diff --git a/src/ecmult_gen_impl.h b/src/ecmult_gen_impl.h index b3e080a58b..4f5ea9f3c0 100644 --- a/src/ecmult_gen_impl.h +++ b/src/ecmult_gen_impl.h @@ -111,7 +111,8 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const overflow = !secp256k1_fe_set_b32(&s, nonce32); overflow |= secp256k1_fe_is_zero(&s); secp256k1_fe_cmov(&s, &secp256k1_fe_one, overflow); - /* Randomize the projection to defend against multiplier sidechannels. */ + /* 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); secp256k1_fe_clear(&s); secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32); @@ -120,6 +121,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const secp256k1_scalar_cmov(&b, &secp256k1_scalar_one, secp256k1_scalar_is_zero(&b)); secp256k1_rfc6979_hmac_sha256_finalize(&rng); memset(nonce32, 0, 32); + /* The random projection in ctx->initial ensures that gb will have a random projection. */ secp256k1_ecmult_gen(ctx, &gb, &b); secp256k1_scalar_negate(&b, &b); ctx->blind = b;