## File: gf2_32.cpp

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libcrypto++ 5.6.4-7
 `123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899` ``````// gf2_32.cpp - written and placed in the public domain by Wei Dai #include "pch.h" #include "misc.h" #include "gf2_32.h" NAMESPACE_BEGIN(CryptoPP) GF2_32::Element GF2_32::Multiply(Element a, Element b) const { word32 table[4]; table[0] = 0; table[1] = m_modulus; if (a & 0x80000000) { table[2] = m_modulus ^ (a<<1); table[3] = a<<1; } else { table[2] = a<<1; table[3] = m_modulus ^ (a<<1); } #if CRYPTOPP_FAST_ROTATE(32) b = rotrFixed(b, 30U); word32 result = table[b&2]; for (int i=29; i>=0; --i) { b = rotlFixed(b, 1U); result = (result<<1) ^ table[(b&2) + (result>>31)]; } return (b&1) ? result ^ a : result; #else word32 result = table[(b>>30) & 2]; for (int i=29; i>=0; --i) result = (result<<1) ^ table[((b>>i)&2) + (result>>31)]; return (b&1) ? result ^ a : result; #endif } GF2_32::Element GF2_32::MultiplicativeInverse(Element a) const { if (a <= 1) // 1 is a special case return a; // warning - don't try to adapt this algorithm for another situation word32 g0=m_modulus, g1=a, g2=a; word32 v0=0, v1=1, v2=1; assert(g1); while (!(g2 & 0x80000000)) { g2 <<= 1; v2 <<= 1; } g2 <<= 1; v2 <<= 1; g0 ^= g2; v0 ^= v2; while (g0 != 1) { if (g1 < g0 || ((g0^g1) < g0 && (g0^g1) < g1)) { assert(BitPrecision(g1) <= BitPrecision(g0)); g2 = g1; v2 = v1; } else { assert(BitPrecision(g1) > BitPrecision(g0)); g2 = g0; g0 = g1; g1 = g2; v2 = v0; v0 = v1; v1 = v2; } while ((g0^g2) >= g2) { assert(BitPrecision(g0) > BitPrecision(g2)); g2 <<= 1; v2 <<= 1; } assert(BitPrecision(g0) == BitPrecision(g2)); g0 ^= g2; v0 ^= v2; } return v0; } NAMESPACE_END ``````