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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_8x32_impl.h
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@@ -147,6 +147,7 @@ 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) {
uint64_t t;
volatile int vflag = flag;
VERIFY_CHECK(flag == 0 || flag == 1);
SECP256K1_SCALAR_VERIFY(r);
VERIFY_CHECK(bit < 256);
@@ -314,6 +315,7 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int 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);
VERIFY_CHECK(flag == 0 || flag == 1);
SECP256K1_SCALAR_VERIFY(r);
r->d[0] = t & nonzero; t >>= 32;
@@ -709,6 +711,7 @@ SECP256K1_INLINE static void secp256k1_scalar_mul_shift_var(secp256k1_scalar *r,
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->d, sizeof(r->d));