diff options
Diffstat (limited to 'src/ecmult_const_impl.h')
| -rw-r--r-- | src/ecmult_const_impl.h | 49 |
1 files changed, 30 insertions, 19 deletions
diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h index 8411752eb0..6d6d354aa4 100644 --- a/src/ecmult_const_impl.h +++ b/src/ecmult_const_impl.h @@ -14,16 +14,22 @@ /* This is like `ECMULT_TABLE_GET_GE` but is constant time */ #define ECMULT_CONST_TABLE_GET_GE(r,pre,n,w) do { \ - int m; \ - int abs_n = (n) * (((n) > 0) * 2 - 1); \ - int idx_n = abs_n / 2; \ + int m = 0; \ + /* Extract the sign-bit for a constant time absolute-value. */ \ + int mask = (n) >> (sizeof(n) * CHAR_BIT - 1); \ + int abs_n = ((n) + mask) ^ mask; \ + int idx_n = abs_n >> 1; \ secp256k1_fe neg_y; \ VERIFY_CHECK(((n) & 1) == 1); \ VERIFY_CHECK((n) >= -((1 << ((w)-1)) - 1)); \ VERIFY_CHECK((n) <= ((1 << ((w)-1)) - 1)); \ VERIFY_SETUP(secp256k1_fe_clear(&(r)->x)); \ VERIFY_SETUP(secp256k1_fe_clear(&(r)->y)); \ - for (m = 0; m < ECMULT_TABLE_SIZE(w); m++) { \ + /* Unconditionally set r->x = (pre)[m].x. r->y = (pre)[m].y. because it's either the correct one \ + * or will get replaced in the later iterations, this is needed to make sure `r` is initialized. */ \ + (r)->x = (pre)[m].x; \ + (r)->y = (pre)[m].y; \ + for (m = 1; m < ECMULT_TABLE_SIZE(w); m++) { \ /* This loop is used to avoid secret data in array indices. See * the comment in ecmult_gen_impl.h for rationale. */ \ secp256k1_fe_cmov(&(r)->x, &(pre)[m].x, m == idx_n); \ @@ -44,11 +50,11 @@ * * Adapted from `The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar * Multiplications Secure against Side Channel Attacks`, Okeya and Tagaki. M. Joye (Ed.) - * CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlagy Berlin Heidelberg 2003 + * CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlag Berlin Heidelberg 2003 * * Numbers reference steps of `Algorithm SPA-resistant Width-w NAF with Odd Scalar` on pp. 335 */ -static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w, int size) { +static int secp256k1_wnaf_const(int *wnaf, const secp256k1_scalar *scalar, int w, int size) { int global_sign; int skew = 0; int word = 0; @@ -59,8 +65,12 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w, int size) int flip; int bit; - secp256k1_scalar neg_s; + secp256k1_scalar s; int not_neg_one; + + VERIFY_CHECK(w > 0); + VERIFY_CHECK(size > 0); + /* Note that we cannot handle even numbers by negating them to be odd, as is * done in other implementations, since if our scalars were specified to have * width < 256 for performance reasons, their negations would have width 256 @@ -75,12 +85,13 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w, int size) * {1, 2} we want to add to the scalar when ensuring that it's odd. Further * complicating things, -1 interacts badly with `secp256k1_scalar_cadd_bit` and * we need to special-case it in this logic. */ - flip = secp256k1_scalar_is_high(&s); + flip = secp256k1_scalar_is_high(scalar); /* We add 1 to even numbers, 2 to odd ones, noting that negation flips parity */ - bit = flip ^ !secp256k1_scalar_is_even(&s); + bit = flip ^ !secp256k1_scalar_is_even(scalar); /* We check for negative one, since adding 2 to it will cause an overflow */ - secp256k1_scalar_negate(&neg_s, &s); - not_neg_one = !secp256k1_scalar_is_one(&neg_s); + secp256k1_scalar_negate(&s, scalar); + not_neg_one = !secp256k1_scalar_is_one(&s); + s = *scalar; secp256k1_scalar_cadd_bit(&s, bit, not_neg_one); /* If we had negative one, flip == 1, s.d[0] == 0, bit == 1, so caller expects * that we added two to it and flipped it. In fact for -1 these operations are @@ -93,7 +104,7 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w, int size) /* 4 */ u_last = secp256k1_scalar_shr_int(&s, w); - while (word * w < size) { + do { int sign; int even; @@ -109,7 +120,7 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w, int size) wnaf[word++] = u_last * global_sign; u_last = u; - } + } while (word * w < size); wnaf[word] = u * global_sign; VERIFY_CHECK(secp256k1_scalar_is_zero(&s)); @@ -132,7 +143,6 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)]; int i; - secp256k1_scalar sc = *scalar; /* build wnaf representation for q. */ int rsize = size; @@ -140,13 +150,13 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons if (size > 128) { rsize = 128; /* split q into q_1 and q_lam (where q = q_1 + q_lam*lambda, and q_1 and q_lam are ~128 bit) */ - secp256k1_scalar_split_lambda(&q_1, &q_lam, &sc); - skew_1 = secp256k1_wnaf_const(wnaf_1, q_1, WINDOW_A - 1, 128); - skew_lam = secp256k1_wnaf_const(wnaf_lam, q_lam, WINDOW_A - 1, 128); + secp256k1_scalar_split_lambda(&q_1, &q_lam, scalar); + skew_1 = secp256k1_wnaf_const(wnaf_1, &q_1, WINDOW_A - 1, 128); + skew_lam = secp256k1_wnaf_const(wnaf_lam, &q_lam, WINDOW_A - 1, 128); } else #endif { - skew_1 = secp256k1_wnaf_const(wnaf_1, sc, WINDOW_A - 1, size); + skew_1 = secp256k1_wnaf_const(wnaf_1, scalar, WINDOW_A - 1, size); #ifdef USE_ENDOMORPHISM skew_lam = 0; #endif @@ -168,6 +178,7 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) { secp256k1_ge_mul_lambda(&pre_a_lam[i], &pre_a[i]); } + } #endif @@ -191,7 +202,7 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons int n; int j; for (j = 0; j < WINDOW_A - 1; ++j) { - secp256k1_gej_double_nonzero(r, r, NULL); + secp256k1_gej_double_nonzero(r, r); } n = wnaf_1[i]; |