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ct: Use volatile trick in scalar_cond_negate

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Tim Ruffing 2023-05-10 15:19:38 +02:00 committed by Jonas Nick
parent edcba04c28
commit 3e94289966
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GPG Key ID: 4861DBF262123605
2 changed files with 4 additions and 2 deletions
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3
src/scalar_4x64_impl.h
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@ -180,7 +180,8 @@ static int secp256k1_scalar_is_high(const secp256k1_scalar *a) {
static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
/* If we are flag = 0, mask = 00...00 and this is a no-op;
* if we are flag = 1, mask = 11...11 and this is identical to secp256k1_scalar_negate */
uint64_t mask = !flag - 1;
volatile int vflag = flag;
uint64_t mask = -vflag;
uint64_t nonzero = (secp256k1_scalar_is_zero(r) != 0) - 1;
uint128_t t = (uint128_t)(r->d[0] ^ mask) + ((SECP256K1_N_0 + 1) & mask);
r->d[0] = t & nonzero; t >>= 64;

3
src/scalar_8x32_impl.h
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@ -253,7 +253,8 @@ static int secp256k1_scalar_is_high(const secp256k1_scalar *a) {
static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
/* If we are flag = 0, mask = 00...00 and this is a no-op;
* if we are flag = 1, mask = 11...11 and this is identical to secp256k1_scalar_negate */
uint32_t mask = !flag - 1;
volatile int vflag = flag;
uint32_t mask = -vflag;
uint32_t nonzero = 0xFFFFFFFFUL * (secp256k1_scalar_is_zero(r) == 0);
uint64_t t = (uint64_t)(r->d[0] ^ mask) + ((SECP256K1_N_0 + 1) & mask);
r->d[0] = t & nonzero; t >>= 32;