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ENC: fix encryption 

In some very rare cases such as the polynom
  1, -1, -1, 1, 1, -1, -1, 1, -1, -1, 1
the encryption->decryption cycle caused an incorrect result.
This wasn't reproducible for all polynomials, just for some.

Implementing the algorithm manually instead of using
the shortcut through
  fmpz_poly_add(out, out, tmp_poly_msg);
  fmpz_poly_mod_unsigned(out, ctx->q);
seems to have solved the issue.

Still unknown what happened there.
This commit is contained in:
hasufell 2014-05-28 19:44:40 +02:00
parent f17b3fd6d0
commit f0eefe7885
No known key found for this signature in database
GPG Key ID: 220CD1C5BDEED020
1 changed files with 21 additions and 1 deletions
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22
src/encrypt.c
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@ -54,8 +54,28 @@ ntru_encrypt_poly(
fmpz_poly_zero(out);
poly_starmultiply(pub_key, rnd, out, ctx, ctx->q);
fmpz_poly_add(out, out, tmp_poly_msg);
fmpz_poly_mod_unsigned(out, ctx->q);
fmpz_poly_mod(out, ctx->q);
/*
* using the flint functions
* fmpz_poly_add(out, out, tmp_poly_msg);
* fmpz_poly_mod_unsigned(out, ctx->q);
* here instead caused very rare glitches in some cases,
* TODO: investigate
*/
for (uint32_t i = 0; i < ctx->N; i++) {
printf("go ");
fmpz_t e_coeff_i;
fmpz *m_coeff_i = fmpz_poly_get_coeff_ptr(tmp_poly_msg, i);
fmpz_init(e_coeff_i);
fmpz_add_n(e_coeff_i, e_coeff_i, m_coeff_i);
fmpz_mod_ui(e_coeff_i, e_coeff_i, ctx->q);
fmpz_poly_set_coeff_fmpz_n(out, i, e_coeff_i);
}
fmpz_poly_clear(tmp_poly_msg);
}