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Improve comments/check for fe_equal{,_var}

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This commit is contained in:
Pieter Wuille 2022-01-31 17:34:36 -05:00
parent c5e788d672
commit 7d7d43c6dd
2 changed files with 22 additions and 2 deletions
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12
src/field.h
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@ -150,10 +150,18 @@ static int secp256k1_fe_is_zero(const secp256k1_fe *a);
*/
static int secp256k1_fe_is_odd(const secp256k1_fe *a);
/** Compare two field elements. Requires magnitude-1 inputs. */
/** Determine whether two field elements are equal.
*
* On input, a and b must be valid field elements with magnitudes not exceeding
* 1 and 31, respectively.
* Returns a = b (mod p).
*/
static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b);
/** Same as secp256k1_fe_equal, but may be variable time. */
/** Determine whether two field elements are equal, without constant-time guarantee.
*
* Identical in behavior to secp256k1_fe_equal, but not constant time in either a or b.
*/
static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b);
/** Compare two field elements. Requires both inputs to be normalized */

12
src/field_impl.h
View File

@ -20,6 +20,12 @@
SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b) {
secp256k1_fe na;
#ifdef VERIFY
secp256k1_fe_verify(a);
secp256k1_fe_verify(b);
VERIFY_CHECK(a->magnitude <= 1);
VERIFY_CHECK(b->magnitude <= 31);
#endif
secp256k1_fe_negate(&na, a, 1);
secp256k1_fe_add(&na, b);
return secp256k1_fe_normalizes_to_zero(&na);
@ -27,6 +33,12 @@ SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp
SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b) {
secp256k1_fe na;
#ifdef VERIFY
secp256k1_fe_verify(a);
secp256k1_fe_verify(b);
VERIFY_CHECK(a->magnitude <= 1);
VERIFY_CHECK(b->magnitude <= 31);
#endif
secp256k1_fe_negate(&na, a, 1);
secp256k1_fe_add(&na, b);
return secp256k1_fe_normalizes_to_zero_var(&na);