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Abstract out verify logic for fe_half

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This commit is contained in:
Pieter Wuille 2022-11-17 11:28:49 -05:00
parent 283cd80ab4
commit 89e324c6b9
4 changed files with 23 additions and 31 deletions
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10
src/field.h
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@ -98,6 +98,7 @@ static const secp256k1_fe secp256k1_const_beta = SECP256K1_FE_CONST(
# define secp256k1_fe_inv secp256k1_fe_impl_inv
# define secp256k1_fe_inv_var secp256k1_fe_impl_inv_var
# define secp256k1_fe_get_bounds secp256k1_fe_impl_get_bounds
# define secp256k1_fe_half secp256k1_fe_impl_half
#endif /* !defined(VERIFY) */
/** Normalize a field element.
@ -302,9 +303,12 @@ static void secp256k1_fe_storage_cmov(secp256k1_fe_storage *r, const secp256k1_f
*/
static void secp256k1_fe_cmov(secp256k1_fe *r, const secp256k1_fe *a, int flag);
/** Halves the value of a field element modulo the field prime. Constant-time.
* For an input magnitude 'm', the output magnitude is set to 'floor(m/2) + 1'.
* The output is not guaranteed to be normalized, regardless of the input. */
/** Halve the value of a field element modulo the field prime in constant-time.
*
* On input, r must be a valid field element.
* On output, r will be normalized and have magnitude floor(m/2) + 1 where m is
* the magnitude of r on input.
*/
static void secp256k1_fe_half(secp256k1_fe *r);
/** Sets r to a field element with magnitude m, normalized if (and only if) m==0.

17
src/field_10x26_impl.h
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@ -1046,17 +1046,12 @@ SECP256K1_INLINE static void secp256k1_fe_impl_cmov(secp256k1_fe *r, const secp2
r->n[9] = (r->n[9] & mask0) | (a->n[9] & mask1);
}
static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
static SECP256K1_INLINE void secp256k1_fe_impl_half(secp256k1_fe *r) {
uint32_t t0 = r->n[0], t1 = r->n[1], t2 = r->n[2], t3 = r->n[3], t4 = r->n[4],
t5 = r->n[5], t6 = r->n[6], t7 = r->n[7], t8 = r->n[8], t9 = r->n[9];
uint32_t one = (uint32_t)1;
uint32_t mask = -(t0 & one) >> 6;
#ifdef VERIFY
secp256k1_fe_verify(r);
VERIFY_CHECK(r->magnitude < 32);
#endif
/* Bounds analysis (over the rationals).
*
* Let m = r->magnitude
@ -1103,10 +1098,8 @@ static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
*
* Current bounds: t0..t8 <= C * (m/2 + 1/2)
* t9 <= D * (m/2 + 1/4)
*/
#ifdef VERIFY
/* Therefore the output magnitude (M) has to be set such that:
*
* Therefore the output magnitude (M) has to be set such that:
* t0..t8: C * M >= C * (m/2 + 1/2)
* t9: D * M >= D * (m/2 + 1/4)
*
@ -1116,10 +1109,6 @@ static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
* and since we want the smallest such integer value for M:
* M == floor(m/2) + 1
*/
r->magnitude = (r->magnitude >> 1) + 1;
r->normalized = 0;
secp256k1_fe_verify(r);
#endif
}
static SECP256K1_INLINE void secp256k1_fe_storage_cmov(secp256k1_fe_storage *r, const secp256k1_fe_storage *a, int flag) {

17
src/field_5x52_impl.h
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@ -375,16 +375,11 @@ SECP256K1_INLINE static void secp256k1_fe_impl_cmov(secp256k1_fe *r, const secp2
r->n[4] = (r->n[4] & mask0) | (a->n[4] & mask1);
}
static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
static SECP256K1_INLINE void secp256k1_fe_impl_half(secp256k1_fe *r) {
uint64_t t0 = r->n[0], t1 = r->n[1], t2 = r->n[2], t3 = r->n[3], t4 = r->n[4];
uint64_t one = (uint64_t)1;
uint64_t mask = -(t0 & one) >> 12;
#ifdef VERIFY
secp256k1_fe_verify(r);
VERIFY_CHECK(r->magnitude < 32);
#endif
/* Bounds analysis (over the rationals).
*
* Let m = r->magnitude
@ -421,10 +416,8 @@ static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
*
* Current bounds: t0..t3 <= C * (m/2 + 1/2)
* t4 <= D * (m/2 + 1/4)
*/
#ifdef VERIFY
/* Therefore the output magnitude (M) has to be set such that:
*
* Therefore the output magnitude (M) has to be set such that:
* t0..t3: C * M >= C * (m/2 + 1/2)
* t4: D * M >= D * (m/2 + 1/4)
*
@ -434,10 +427,6 @@ static SECP256K1_INLINE void secp256k1_fe_half(secp256k1_fe *r) {
* and since we want the smallest such integer value for M:
* M == floor(m/2) + 1
*/
r->magnitude = (r->magnitude >> 1) + 1;
r->normalized = 0;
secp256k1_fe_verify(r);
#endif
}
static SECP256K1_INLINE void secp256k1_fe_storage_cmov(secp256k1_fe_storage *r, const secp256k1_fe_storage *a, int flag) {

10
src/field_impl.h
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@ -384,6 +384,16 @@ SECP256K1_INLINE static void secp256k1_fe_get_bounds(secp256k1_fe* r, int m) {
secp256k1_fe_verify(r);
}
static void secp256k1_fe_impl_half(secp256k1_fe *r);
SECP256K1_INLINE static void secp256k1_fe_half(secp256k1_fe *r) {
secp256k1_fe_verify(r);
VERIFY_CHECK(r->magnitude < 32);
secp256k1_fe_impl_half(r);
r->magnitude = (r->magnitude >> 1) + 1;
r->normalized = 0;
secp256k1_fe_verify(r);
}
#endif /* defined(VERIFY) */
#endif /* SECP256K1_FIELD_IMPL_H */