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gerrit-public.fairphone.software / platform / external / ImageMagick / 7c0ad764b61525786ae18bdc418a02c2482143e1 / . / www / cipher.html
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3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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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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cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]29 <link rel="icon" href="../images/wand.png"/>
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cristyad41c0c2012-10-31 00:49:28 +0000[diff] [blame]31 <link rel="canonical" href="http://www.imagemagick.org" />
32 <link rel="meta" type="application/rdf+xml" title="ICI" href="http://imagemagick.org/ici.rdf"/>
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36 <!-- Optionaly include easing and/or mousewheel plugins -->
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50 <script type="text/javascript">
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55 <!-- ImageMagick style -->
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cristy2bd2a982011-02-15 00:52:58 +0000[diff] [blame]240<h1>Enciphering and Deciphering Images</h1>
cristy751980d2012-06-03 23:18:35 +0000[diff] [blame]241<p class="navigation-index">[<a href="cipher.html#encipher">Encipher an Image</a> &bull; <a href="cipher.html#decipher">Decipher an Image</a> &bull; <a href="cipher.html#caveats">Encipher and Decipher Caveats</a>]</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]242
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]243<div class="doc-section">
244
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]245<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]246
247<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>
248
249</div>
250
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]251<h2><a id="encipher"></a>Encipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]252<div class="doc-section">
253
cristy751980d2012-06-03 23:18:35 +0000[diff] [blame]254<p>Use the <a href="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]255
cristyeed6cb62012-01-28 19:38:34 +0000[diff] [blame]256<p class='crt'><span class="crtprompt"> $ </span><span class='crtin'>convert rose.jpg -encipher passphrase.txt rose.png</span></p>
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]257<p>Here we encipher an image using another image as the passphrase:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]258
cristyeed6cb62012-01-28 19:38:34 +0000[diff] [blame]259<p class='crt'><span class="crtprompt"> $ </span><span class='crtin'>convert rose.jpg -encipher smiley.gif rose.png</span></p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]260</div>
261
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]262<h2><a id="decipher"></a>Decipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]263<div class="doc-section">
264
cristy751980d2012-06-03 23:18:35 +0000[diff] [blame]265<p>Use the <a href="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]266
cristyeed6cb62012-01-28 19:38:34 +0000[diff] [blame]267<p class='crt'><span class="crtprompt"> $ </span><span class='crtin'>convert rose.png -decipher passphrase.txt rose.jpg</span></p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]268</div>
269
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame]270<h2><a id="caveats"></a>Encipher and Decipher Caveats</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]271<div class="doc-section">
272
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]273<p>Some formats do not support enciphered pixels-- the JPEG or GIF format, for
274example. To ensure your image format is supported, encipher a test image and
275verify you can restore its original content <em>before</em> you encipher any
276additional images in that format.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]277
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]278<p>The image format also should be 8-bit and RGB (TrueColor). As such you may
279like to include the options "-depth 8 -type TrueColor" before the output
280filename. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]281
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]282<p>The passphrase can be any combinations of letters and symbols. It should
283be a minimum of 12 character combinations to help ensure your image remains
284private. Also make sure your passphrase file permissions prevent others from
285reading it otherwise unintended users may be able to view the original image
286content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]287
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]288<p>You can only restore the original image content if you know your
289passphrase. If you lose or forget it, your original image content is lost
290forever.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]291
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]292<p>ImageMagick only scrambles the image pixels. The image metadata remains
293untouched and readable by anyone with access to the image file.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]294
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]295<p>ImageMagick uses the <a
296href="http://en.wikipedia.org/wiki/Advanced_Encryption_Standard">AES</a>
297cipher in cipher feedback mode. We use the passphrase to derive the cipher
298key and the required nonce.</p>
299
300<p>Currently only ImageMagick can restore your enciphered image content. We
301use a standard cipher and mode so other vendors are encouraged to support
302enciphered image content.</p>
303
304<p>Some small practical examples of image enciphering can be found in IM
cristy2bd2a982011-02-15 00:52:58 +0000[diff] [blame]305Examples <a href="http://www.imagemagick.org/Usage/transform/#encipher"
306>Encrypting Image Data</a>. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]307
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]308</div>
cristyad41c0c2012-10-31 00:49:28 +0000[diff] [blame]309
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