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Add VERIFY_CHECKs that flags are 0 or 1

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Flags for constant-time masking rely
on the values being exactly 0 or 1 rather
than 0 or true. Add VERIFY_CHECKs to enforce
in VERIFY builds as a preventative
measure and add documentation where relevant.
This commit is contained in:
John Moffett
2025-12-09 12:43:49 -05:00
parent e7f7083b53
commit ae00c552df
11 changed files with 30 additions and 8 deletions
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3
src/scalar_low_impl.h
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@@ -56,6 +56,7 @@ static int secp256k1_scalar_add(secp256k1_scalar *r, const secp256k1_scalar *a,
static void secp256k1_scalar_cadd_bit(secp256k1_scalar *r, unsigned int bit, int flag) {
SECP256K1_SCALAR_VERIFY(r);
VERIFY_CHECK(flag == 0 || flag == 1);
if (flag && bit < 32)
*r += ((uint32_t)1 << bit);
@@ -121,6 +122,7 @@ static int secp256k1_scalar_is_high(const secp256k1_scalar *a) {
static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
SECP256K1_SCALAR_VERIFY(r);
VERIFY_CHECK(flag == 0 || flag == 1);
if (flag) secp256k1_scalar_negate(r, r);
@@ -157,6 +159,7 @@ SECP256K1_INLINE static int secp256k1_scalar_eq(const secp256k1_scalar *a, const
static SECP256K1_INLINE void secp256k1_scalar_cmov(secp256k1_scalar *r, const secp256k1_scalar *a, int flag) {
uint32_t mask0, mask1;
volatile int vflag = flag;
VERIFY_CHECK(flag == 0 || flag == 1);
SECP256K1_SCALAR_VERIFY(a);
SECP256K1_CHECKMEM_CHECK_VERIFY(r, sizeof(*r));