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Remove secret-dependant non-constant time operation in ecmult_const.

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ECMULT_CONST_TABLE_GET_GE was branching on its secret input.

Also makes secp256k1_gej_double_var implemented as a wrapper
 on secp256k1_gej_double_nonzero instead of the other way
 around.  This wasn't a constant time bug but it was fragile
 and could easily become one in the future if the double_var
 algorithm is changed.
This commit is contained in:
Gregory Maxwell
2020-01-08 14:58:28 +00:00
parent f45d897101
commit 2241ae6d14
5 changed files with 38 additions and 33 deletions
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8
src/ecmult_const_impl.h
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@@ -15,8 +15,9 @@
/* 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 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)); \
@@ -172,6 +173,7 @@ 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
@@ -195,7 +197,7 @@ 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];