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POLY: add pb_mod2_to_modq()

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This should make pb_inverse_poly_q() a bit more readable.
TODO: make the algorithm more descriptive in general.
This commit is contained in:
hasufell 2014-04-20 19:57:45 +02:00
parent 1968b8207f
commit cabc81dfd9
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GPG Key ID: 220CD1C5BDEED020
1 changed files with 42 additions and 23 deletions
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65
src/poly.c
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@ -36,6 +36,9 @@
* static declarations
*/
static unsigned int get_degree(pb_poly const * const poly);
static void pb_mod2_to_modq(pb_poly * const a,
pb_poly *Fq,
ntru_context *ctx);
/**
@ -283,6 +286,43 @@ static unsigned int get_degree(pb_poly const * const poly)
return count;
}
/**
* Find the inverse polynomial modulo a power of 2,
* which is q.
*
* @param a polynomial to invert (is allowed to be the same as param Fq)
* @param Fq polynomial [out]
* @param ctx NTRU context
* @return true/false for success/failure
*/
static void pb_mod2_to_modq(pb_poly * const a,
pb_poly *Fq,
ntru_context *ctx)
{
int v = 2;
while (v < (int)(ctx->q)) {
pb_poly *pb_tmp,
*pb_tmp2;
mp_int tmp_v;
pb_tmp = build_polynom(NULL, ctx->N, ctx);
v = v * 2;
init_integer(&tmp_v);
MP_SET_INT(&tmp_v, v);
pb_tmp2 = build_polynom(NULL, ctx->N, ctx);
MP_SET_INT(&(pb_tmp2->terms[0]), 2);
/* mod after sub or before? */
pb_starmultiply(a, Fq, pb_tmp, ctx, v);
PB_SUB(pb_tmp2, pb_tmp, pb_tmp);
PB_MOD(pb_tmp, &tmp_v, pb_tmp, ctx->N);
pb_starmultiply(Fq, pb_tmp, Fq, ctx, v);
mp_clear(&tmp_v);
delete_polynom_multi(pb_tmp, pb_tmp2, NULL);
}
}
/**
* Invert the polynomial a modulo q.
*
@ -296,8 +336,7 @@ bool pb_inverse_poly_q(pb_poly * const a,
ntru_context *ctx)
{
int k = 0,
j = 0,
v = 2;
j = 0;
pb_poly *a_tmp, *b, *c, *f, *g;
b = build_polynom(NULL, ctx->N + 1, ctx);
@ -347,27 +386,7 @@ OUT_OF_LOOP:
MP_COPY(&(b->terms[i]), &(Fq->terms[j]));
}
while (v < (int)(ctx->q)) {
pb_poly *pb_tmp,
*pb_tmp2;
mp_int tmp_v;
pb_tmp = build_polynom(NULL, ctx->N, ctx);
v = v * 2;
init_integer(&tmp_v);
mp_set_int(&tmp_v, v);
pb_tmp2 = build_polynom(NULL, ctx->N, ctx);
mp_set_int(&(pb_tmp2->terms[0]), 2);
/* hope this does not blow up in our face */
pb_starmultiply(a_tmp, Fq, pb_tmp, ctx, v);
PB_SUB(pb_tmp2, pb_tmp, pb_tmp);
PB_MOD(pb_tmp, &tmp_v, pb_tmp, ctx->N);
pb_starmultiply(Fq, pb_tmp, Fq, ctx, v);
mp_clear(&tmp_v);
delete_polynom(pb_tmp);
delete_polynom(pb_tmp2);
}
pb_mod2_to_modq(a_tmp, Fq, ctx);
for (int i = ctx->N - 1; i >= 0; i--)
if (mp_cmp_d(&(Fq->terms[i]), 0) == MP_LT) {