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gerrit-public.fairphone.software / platform / external / ImageMagick / 0113ca241ec6d58d2227aa3cfcf82a9cc139a47d / . / www / cipher.html
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cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]1
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
cristy2bd2a982011-02-15 00:52:58 +0000[diff] [blame]4<html version="-//W3C//DTD XHTML 1.1//EN"
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cristyffb69ed2010-12-25 00:06:48 +0000[diff] [blame]12 <title>ImageMagick: Encipher or Decipher an Image</title>
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cristye6e84152011-06-06 14:16:49 +0000[diff] [blame]17 <meta name="Description" content="Use ImageMagick to convert, edit, or compose bitmap images in a variety of formats. In addition resize, rotate, shear, distort and transform images automagically."/>
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cristy3eaa0ef2010-03-06 20:35:26 +0000[diff] [blame]51</head>
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217<div class="main">
218
cristy2bd2a982011-02-15 00:52:58 +0000[diff] [blame]219<h1>Enciphering and Deciphering Images</h1>
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]220<p class="navigation-index">[<a href="#encipher">Encipher an Image</a> &bull; <a href="#decipher">Decipher an Image</a> &bull; <a href="#caveats">Encipher and Decipher Caveats</a>]</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]221
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]222<div class="doc-section">
223
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]224<p>Most images, by design, are made to be viewed often and by many people. Web images, for example, may be viewed hundreds of times a day by a multitude of web vistors. However, in some cases, you may want to keep a particular image private so that only you or perhaps a select group of your friends or web visitors can view it. ImageMagick permits you to scramble your images such that unless someone knows your passphrase, they will be unable to view the original content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]225
226<p>You could use an <a href="http://www.wizards-toolkit.org/www/encipher.html">enciphering</a> utility to scramble your image but they typically scramble the entire file making it unrecognizable as an image format. With ImageMagick, only the pixels are scrambled. The scrambled image continues to be recognized as an image and will even display in your web page. However, the content appears as gibberish, nothing like the original content.</p>
227
228</div>
229
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]230<h2><a id="encipher"></a>Encipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]231<div class="doc-section">
232
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]233<p>Use the <a href="../www/command-line-options.html#encipher">-encipher</a> option to scramble your image so that it is unrecognizable. The option requires a filename that contains your passphrase. In this example we scramble an image and save it in the PNG format:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]234
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]235<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher passphrase.txt rose.png</span></p>
236<p>Here we encipher an image using another image as the passphrase:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]237
238<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher smiley.gif rose.png</span></p>
239</div>
240
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]241<h2><a id="decipher"></a>Decipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]242<div class="doc-section">
243
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]244<p>Use the <a href="../www/command-line-options.html#decipher">-decipher</a> option to unscramble your image so that it is recognizable once again. The option requires a filename that contains your passphrase. In this example we unscramble an image and save it in the JPEG format:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]245
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]246<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.png -decipher passphrase.txt rose.jpg</span></p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]247</div>
248
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]249<h2><a id="caveats"></a>Encipher and Decipher Caveats</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]250<div class="doc-section">
251
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]252<p>Some formats do not support enciphered pixels-- the JPEG or GIF format, for
253example. To ensure your image format is supported, encipher a test image and
254verify you can restore its original content <em>before</em> you encipher any
255additional images in that format.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]256
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]257<p>The image format also should be 8-bit and RGB (TrueColor). As such you may
258like to include the options "-depth 8 -type TrueColor" before the output
259filename. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]260
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]261<p>The passphrase can be any combinations of letters and symbols. It should
262be a minimum of 12 character combinations to help ensure your image remains
263private. Also make sure your passphrase file permissions prevent others from
264reading it otherwise unintended users may be able to view the original image
265content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]266
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]267<p>You can only restore the original image content if you know your
268passphrase. If you lose or forget it, your original image content is lost
269forever.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]270
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]271<p>ImageMagick only scrambles the image pixels. The image metadata remains
272untouched and readable by anyone with access to the image file.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]273
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]274<p>ImageMagick uses the <a
275href="http://en.wikipedia.org/wiki/Advanced_Encryption_Standard">AES</a>
276cipher in cipher feedback mode. We use the passphrase to derive the cipher
277key and the required nonce.</p>
278
279<p>Currently only ImageMagick can restore your enciphered image content. We
280use a standard cipher and mode so other vendors are encouraged to support
281enciphered image content.</p>
282
283<p>Some small practical examples of image enciphering can be found in IM
cristy2bd2a982011-02-15 00:52:58 +0000[diff] [blame]284Examples <a href="http://www.imagemagick.org/Usage/transform/#encipher"
285>Encrypting Image Data</a>. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]286
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