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171
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173
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]174<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>
175
176<h2><a name="intro"></a>Introduction to Enciphering and Deciphering Images</h2>
177<div class="doc-section">
178
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]179<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]180
181<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>
182
183</div>
184
185<h2><a name="encipher"></a>Encipher an Image</h2>
186<div class="doc-section">
187
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]188<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]189
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]190<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher passphrase.txt rose.png</span></p>
191<p>Here we encipher an image using another image as the passphrase:</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]192
193<p class='crt'><span class="crtprompt"> $magick&gt; </span><span class='crtin'>convert rose.jpg -encipher smiley.gif rose.png</span></p>
194</div>
195
196<h2><a name="decipher"></a>Decipher an Image</h2>
197<div class="doc-section">
198
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]199<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]200
cristybebec402010-04-23 19:19:08 +0000[diff] [blame]201<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]202</div>
203
204<h2><a name="caveats"></a>Encipher and Decipher Caveats</h2>
205<div class="doc-section">
206
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]207<p>Some formats do not support enciphered pixels-- the JPEG or GIF format, for
208example. To ensure your image format is supported, encipher a test image and
209verify you can restore its original content <em>before</em> you encipher any
210additional images in that format.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]211
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]212<p>The image format also should be 8-bit and RGB (TrueColor). As such you may
213like to include the options "-depth 8 -type TrueColor" before the output
214filename. </p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]215
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]216<p>The passphrase can be any combinations of letters and symbols. It should
217be a minimum of 12 character combinations to help ensure your image remains
218private. Also make sure your passphrase file permissions prevent others from
219reading it otherwise unintended users may be able to view the original image
220content.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]221
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]222<p>You can only restore the original image content if you know your
223passphrase. If you lose or forget it, your original image content is lost
224forever.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]225
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]226<p>ImageMagick only scrambles the image pixels. The image metadata remains
227untouched and readable by anyone with access to the image file.</p>
cristy3ed852e2009-09-05 21:47:34 +0000[diff] [blame]228
cristyd0bda5e2010-09-21 16:22:45 +0000[diff] [blame]229<p>ImageMagick uses the <a
230href="http://en.wikipedia.org/wiki/Advanced_Encryption_Standard">AES</a>
231cipher in cipher feedback mode. We use the passphrase to derive the cipher
232key and the required nonce.</p>
233
234<p>Currently only ImageMagick can restore your enciphered image content. We
235use a standard cipher and mode so other vendors are encouraged to support
236enciphered image content.</p>
237
238<p>Some small practical examples of image enciphering can be found in IM
239Examples <A HREF="http://www.imagemagick.org/Usage/transform/#encipher"
240>Encrypting Image Data</A>. </p>
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