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# gf256.cpp

    00001 // gf256.cpp - written and placed in the public domain by Wei Dai
    00002
    00003 #include "pch.h"
    00004 #include "gf256.h"
    00005
    00006 NAMESPACE_BEGIN(CryptoPP)
    00007
    00008 GF256::Element GF256::Multiply(Element a, Element b) const
    00009 {
    00010         word result = 0, t = b;
    00011
    00012         for (unsigned int i=0; i<8; i++)
    00013         {
    00014                 result <<= 1;
    00015                 if (result & 0x100)
    00016                         result ^= m_modulus;
    00017
    00018                 t <<= 1;
    00019                 if (t & 0x100)
    00020                         result ^= a;
    00021         }
    00022
    00023         return (GF256::Element) result;
    00024 }
    00025
    00026 GF256::Element GF256::MultiplicativeInverse(Element a) const
    00027 {
    00028         Element result = a;
    00029         for (int i=1; i<7; i++)
    00030                 result = Multiply(Square(result), a);
    00031         return Square(result);
    00032 }
    00033
    00034 NAMESPACE_END