diff options
Diffstat (limited to 'src/field_impl.h')
| -rw-r--r-- | src/field_impl.h | 46 |
1 files changed, 33 insertions, 13 deletions
diff --git a/src/field_impl.h b/src/field_impl.h index e6ec11e8f2..77f4aae2f9 100644 --- a/src/field_impl.h +++ b/src/field_impl.h @@ -21,15 +21,24 @@ #error "Please select field implementation" #endif -SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe_t *a, const secp256k1_fe_t *b) { - secp256k1_fe_t na; +SECP256K1_INLINE static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b) { + secp256k1_fe na; secp256k1_fe_negate(&na, a, 1); secp256k1_fe_add(&na, b); return secp256k1_fe_normalizes_to_zero_var(&na); } -static int secp256k1_fe_sqrt_var(secp256k1_fe_t *r, const secp256k1_fe_t *a) { - secp256k1_fe_t x2, x3, x6, x9, x11, x22, x44, x88, x176, x220, x223, t1; +static int secp256k1_fe_sqrt_var(secp256k1_fe *r, const secp256k1_fe *a) { + /** Given that p is congruent to 3 mod 4, we can compute the square root of + * a mod p as the (p+1)/4'th power of a. + * + * As (p+1)/4 is an even number, it will have the same result for a and for + * (-a). Only one of these two numbers actually has a square root however, + * so we test at the end by squaring and comparing to the input. + * Also because (p+1)/4 is an even number, the computed square root is + * itself always a square (a ** ((p+1)/4) is the square of a ** ((p+1)/8)). + */ + secp256k1_fe x2, x3, x6, x9, x11, x22, x44, x88, x176, x220, x223, t1; int j; /** The binary representation of (p + 1)/4 has 3 blocks of 1s, with lengths in @@ -117,8 +126,8 @@ static int secp256k1_fe_sqrt_var(secp256k1_fe_t *r, const secp256k1_fe_t *a) { return secp256k1_fe_equal_var(&t1, a); } -static void secp256k1_fe_inv(secp256k1_fe_t *r, const secp256k1_fe_t *a) { - secp256k1_fe_t x2, x3, x6, x9, x11, x22, x44, x88, x176, x220, x223, t1; +static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *a) { + secp256k1_fe x2, x3, x6, x9, x11, x22, x44, x88, x176, x220, x223, t1; int j; /** The binary representation of (p - 2) has 5 blocks of 1s, with lengths in @@ -207,11 +216,15 @@ static void secp256k1_fe_inv(secp256k1_fe_t *r, const secp256k1_fe_t *a) { secp256k1_fe_mul(r, a, &t1); } -static void secp256k1_fe_inv_var(secp256k1_fe_t *r, const secp256k1_fe_t *a) { +static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *a) { #if defined(USE_FIELD_INV_BUILTIN) secp256k1_fe_inv(r, a); #elif defined(USE_FIELD_INV_NUM) - secp256k1_num_t n, m; + secp256k1_num n, m; + static const secp256k1_fe negone = SECP256K1_FE_CONST( + 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, + 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFEUL, 0xFFFFFC2EUL + ); /* 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, @@ -220,21 +233,28 @@ static void secp256k1_fe_inv_var(secp256k1_fe_t *r, const secp256k1_fe_t *a) { 0xFF,0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F }; unsigned char b[32]; - secp256k1_fe_t c = *a; + int res; + 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); secp256k1_num_mod_inverse(&n, &n, &m); secp256k1_num_get_bin(b, 32, &n); - VERIFY_CHECK(secp256k1_fe_set_b32(r, b)); + res = secp256k1_fe_set_b32(r, b); + (void)res; + VERIFY_CHECK(res); + /* Verify the result is the (unique) valid inverse using non-GMP code. */ + secp256k1_fe_mul(&c, &c, r); + secp256k1_fe_add(&c, &negone); + CHECK(secp256k1_fe_normalizes_to_zero_var(&c)); #else #error "Please select field inverse implementation" #endif } -static void secp256k1_fe_inv_all_var(size_t len, secp256k1_fe_t *r, const secp256k1_fe_t *a) { - secp256k1_fe_t u; +static void secp256k1_fe_inv_all_var(size_t len, secp256k1_fe *r, const secp256k1_fe *a) { + secp256k1_fe u; size_t i; if (len < 1) { return; @@ -252,7 +272,7 @@ static void secp256k1_fe_inv_all_var(size_t len, secp256k1_fe_t *r, const secp25 secp256k1_fe_inv_var(&u, &r[--i]); while (i > 0) { - int j = i--; + size_t j = i--; secp256k1_fe_mul(&r[j], &r[i], &u); secp256k1_fe_mul(&u, &u, &a[j]); } |