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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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2
src/field_5x52_impl.h
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@@ -349,6 +349,7 @@ SECP256K1_INLINE static void secp256k1_fe_impl_sqr(secp256k1_fe *r, const secp25
SECP256K1_INLINE static void secp256k1_fe_impl_cmov(secp256k1_fe *r, const secp256k1_fe *a, int flag) {
uint64_t mask0, mask1;
volatile int vflag = flag;
VERIFY_CHECK(flag == 0 || flag == 1);
SECP256K1_CHECKMEM_CHECK_VERIFY(r->n, sizeof(r->n));
mask0 = vflag + ~((uint64_t)0);
mask1 = ~mask0;
@@ -416,6 +417,7 @@ static SECP256K1_INLINE void secp256k1_fe_impl_half(secp256k1_fe *r) {
static SECP256K1_INLINE void secp256k1_fe_storage_cmov(secp256k1_fe_storage *r, const secp256k1_fe_storage *a, int flag) {
uint64_t mask0, mask1;
volatile int vflag = flag;
VERIFY_CHECK(flag == 0 || flag == 1);
SECP256K1_CHECKMEM_CHECK_VERIFY(r->n, sizeof(r->n));
mask0 = vflag + ~((uint64_t)0);
mask1 = ~mask0;