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<h1>gf2_32.cpp</h1><pre class="fragment"><div>00001 <span class="comment">// gf2_32.cpp - written and placed in the public domain by Wei Dai</span>
00002 
00003 <span class="preprocessor">#include "pch.h"</span>
00004 <span class="preprocessor">#include "misc.h"</span>
00005 <span class="preprocessor">#include "gf2_32.h"</span>
00006 
00007 NAMESPACE_BEGIN(CryptoPP)
00008 
00009 <a class="code" href="class_g_f2__32.html">GF2_32</a>::Element <a class="code" href="class_g_f2__32.html">GF2_32</a>::Multiply(Element a, Element b)<span class="keyword"> const</span>
00010 <span class="keyword"></span>{
00011         word32 table[4];
00012         table[0] = 0;
00013         table[1] = m_modulus;
00014         <span class="keywordflow">if</span> (a &amp; 0x80000000)
00015         {
00016                 table[2] = m_modulus ^ (a&lt;&lt;1);
00017                 table[3] = a&lt;&lt;1;
00018         }
00019         <span class="keywordflow">else</span>
00020         {
00021                 table[2] = a&lt;&lt;1;
00022                 table[3] = m_modulus ^ (a&lt;&lt;1);
00023         }
00024 
00025 <span class="preprocessor">#ifdef FAST_ROTATE</span>
00026 <span class="preprocessor"></span>        b = rotrFixed(b, 30U);
00027         word32 result = table[b&amp;2];
00028 
00029         <span class="keywordflow">for</span> (<span class="keywordtype">int</span> i=29; i&gt;=0; --i)
00030         {
00031                 b = rotlFixed(b, 1U);
00032                 result = (result&lt;&lt;1) ^ table[(b&amp;2) + (result&gt;&gt;31)];
00033         }
00034 
00035         <span class="keywordflow">return</span> (b&amp;1) ? result ^ a : result;
00036 <span class="preprocessor">#else</span>
00037 <span class="preprocessor"></span>        word32 result = table[(b&gt;&gt;30) &amp; 2];
00038 
00039         <span class="keywordflow">for</span> (<span class="keywordtype">int</span> i=29; i&gt;=0; --i)
00040                 result = (result&lt;&lt;1) ^ table[((b&gt;&gt;i)&amp;2) + (result&gt;&gt;31)];
00041 
00042         <span class="keywordflow">return</span> (b&amp;1) ? result ^ a : result;
00043 <span class="preprocessor">#endif</span>
00044 <span class="preprocessor"></span>}
00045 
00046 GF2_32::Element GF2_32::MultiplicativeInverse(Element a)<span class="keyword"> const</span>
00047 <span class="keyword"></span>{
00048         <span class="keywordflow">if</span> (a &lt;= 1)             <span class="comment">// 1 is a special case</span>
00049                 <span class="keywordflow">return</span> a;
00050 
00051         <span class="comment">// warning - don't try to adapt this algorithm for another situation</span>
00052         word32 g0=m_modulus, g1=a, g2=a;
00053         word32 v0=0, v1=1, v2=1;
00054 
00055         assert(g1);
00056 
00057         <span class="keywordflow">while</span> (!(g2 &amp; 0x80000000))
00058         {
00059                 g2 &lt;&lt;= 1;
00060                 v2 &lt;&lt;= 1;
00061         }
00062 
00063         g2 &lt;&lt;= 1;
00064         v2 &lt;&lt;= 1;
00065 
00066         g0 ^= g2;
00067         v0 ^= v2;
00068 
00069         <span class="keywordflow">while</span> (g0 != 1)
00070         {
00071                 <span class="keywordflow">if</span> (g1 &lt; g0 || ((g0^g1) &lt; g0 &amp;&amp; (g0^g1) &lt; g1))
00072                 {
00073                         assert(BitPrecision(g1) &lt;= BitPrecision(g0));
00074                         g2 = g1;
00075                         v2 = v1;
00076                 }
00077                 <span class="keywordflow">else</span>
00078                 {
00079                         assert(BitPrecision(g1) &gt; BitPrecision(g0));
00080                         g2 = g0; g0 = g1; g1 = g2;
00081                         v2 = v0; v0 = v1; v1 = v2;
00082                 }
00083 
00084                 <span class="keywordflow">while</span> ((g0^g2) &gt;= g2)
00085                 {
00086                         assert(BitPrecision(g0) &gt; BitPrecision(g2));
00087                         g2 &lt;&lt;= 1;
00088                         v2 &lt;&lt;= 1;
00089                 }
00090 
00091                 assert(BitPrecision(g0) == BitPrecision(g2));
00092                 g0 ^= g2;
00093                 v0 ^= v2;
00094         }
00095 
00096         <span class="keywordflow">return</span> v0;
00097 }
00098 
00099 NAMESPACE_END
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