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Author SHA1 Message Date
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37a4e4297f Steghide advanced usage 2024-09-11 22:16:36 +02:00
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2b6d14c354 Steghide basic info 2024-09-08 21:14:28 +02:00
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7c2e4e6b0f steghide 2024-09-05 21:28:44 +02:00
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74f180e26b Intro Steganography 2024-08-30 19:55:56 +02:00
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<a href="../index.html">Previous Page</a></br></br><p><img src="../../assets/img/user.png" width="50px" height="50px"> <ba>Zesc - 2024-08-30</ba></p>
<h1>Other sources of Plausible Deniability: Steganography</h1>
<p>Steganography is the craft of hiding messages. It is a close relative of cryptography, but where cryptography strives to conceal the contents of a messages, steganography attempts to conceal its presence in the first place. Therefore <b>steganography helps avoiding suspicion and providing deniability</b>.</p>
<p>The important difference between the two fields is adherence to <i>Kerckhoffs's principle</i>: the assumption that an algorithm must be publicly known to guarantee that it has no flaws or backdoors. (This is the reason why you should only use open source crypto software and never roll your own crypto.)<br/>Steganography does the opposite by relying on <i>security by obscurity</i>: the method by which you hid your data must be kept secret.</p>
<p>That means that whilst complementary to cryptography, steganography on itself is less secure than the mathematically provable security provided by cryptography. Think of it as tucking away your valuables in secret location versus putting them into a sturdy safe. The safe may draw immediate attention by burglars, but provides reliable resistance to attacks, whilst whether they find your hidden stash is up to chance.</p>
<h2><b>Then why use steganography at all?</b></h2>
<p>In military science, there is the concept of the <i>Integrated Survivability Onion</i> &mdash; in short, it describes the idea that they can't kill you if they don't hit you, that they can't hit you if they don't shoot at you and that they can't shoot at you if they don't see you. The same thing applies to every good digital defense-in-depth approach. Using steganography can't harm you, <b>it just shouldn't be all your rely on</b>. In our example, a hidden safe is better than either option on its own.</p>
<p>The main strength of it is that <b>steganography can conceal metadata</b> to some extent. Metadata (i.e. data about data and communications) is the primary way that state actors identify targets. When you can become guilty by association, <b>your primary concern may be communicating in public without anyone noticing</b> and not the confidentiality of your communications. (In fact, since many cryptographic schemes attest the identity of the sender, e.g. signatures, you should avoid those when looking for <u>plausible</u> deniability in case of compromise.)</p>
<p>However, <b>some form of communication event must always occur</b>, so steganography exploits various side-channels in order to embed additional concealed data. <b>In order to thwart analysis of metadata, communicate through one or multiple uninvolved third parties as dead-drop</b>, preferably such with broad distribution (like popular websites).<!-- Weirdly worded? --></p>
<p>When relying on such third parties, <b>steganography can help circumventing censors</b>. If in adversary controls a critical link in the network and blocks all communications they can't inspect, you need to conceal your encrypted traffic inside of superficially innocuous traffic.</p>
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<h1>Hiding files in images with steghide</h1>
<a href="../index.html">Previous Page</a></br></br><p><img src="../../assets/img/user.png" width="50px" height="50px"> <ba>Zesc - 2024-08-30</ba></p>
<h2><b>Basic Use</b></h2>
<p>steghide is a mature GPL-licensed CLI tool for hiding arbitrary data inside of of image files (and some archaic audio formats). Its official web presence is located at <a href="https://steghide.sourceforge.net/">https://steghide.sourceforge.net/</a>, but it is ubiquitously mirrored in various repositories and package managers (<a href="https://github.com/StegHigh/steghide"></a>. If you use APT, simply install it with</p>
<pre><code class="nim"># apt install steghide</code></pre>
<!-- <p>steghide conceals data inside a larger coverfile in a way that is indistinguishable to first-order statistical analysis. This means that without comparing to exact copy of your original coverfile, there are no anomalies to it.</p> -->
<p>steghide uses subcommands, and the three most important ones are <code class="nim">info</code>, <code class="nim">embed</code> and <code class="nim">extract</code> which will be explained in sequence. For this tutorial, we are going to use the <a href="https://apod.nasa.gov/apod/astropix.html">Astronomy Picture of the Day</a> of 2024-09-08, a beautiful image of the Andromeda galaxy:</p>
<div style="text-align: center"><img src="apod20240824.jpg" style="width:250px;"></div>
<p>Our example file to hide is going to be <a href="https://bitcoin.org/bitcoin.pdf">Bitcoin: A Peer-to-Peer Electronic Cash System</a>, the original paper by Satoshi Nakamoto.</p>
<p>As mentioned above, the cover image must be larger than the file you wish to embed, <b>as a rule of thumb your coverfile should be 20 times larger</b>. We can check how many bytes steghide can embed by using <code class="nim">steghide info <i>file</i></code>. In our case, it tells us that it can embed 232 KB and asks whether an attempt should be made at reading embed data without extracting. As there is nothing embedded yet, we decline with <kbd>n</kbd>.</p>
<pre><samp>"apod20240824.jpg":
format: jpeg
capacity: 232.6 KB
Try to get information about embedded data ? (y/n)
</samp></pre>
<p>The PDF is only 180 KB, so it fits. Next, we use <code class="nim">steghide embed -cf <i>coverfile</i> -ef <i>embedfile</i> -sf <i>destination</i></code> and get asked for a passphrase (make sure to remember it or safe it in a password manager). After re-entering the passphrase (you can set it beforehand with the <code class="nim">-p</code> option) it will write the processed file to the specified destination. You can also leave out the <code class="nim">-sf</code> option, in which case it defaults to overwriting the coverfile. Look at it and try to find a visual difference to the original:</p>
<div style="text-align: center"><img src="output1.jpg" style="width:250px"></div>
<p>It's completely lost in the visual noise. As an experiment, let's try taking the difference of both images with a program like GIMP. This is what it looks like:</p>
<div style="text-align: center"><img src="difference.jpg" style="width:250px"></div>
<p>A difference of zero means a fully black pixel. Try zooming in and you'll see that only a few pixels are slightly lighter than black. These are the pixels that contain parts of your embedded file in their least significant bits..</p>
<div><b>WARNING: changing the output file in anyway will make the embedded file irretrievable. This is especially important for every form of lossy compression!</b></div>
<p>Now let's try to extract the file we've just hid. For this, use <code class="nim">steghide embed -sf <i>input</i></code></p>. This will prompt you for the passphrase. Enter it, and the contents will be extracted into the working directory. In our case we will be asked whether we want to overwrite the file with the same name that already exists. (You can suppress this warning with <code class="nim">-f</code> or by specifying an alternative output destination with <code class="nim">-xf</code>. Here, I used the later so we can confirm the files to be identical with md5sum:
<pre><samp>d56d71ecadf2137be09d8b1d35c6c042 bitcoin.pdf
d56d71ecadf2137be09d8b1d35c6c042 output.pdf</samp></pre>
<p>And they are! This concludes the basic introduction to steghide. Some advanced details follow below, but in most cases, reading the well-written man page suffices.</p>
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<h2><b>Advanced Use</b></h2>
<h3><b>Changing the encryption</b></h3>
<p>As you've seen, steghide needs a passphrase to embed and extract data. (Note: using an empty passphrase can lead to issues extracting.) This passphrase is used to both encrypt your file before embedding and to seed the PRNG by which it selects pixel positions for embedding data. By default, it uses AES-128. You should not rely on steghide for securing your data cryptographically (see the <a href="../index.html">introduction to steganography</a>), and when in doubt you can separately encrypt the file before embedding, but you may still want to change the used algorithm. For this, simply use <code class="nim">-e <i>algorithm</i> <i>mode</i></code></p>. You can list all available modes with <code class="nim">steghide encinfo</code>. So as example, to embed our previous picture using AES-256 in cipher feedback mode, you would use this:
<pre><code class="nim">$ steghide embed -cf apod20240824.jpg -ef bitcoin.pdf -e rijandael-256 cfb </code></pre>
<h3><b>Use steghide recursively</b></h3>
<p>Since steghide works on arbitrary binary data, you can embed steghide's output into another picture, provided the size is sufficient. The 4.8 MB picture below contains a 260 KB waveform audio file which in turns contains 2.7 KB UTF-8 encoded text file. (The passphrase is <kbd>moon</kbd> for both.)</p>
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<h2><b>Setup</b></h2> </br> </br>
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