exa/codecrypt
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gf2m: faster operations using a lookup table

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This commit is contained in:
Mirek Kratochvil 2012-05-13 19:45:16 +02:00
parent 1c2e807f69
commit 022d20aa30
2 changed files with 29 additions and 6 deletions
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2
include/codecrypt.h
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@ -118,6 +118,8 @@ public:
bool create (uint m);
std::vector<uint> log, antilog;
uint add (uint, uint);
uint mult (uint, uint);
uint exp (uint, sint);

33
lib/gf2m.cpp
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@ -70,18 +70,39 @@ bool is_irreducible_gf2_poly (uint p)
return true;
}
uint gf2p_tablemult (uint a, uint b, uint n,
const std::vector<uint>&log,
const std::vector<uint>&antilog)
{
if (! (a && b) ) return 0;
return antilog[ (log[a] + log[b]) % (n - 1) ];
}
bool gf2m::create (uint M)
{
if (M < 1) return false; //too small.
m = M;
n = 1 << m;
if (!n) return false; //too big.
for (uint t = (1 << m)+1, e = 1 << (m+1); t < e; t += 2)
for (uint t = (1 << m) + 1, e = 1 << (m + 1); t < e; t += 2)
if (is_irreducible_gf2_poly (t) ) {
poly = t;
return true;
break;
}
log.resize (n);
antilog.resize (n);
log[0] = n - 1;
antilog[n-1] = 0;
uint xi = 1; //x^0
for (uint i = 0; i < n - 1; ++i) {
log[xi] = i;
antilog[i] = xi;
xi <<= 1; //multiply by x
xi = gf2p_mod (xi, poly);
}
return false;
}
uint gf2m::add (uint a, uint b)
@ -91,7 +112,7 @@ uint gf2m::add (uint a, uint b)
uint gf2m::mult (uint a, uint b)
{
return gf2p_modmult (a, b, poly);
return gf2p_tablemult (a, b, n, log, antilog);
}
uint gf2m::exp (uint a, sint k)
@ -113,8 +134,8 @@ uint gf2m::exp (uint a, sint k)
uint gf2m::inv (uint a)
{
if (n == 2) return a;
return exp (a, n - 2);
if (!a) return 0;
return antilog[ (n-1-log[a]) % (n - 1) ];
}
uint gf2m::sq_root (uint a)