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Do not invoke fe_is_zero on failed set_b32_limit #1316

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merged 1 commit into from
May 23, 2023
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Do not invoke fe_is_zero on failed set_b32_limit #1316

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7 changes: 2 additions & 5 deletions src/ecmult_gen_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -87,7 +87,6 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
secp256k1_fe s;
unsigned char nonce32[32];
secp256k1_rfc6979_hmac_sha256 rng;
int overflow;
unsigned char keydata[64];
if (seed32 == NULL) {
/* When seed is NULL, reset the initial point and blinding value. */
Expand All @@ -106,11 +105,9 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
memcpy(keydata + 32, seed32, 32);
secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, 64);
memset(keydata, 0, sizeof(keydata));
/* Accept unobservably small non-uniformity. */
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@real-or-random real-or-random May 19, 2023 •
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nit: maybe bring that comment back (ideally one line below, i.e., above the secp256k1_fe_set_b32_mod call )

edit: Okay, ignore me. Now that we have two separate functions, calling the _mod variant makes the fact that we accept non-uniformity always explicit. That is, the code is self-documenting and the comment is superfluous.

secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
overflow = !secp256k1_fe_set_b32_limit(&s, nonce32);
overflow |= secp256k1_fe_is_zero(&s);
secp256k1_fe_cmov(&s, &secp256k1_fe_one, overflow);
secp256k1_fe_set_b32_mod(&s, nonce32);
secp256k1_fe_cmov(&s, &secp256k1_fe_one, secp256k1_fe_normalizes_to_zero(&s));
/* Randomize the projection to defend against multiplier sidechannels.
Do this before our own call to secp256k1_ecmult_gen below. */
secp256k1_gej_rescale(&ctx->initial, &s);
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