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170
cristyffb69ed2010-12-25 00:06:48 +0000[diff] [blame^]171<h1>ImageMagikc Enciphering and Deciphering Images</h2>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]172<p class="navigation-index">[<a href="#intro">Introduction to Enciphering and Deciphering Images</a> &bull; <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>
173
174<h2><a name="intro"></a>Introduction to Enciphering and Deciphering Images</h2>
175<div class="doc-section">
176
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]177<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]178
179<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>
180
181</div>
182
183<h2><a name="encipher"></a>Encipher an Image</h2>
184<div class="doc-section">
185
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]186<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]187
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]188<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher passphrase.txt rose.png</span></p>
189<p>Here we encipher an image using another image as the passphrase:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]190
191<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher smiley.gif rose.png</span></p>
192</div>
193
194<h2><a name="decipher"></a>Decipher an Image</h2>
195<div class="doc-section">
196
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]197<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]198
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]199<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]200</div>
201
202<h2><a name="caveats"></a>Encipher and Decipher Caveats</h2>
203<div class="doc-section">
204
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]205<p>Some formats do not support enciphered pixels-- the JPEG or GIF format, for
206example. To ensure your image format is supported, encipher a test image and
207verify you can restore its original content <em>before</em> you encipher any
208additional images in that format.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]209
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]210<p>The image format also should be 8-bit and RGB (TrueColor). As such you may
211like to include the options "-depth 8 -type TrueColor" before the output
212filename. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]213
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]214<p>The passphrase can be any combinations of letters and symbols. It should
215be a minimum of 12 character combinations to help ensure your image remains
216private. Also make sure your passphrase file permissions prevent others from
217reading it otherwise unintended users may be able to view the original image
218content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]219
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]220<p>You can only restore the original image content if you know your
221passphrase. If you lose or forget it, your original image content is lost
222forever.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]223
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]224<p>ImageMagick only scrambles the image pixels. The image metadata remains
225untouched and readable by anyone with access to the image file.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]226
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]227<p>ImageMagick uses the <a
228href="http://en.wikipedia.org/wiki/Advanced_Encryption_Standard">AES</a>
229cipher in cipher feedback mode. We use the passphrase to derive the cipher
230key and the required nonce.</p>
231
232<p>Currently only ImageMagick can restore your enciphered image content. We
233use a standard cipher and mode so other vendors are encouraged to support
234enciphered image content.</p>
235
236<p>Some small practical examples of image enciphering can be found in IM
237Examples <A HREF="http://www.imagemagick.org/Usage/transform/#encipher"
238>Encrypting Image Data</A>. </p>
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