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Add function for testing quadratic residue field/group elements.

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This commit is contained in:
Pieter Wuille
2015-11-25 17:38:44 +01:00
parent efd953a7a7
commit e6e9805fc4
6 changed files with 76 additions and 4 deletions
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25
src/field_impl.h
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@@ -280,4 +280,29 @@ static void secp256k1_fe_inv_all_var(size_t len, secp256k1_fe *r, const secp256k
r[0] = u;
}
static int secp256k1_fe_is_quad_var(const secp256k1_fe *a) {
#ifndef USE_NUM_NONE
unsigned char b[32];
secp256k1_num n;
secp256k1_num m;
/* secp256k1 field prime, value p defined in "Standards for Efficient Cryptography" (SEC2) 2.7.1. */
static const unsigned char prime[32] = {
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
0xFF,0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F
};
secp256k1_fe c = *a;
secp256k1_fe_normalize_var(&c);
secp256k1_fe_get_b32(b, &c);
secp256k1_num_set_bin(&n, b, 32);
secp256k1_num_set_bin(&m, prime, 32);
return secp256k1_num_jacobi(&n, &m) >= 0;
#else
secp256k1_fe r;
return secp256k1_fe_sqrt_var(&r, a) == 1;
#endif
}
#endif