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mce_qd: compute check matrix on the fly

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codecrypt is usually invoked only with one prepare() and decrypt(), so
this basically saves several megs of memory and cuts needed computation
time in half.
This commit is contained in:
Mirek Kratochvil 2013-05-18 10:52:57 +02:00
parent 79b17b8724
commit 8c185c51ad
2 changed files with 22 additions and 21 deletions
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35
src/mce_qd.cpp
View file

@ -241,7 +241,7 @@ int privkey::prepare()
std::set<uint> used;
used.clear();
polynomial g, tmp;
polynomial tmp;
g.clear();
g.resize (1, 1); //g(x)=1
tmp.clear();
@ -294,7 +294,7 @@ int privkey::prepare()
// prepare permuted support, from that prepare permuted check matrix
// (so that it can be applied directly)
uint pos, blk_perm;
std::vector<uint> sbl1, sbl2, permuted_support;
std::vector<uint> sbl1, sbl2;
sbl1.resize (block_size);
sbl2.resize (block_size);
@ -318,17 +318,6 @@ int privkey::prepare()
permuted_support[j + pos * block_size] = sbl2[j];
}
//prepare Hc
Hc.resize (block_size * block_count);
for (i = 0; i < block_size * block_count; ++i) {
Hc[i].resize (block_size * 2);
Hc[i][0] = fld.inv (g.eval (permuted_support[i], fld) );
Hc[i][0] = fld.mult (Hc[i][0], Hc[i][0]);
for (j = 1; j < 2 * block_size; ++j)
Hc[i][j] = fld.mult (permuted_support[i],
Hc[i][j - 1]);
}
//convert the permuted support to actual lookup
support_pos.clear();
//fld.n in support lookup means that it isn't there (we don't have -1)
@ -419,15 +408,27 @@ int privkey::decrypt (const bvector & in, bvector & out, bvector & errors)
{
if (in.size() != cipher_size() ) return 2;
polynomial synd;
uint i;
uint i, j, tmp;
/*
* compute the syndrome from alternant check matrix
* that is H_alt = Vdm(L) * Diag(g(L_i)^{-2})
*/
uint h_size = 1 << (T + 1); //= 2*block_size
synd.clear();
for (i = 0; i < cipher_size(); ++i)
if (in[i]) synd.add (Hc[i], fld);
synd.resize (h_size, 0);
for (i = 0; i < cipher_size(); ++i) if (in[i]) {
tmp = fld.inv (g.eval (permuted_support[i], fld) );
tmp = fld.mult (tmp, tmp); //g(Li)^{-2}
synd[0] = fld.add (synd[0], tmp);
for (j = 1; j < h_size; ++j) {
tmp = fld.mult (tmp, permuted_support[i]);
synd[j] = fld.add (synd[j], tmp);
}
}
//decoding
polynomial loc;
//compute_alternant_error_locator (synd, fld, g, loc);
compute_alternant_error_locator (synd, fld, 1 << T, loc);
bvector ev;

8
src/mce_qd.h
View file

@ -52,10 +52,10 @@ public:
permutation hperm; //block permutation of H block used to get G
//derivable stuff
//cols of check matrix of g^2(x)
std::vector<polynomial> Hc;
//pre-permuted positions of support rows
std::vector<uint> support_pos;
//pre-permuted positions of support rows and support content
std::vector<uint> support_pos, permuted_support;
//generating polynomial
polynomial g;
int decrypt (const bvector&, bvector&);
int decrypt (const bvector&, bvector&, bvector&);