best one could do would IMHO be a software that employs AI techniques (NLP) to determine the required grammatical informations from given sentences and that provides good interactions with the user to deal with the actual issues of stego encoding.

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how come you have chose 5 bits? for a more secure transmission i would probably want to send the output from a command like gpg -ca file.txt hidden steganographically within the text. since this output is ascii and usually contains characters like '+', '=' and '/' then i'm guessing 8 bits would be needed per character (since ascii is 0-255). it seems to me like efficiency (measured as something like the ratio of hidden characters to visible sentences) is going to be the main challenge. –  mulllhausen Sep 26 '12 at 5:06
The 5-bit coding corresponds to cases e.g. in classical crypto where the text is confined to be in an alphabet of 26 characters and without spaces. In the general case you simply have a bit string of total length n. Then you could just take 5 bit groups from it. Anyway the example rule I gave serves to illustrate the basic idea. I suppose one could modify or generalize it to better suit one's particular needs. –  Mok-Kong Shen Sep 26 '12 at 20:14
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