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Rhys Weatherley
2015-03-25 19:36:25 +10:00
parent fba6ac9417
commit efb2741e4a
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crypto_rng.html
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@@ -110,12 +110,15 @@ Initializing the random number generator</h1>
<div class="line"><span class="preprocessor">#include &lt;TransistorNoiseSource.h&gt;</span></div>
</div><!-- fragment --><p>Next we create a global variable for the noise source and specify the I/O pin that the noise circuit is connected to:</p>
<div class="fragment"><div class="line"><a class="code" href="classTransistorNoiseSource.html">TransistorNoiseSource</a> noise(A1);</div>
</div><!-- fragment --><p>Then in the setup() function we call <a class="el" href="classRNGClass.html#a7f1aab3c324f8e8a424d683425e0fcf8">RNG.begin()</a> to start the random number generator running:</p>
</div><!-- fragment --><p>Then in the setup() function we call <a class="el" href="classRNGClass.html#a7f1aab3c324f8e8a424d683425e0fcf8">RNG.begin()</a> to start the random number generator running and call <a class="el" href="classRNGClass.html#aacf23b192b0e4cc8726d9abe05f5a9db">RNG.addNoiseSource()</a> to register all of the application's noise sources:</p>
<div class="fragment"><div class="line"><span class="keywordtype">void</span> setup() {</div>
<div class="line"> <span class="comment">// Initialize the random number generator with the application tag</span></div>
<div class="line"> <span class="comment">// &quot;MyApp 1.0&quot; and load the previous seed from EEPROM address 500.</span></div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#a7f1aab3c324f8e8a424d683425e0fcf8">begin</a>(<span class="stringliteral">&quot;MyApp 1.0&quot;</span>, 500);</div>
<div class="line"></div>
<div class="line"> <span class="comment">// Add the noise source to the list of sources known to RNG.</span></div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#aacf23b192b0e4cc8726d9abe05f5a9db">addNoiseSource</a>(noise);</div>
<div class="line"></div>
<div class="line"> <span class="comment">// ...</span></div>
<div class="line">}</div>
</div><!-- fragment --><p>The begin() function is passed two arguments: a tag string that should be different for every application and an EEPROM address to use to load and save the random number seed. The tag string ensures that different applications and versions will generate different random numbers upon first boot before the noise source has collected any entropy. If the device also has a unique serial number or a MAC address, then those can be mixed in during the setup() function after calling begin():</p>
@@ -123,17 +126,15 @@ Initializing the random number generator</h1>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#a7f1aab3c324f8e8a424d683425e0fcf8">begin</a>(<span class="stringliteral">&quot;MyApp 1.0&quot;</span>, 500);</div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#ad99535ea23ae2fec55bdebb8c24def02">stir</a>(serial_number, <span class="keyword">sizeof</span>(serial_number));</div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#ad99535ea23ae2fec55bdebb8c24def02">stir</a>(mac_address, <span class="keyword">sizeof</span>(mac_address));</div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#aacf23b192b0e4cc8726d9abe05f5a9db">addNoiseSource</a>(noise);</div>
<div class="line"> ...</div>
<div class="line">}</div>
</div><!-- fragment --><p>The random number generator needs 49 bytes of EEPROM space at the specified address to store the previous seed. When the system is started next time, the previous saved seed is loaded and then deliberately overwritten with a new seed. This ensures that the device will not accidentally generate the same sequence of random numbers if it is restarted before a new seed can be saved.</p>
<p>By default the seed is saved once an hour, although this can be changed with <a class="el" href="classRNGClass.html#a5848e87a5f2f0302c88b0377f0e3366d">RNG.setAutoSaveTime()</a>. Because the device may be restarted before the first hour expires, there is a special case in the code: the first time that the entropy pool fills up, a save will be automatically forced.</p>
<p>To use the random number generator properly, there are some regular tasks that must be performed every time around the application's main loop(). Newly accumulated noise must be mixed in with <a class="el" href="classRNGClass.html#ad99535ea23ae2fec55bdebb8c24def02">RNG.stir()</a> and the <a class="el" href="classRNGClass.html#a8cb91e39f0c4591de5bf98b1e2880b13">RNG.loop()</a> function must be called to perform auto-saves:</p>
<p>To use the random number generator properly, there are some regular tasks that must be performed every time around the application's main loop(). Newly accumulated noise must be mixed in and auto-saves must be performed on a regular basis. The <a class="el" href="classRNGClass.html#a8cb91e39f0c4591de5bf98b1e2880b13">RNG.loop()</a> function takes care of these tasks for us:</p>
<div class="fragment"><div class="line"><span class="keywordtype">void</span> loop() {</div>
<div class="line"> <span class="comment">// ...</span></div>
<div class="line"></div>
<div class="line"> <span class="comment">// If the noise source has accumulated new entropy, then stir it in.</span></div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#ad99535ea23ae2fec55bdebb8c24def02">stir</a>(noise);</div>
<div class="line"></div>
<div class="line"> <span class="comment">// Perform regular housekeeping on the random number generator.</span></div>
<div class="line"> RNG.<a class="code" href="classRNGClass.html#a8cb91e39f0c4591de5bf98b1e2880b13">loop</a>();</div>
<div class="line"></div>
@@ -181,7 +182,7 @@ Destroying secret data</h1>
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