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gerrit-public.fairphone.software / platform / external / ImageMagick / dcca3fa3bc535a40a90c681bf07f2379fc2f3f76 / . / www / cipher.html
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cristyffb69ed2010-12-25 00:06:48 +0000[diff] [blame]8 <title>ImageMagick: Encipher or Decipher an Image</title>
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174
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame^]175<h1>Enciphering and Deciphering Images</h2>
176<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]177
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]178<div class="doc-section">
179
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]180<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]181
182<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>
183
184</div>
185
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame^]186<h2><a id="encipher"></a>Encipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]187<div class="doc-section">
188
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]189<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]190
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]191<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher passphrase.txt rose.png</span></p>
192<p>Here we encipher an image using another image as the passphrase:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]193
194<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher smiley.gif rose.png</span></p>
195</div>
196
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame^]197<h2><a id="decipher"></a>Decipher an Image</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]198<div class="doc-section">
199
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]200<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]201
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]202<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]203</div>
204
cristydcca3fa2010-12-29 21:50:07 +0000[diff] [blame^]205<h2><a id="caveats"></a>Encipher and Decipher Caveats</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]206<div class="doc-section">
207
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]208<p>Some formats do not support enciphered pixels-- the JPEG or GIF format, for
209example. To ensure your image format is supported, encipher a test image and
210verify you can restore its original content <em>before</em> you encipher any
211additional images in that format.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]212
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]213<p>The image format also should be 8-bit and RGB (TrueColor). As such you may
214like to include the options "-depth 8 -type TrueColor" before the output
215filename. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]216
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]217<p>The passphrase can be any combinations of letters and symbols. It should
218be a minimum of 12 character combinations to help ensure your image remains
219private. Also make sure your passphrase file permissions prevent others from
220reading it otherwise unintended users may be able to view the original image
221content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]222
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]223<p>You can only restore the original image content if you know your
224passphrase. If you lose or forget it, your original image content is lost
225forever.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]226
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]227<p>ImageMagick only scrambles the image pixels. The image metadata remains
228untouched and readable by anyone with access to the image file.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]229
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]230<p>ImageMagick uses the <a
231href="http://en.wikipedia.org/wiki/Advanced_Encryption_Standard">AES</a>
232cipher in cipher feedback mode. We use the passphrase to derive the cipher
233key and the required nonce.</p>
234
235<p>Currently only ImageMagick can restore your enciphered image content. We
236use a standard cipher and mode so other vendors are encouraged to support
237enciphered image content.</p>
238
239<p>Some small practical examples of image enciphering can be found in IM
240Examples <A HREF="http://www.imagemagick.org/Usage/transform/#encipher"
241>Encrypting Image Data</A>. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]242
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