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Class: Stegger
Hide encrypted data in images using steganography
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Date: 15 years ago
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Stegger - Simple But Secure Steganography For PHP Introduction Steganography is the art (and science) of hiding data within an image file. The reasons should be fairly obvious, you have something you want to hide, and the best place to hide something is in the last place anyone will look for it, right under thier nose. Simply put, a secret is not a secret if everyone knows about it. How to use it This is a short run through of how to use the class, for anything not covered here read the source code, it is heavily commented. 1. First things first you should either include the Stegger class in your script or paste the contents of the Stegger file into your PHP file, its as simple as.. include_once(''); 2. Now we need to create the Stegger object so we can use it, you can call the object whatever you want, for this example I will be calling it $Stegger. $Stegger = new Stegger(); 3. Before we can get to hiding data in an image we need to know where or what it is, Stegger will take data as a file upload, path or URL to file, plain text messages or multiple files through a glob style string (/*.txt). - Path to local file(s) eg: Linux: /home/something/secretfile.txt /path/to/all/files/like/*.txt Windows: C:/Windows/but/with/bars/secretfile.txt C:/Windows/can/do/this/too/*.txt - URL to remote file ssh2.s - File upload through PHP $_FILES['secretfile'] - Text messages Hey, Meet me at the place at 12! For this example we will be hiding a file uploaded through a form, for more examples please check the optional extras section of this file, please note that you can just pass the file directly and do not need to assign it to another variable, I am just doing it for clarity. $SecretData = $_FILES['secretfile']; 4. Now that we have the data we want to hide we need to get the image, just like with the secret data you can supply an image in different ways, for this example we will be using a URL but you can use paths or file uploads as well. $Image = ''; 5. Stegger allows encryption using a build in encryption scheme that uses a public and private key, the public key can be set in the Stegger source file but you need to provide the private key at run time. $Key = 'Wow this is a very very very long key that I am typing'; 6. Ok its time to let Stegger work its magic, there is only one method to call. $Stegger->Put($SecretData, $Image, $Key); The image will automatically be output to the users browser to be downloaded. If your script is used from the command line or you just want to save the encoded image somewhere on the server you can supply a path to save the image to, if you use the script from the command line you must provide the output path. $Stegger->Put($SecretData, $Image, $Key, '/some/local/path/image.png'); The key and output path parameters are optional, at the very least you just have to provide an image and something to hide. 7. To get our secret data back out of the image we use a method that works much like the Put() method except it does not need the secret data parameter. $Stegger->Get($Image, $Key); If you have multiple files or if you are using a script from the command line you need to specify an output path. $Stegger->Get($Image, $Key, '/save/files/to/this/path/'); Optional extras As you may have noticed above, Stegger is pretty flexible in the way it takes certain parameters, you can provide PHP file upload variables, paths and URLs but there is one more trick up Stegger's sleeve, you can supply more than one file upload, URL, message or path by passing them as an array, for a better understanding look at the small example below. // Instantiate the Stegger object $Stegger = new Stegger(); // We want to hide all the .txt files in this directory and a php file from another $SecretData[] = '/some/path/to/files/*.txt'; $SecretData[] = '/some/path/to/files/file.php'; // Download two files from the internet and hide them $SecretData[] = ''; $SecretData[] = ''; // We will also be hiding a file that was uploaded through a form $SecretData[] = $_FILES['secretfile']; // And we will encode a little message to the person who will decode this $SecretData[] = "Heya,\n You can find the stuff in the thing\nThanks"; // And that is all there is to it $Stegger->Put($SecretData, ''); How it works Steganography isn't as hard as it sounds, there are many methods one can use to hide data within an image, none of them are right or wrong but some have more limitations than others. Before I can get into how it's done in this class I feel it is important to discuss the other methods. By far the easiest way to hide data within an image is to store it in the comment field of the image file, this is not an ideal method at all. Even if you could get past the fact that the data will be trivial to find you will probably find that the 255 bytes of space (on average) is not enough to store anything worthwhile. This method also requires special knowledge of the specific image type of the image you wish to hide the data inside. Another method that is frequently employed for steganography is to utilise the unused space allocated for colour information (a 32 bit colour often only uses 24 bits of space). A variation of this method is to use the space allocated for transparency (the alpha channel, which yields 7 bits of data per pixel) but that depends on the image type and if it uses alpha blending or a real alpha channel. Although data hidden with these methods is harder to find it can still be detected by the trained eye or some specialist tools, once someone finds that the unused space has been used they know there is secret data, which means even if your data is protected with encryption, we have failed since all this steganography stuff is about being evasive and keeping the data hidden. The method employed by the Stegger class is different to the above methods in that it does not store data in any empty space within the image. If that does not make much sense to you yet, don't worry, read on. The best way to learn about something is to understand the fundementals of its seperate parts, in steganography there are two parts we need to learn about, the image (in which we will be hiding data) and the data (which we will be hiding inside the image). At the most basic level, all images are made up of pixels (also known by thier not so cool name, picture elements), each pixel is basically a tiny little block that displays a colour and all colours are made up from a combination of the three primary colours, red, green and blue. Each pixel holds information about these three primary colours to work out what colour it should be, each primary colour in a (32 bit or 24 bit) pixel takes up 1 byte of data which means each primary colour is represented by a number between 0 (no colour) and 255 (full colour). So the colour white on your computer screen is not a lack of all colour, rather, it is all the primary colours turned on full blast (red: 255, green: 255, blue: 255). Stegger takes advantage of these image basics to hide data by altering the value of each primary colour in each pixel to either reflect a 1 or a 0, because as you should know, all digital data is just a sequence of 1's and 0's, binary. There are a number of ways to alter a number to represent a 1 or a 0, the method that Stegger (and some other tools) uses is to alter the number to be either odd or even, this allows us to store 1 bit of data in every primary colour which means we store 3 bits of data symbolically within each pixel. Now 3 bits per pixel might not sound like enough to hide data but if you consider that an 800 x 600 pixel image has 480 000 pixels and that we can spread the data through-out some or all of those pixels, we have a viable way of storing data in an image (that 800 x 600 pixel image would allow us to store about 175KB of data with compression that could mean over a Megabyte of text data). The advantage of this method is that the data is stored symbolically which means an attacker has no way of knowing for sure if each pixel is representing some data or if that is infact the way the image was made. There are limitations to this method however, namely, the image needs to be saved in a lossless format or the data we have symbolically stored inside the image will be lost or mangled. This does not mean this method cannot be used for formats like JPEG and GIF, but to work with those formats rather than assigning an odd or even number per primary colour in a pixel you would need to look for the closest odd or even number in the colour index of the image and you would need to make sure there were not too many similar colours in the colour index or they will be lost when the image is saved, it is for this reason that Stegger saves all encoded images as a PNG, which is lossless. Please look at the images in the docs/images/ directory for a graphical summary of what was said above. How is this useful Steganography has a stigma attached to it, its cool but ultimately useless, that may be the case if you have nothing to be evasive about or no one to evade (which means you're either a saint or naive, possibly both) but for everyone else steganography can be not only a cool tool, but a useful one too. Sometimes encryption is not enough, sometimes we want to hide the fact that we even have data to encrypt. Imagine for a minute that you have reason to believe your connection is being sniffed, perhaps its part of your work/univeristy/isp's security policy, if you were to just merely encrypt the data and it was to be intercepted between point A and point B, sure your data would be relatively safe (assuming the person who intercepted it isn't the type to link up a few boxes and try crack the data for 8 months) but you would have let the attacker know something very valuable, that you have something to hide and whilst they may not be able to get the decrypted data, they will have an incentive to keep digging and watch you. If we were to use the same analogy but this time instead of sending an encrypted file you send a joke image to a friend that has your secret data encrypted and encoded inside, the email with the image attached would be sniffed but the attacker would simply laugh it off as one of those silly joke emails that everyone hates and spend his time looking for more obvious targets. The most exciting thing about steganography is the fact that web is littered with images, you could take an encoded image and send it to one of the large free image hosting sites like flickr and imageshack, or make a page on myspace about a fictional person and encode secret data into a misc image on the page and no one would be the wiser, it could be the most public of places, a million people could look at the image everyday and nobody would even look twice. Security It has been said that security through obscurity is no security at all, whilst I think we can all fundementally agree with that point, it needs to be said that obscurity does help. Evasion can be a key factor in keeping anything secure, it is hard for an attacker to blow something up if they can't find it and even harder if they don't know for sure that it exists. How many digital images would you say you have seen in your life time? How many times when looking at or glancing over a digital image did you wonder what sort of secret data was encoded inside it? If you are a normal person (which you might not be if you google for steganography applications) you have never wondered about secret data encoded in any images, even if you thought about steganography regularly, there are just too many images on the internet to know which ones to examine closer. That being said, evasion is not enough, and this is why every little piece of data encoded into an image with Stegger is encrypted with a custom (and that means obscure) two way encryption algorithm called Secrypt. Although I can't guarantee anything (as the GNU license states), I would be extremely suprised if an untrusted party was able to discover your secret data inside your image file. I would be even more suprised if that untrusted party was able to crack the encryption (assuming you used a strong key). If the data was discovered by an attacker I would think that the weak link in the security chain was a person and not the steganography method or encyrption scheme used. Disclaimer Unfortunately we aren't all bound by the same laws as we are not governed by the same people with the same ideals (which is probably a good thing). The country in which this application was written and the country in which the author resides have no laws against its citizens possessing knowledge of, or developing tools for, encryption, steganography and other fields of computer science that aid in evasion and/or security. If you are living in a country or state where the use of an application like this is considered illegal (some states in the US might) then no matter how just you may think your reasoning is in believing the laws are prehistoric and ignorant, I emplore you, DO NOT break the law. The author cannot and will not take any responsibility for your actions. Some people seem to consistantly confuse certain technlogies with illicit activities, and like those technologies, this application can be used for good or bad, its use is a reflection apon the user rather than the author, so if you and your drug cartel buddies get caught posting images with secret infromation inside on flickr, do not drag me into court, it is all on you, and if you happen to save the world or something using this application (ok now I'm really stretching it) I can't take responsibility for that either. They are your actions and your consequences. About The Author My name is Warren Smith and if you have anything more worthwhile or interesting for me to be doing other than making stuff like this, I am available. If you have any bug reports, improvements or cool ideas about this application feel free to drop me an email. I can be contacted at smythinc (at) gmail (dot) com Requirements PHP >= 4.3.0 with the Zlib extension