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ecmult_multi: reduce strauss memory usage by 30% #1761

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23 changes: 19 additions & 4 deletions src/ecmult_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -220,9 +220,24 @@ static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a,
return last_set_bit + 1;
}

/* Same as secp256k1_ecmult_wnaf, but stores to int8_t array. Requires w <= 8. */
static int secp256k1_ecmult_wnaf_small(int8_t *wnaf, int len, const secp256k1_scalar *a, int w) {
int wnaf_tmp[256];
int ret, i;

VERIFY_CHECK(2 <= w && w <= 8);
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@real-or-random real-or-random Nov 7, 2025

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Suggested change
VERIFY_CHECK(2 <= w && w <= 8);
VERIFY_CHECK(2 <= w && w <= 7);

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@jonasnick jonasnick Nov 12, 2025

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I don't see why w = 8 wouldn't work. The documentation of wnaf says

 *  - each wnaf[i] is either 0, or an odd integer between -(1<<(w-1) - 1) and (1<<(w-1) - 1)

So for w = 8, wnaf[i] is in [-127, 127] which fits in an int8_t.

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@real-or-random real-or-random Nov 12, 2025

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Sorry, yes, you're right. I was getting confused. secp256k1_ecmult_wnaf itself needs w <= 31 (and not 32), if only because it performs a carry << w shift (for int carry) which is certainly UB if int is 32 bits. (In fact, if carry == 1, then even 1 << 31 is UB. This is another edge case that we should fix! Let me add this to the other issue.)

But since your function only copies the results, everything is fine.

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@jonasnick jonasnick Nov 12, 2025

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In fact, if carry == 1, then even 1 << 31 is UB. This is another edge case that we should fix! Let me add this to the other issue

Oh, great catch!

ret = secp256k1_ecmult_wnaf(wnaf_tmp, len, a, w);

for (i = 0; i < len; i++) {
wnaf[i] = (int8_t)wnaf_tmp[i];
}

return ret;
}

struct secp256k1_strauss_point_state {
int wnaf_na_1[129];
int wnaf_na_lam[129];
int8_t wnaf_na_1[129];
int8_t wnaf_na_lam[129];
int bits_na_1;
int bits_na_lam;
};
Expand Down Expand Up @@ -259,8 +274,8 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
secp256k1_scalar_split_lambda(&na_1, &na_lam, &na[np]);

/* build wnaf representation for na_1 and na_lam. */
state->ps[no].bits_na_1 = secp256k1_ecmult_wnaf(state->ps[no].wnaf_na_1, 129, &na_1, WINDOW_A);
state->ps[no].bits_na_lam = secp256k1_ecmult_wnaf(state->ps[no].wnaf_na_lam, 129, &na_lam, WINDOW_A);
state->ps[no].bits_na_1 = secp256k1_ecmult_wnaf_small(state->ps[no].wnaf_na_1, 129, &na_1, WINDOW_A);
state->ps[no].bits_na_lam = secp256k1_ecmult_wnaf_small(state->ps[no].wnaf_na_lam, 129, &na_lam, WINDOW_A);
VERIFY_CHECK(state->ps[no].bits_na_1 <= 129);
VERIFY_CHECK(state->ps[no].bits_na_lam <= 129);
if (state->ps[no].bits_na_1 > bits) {
Expand Down