group_impl.h

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/***********************************************************************
 * Copyright (c) 2013, 2014 Pieter Wuille                              *
 * Distributed under the MIT software license, see the accompanying    *
 * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
 ***********************************************************************/
 
#ifndef SECP256K1_GROUP_IMPL_H
#define SECP256K1_GROUP_IMPL_H
 
#include "field.h"
#include "group.h"
 
#define SECP256K1_G_ORDER_13 SECP256K1_GE_CONST(\
    0xc3459c3d, 0x35326167, 0xcd86cce8, 0x07a2417f,\
    0x5b8bd567, 0xde8538ee, 0x0d507b0c, 0xd128f5bb,\
    0x8e467fec, 0xcd30000a, 0x6cc1184e, 0x25d382c2,\
    0xa2f4494e, 0x2fbe9abc, 0x8b64abac, 0xd005fb24\
)
#define SECP256K1_G_ORDER_199 SECP256K1_GE_CONST(\
    0x226e653f, 0xc8df7744, 0x9bacbf12, 0x7d1dcbf9,\
    0x87f05b2a, 0xe7edbd28, 0x1f564575, 0xc48dcf18,\
    0xa13872c2, 0xe933bb17, 0x5d9ffd5b, 0xb5b6e10c,\
    0x57fe3c00, 0xbaaaa15a, 0xe003ec3e, 0x9c269bae\
)
#define SECP256K1_G SECP256K1_GE_CONST(\
    0x79BE667EUL, 0xF9DCBBACUL, 0x55A06295UL, 0xCE870B07UL,\
    0x029BFCDBUL, 0x2DCE28D9UL, 0x59F2815BUL, 0x16F81798UL,\
    0x483ADA77UL, 0x26A3C465UL, 0x5DA4FBFCUL, 0x0E1108A8UL,\
    0xFD17B448UL, 0xA6855419UL, 0x9C47D08FUL, 0xFB10D4B8UL\
)
/* These exhaustive group test orders and generators are chosen such that:
 * - The field size is equal to that of secp256k1, so field code is the same.
 * - The curve equation is of the form y^2=x^3+B for some constant B.
 * - The subgroup has a generator 2*P, where P.x=1.
 * - The subgroup has size less than 1000 to permit exhaustive testing.
 * - The subgroup admits an endomorphism of the form lambda*(x,y) == (beta*x,y).
 *
 * These parameters are generated using sage/gen_exhaustive_groups.sage.
 */
#if defined(EXHAUSTIVE_TEST_ORDER)
#  if EXHAUSTIVE_TEST_ORDER == 13
static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G_ORDER_13;
 
static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(
    0x3d3486b2, 0x159a9ca5, 0xc75638be, 0xb23a69bc,
    0x946a45ab, 0x24801247, 0xb4ed2b8e, 0x26b6a417
);
#  elif EXHAUSTIVE_TEST_ORDER == 199
static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G_ORDER_199;
 
static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(
    0x2cca28fa, 0xfc614b80, 0x2a3db42b, 0x00ba00b1,
    0xbea8d943, 0xdace9ab2, 0x9536daea, 0x0074defb
);
#  else
#    error No known generator for the specified exhaustive test group order.
#  endif
#else
static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G;
 
static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 7);
#endif
 
static void secp256k1_ge_set_gej_zinv(secp256k1_ge *r, const secp256k1_gej *a, const secp256k1_fe *zi) {
    secp256k1_fe zi2;
    secp256k1_fe zi3;
    VERIFY_CHECK(!a->infinity);
    secp256k1_fe_sqr(&zi2, zi);
    secp256k1_fe_mul(&zi3, &zi2, zi);
    secp256k1_fe_mul(&r->x, &a->x, &zi2);
    secp256k1_fe_mul(&r->y, &a->y, &zi3);
    r->infinity = a->infinity;
}
 
static void secp256k1_ge_set_xy(secp256k1_ge *r, const secp256k1_fe *x, const secp256k1_fe *y) {
    r->infinity = 0;
    r->x = *x;
    r->y = *y;
}
 
static int secp256k1_ge_is_infinity(const secp256k1_ge *a) {
    return a->infinity;
}
 
static void secp256k1_ge_neg(secp256k1_ge *r, const secp256k1_ge *a) {
    *r = *a;
    secp256k1_fe_normalize_weak(&r->y);
    secp256k1_fe_negate(&r->y, &r->y, 1);
}
 
static void secp256k1_ge_set_gej(secp256k1_ge *r, secp256k1_gej *a) {
    secp256k1_fe z2, z3;
    r->infinity = a->infinity;
    secp256k1_fe_inv(&a->z, &a->z);
    secp256k1_fe_sqr(&z2, &a->z);
    secp256k1_fe_mul(&z3, &a->z, &z2);
    secp256k1_fe_mul(&a->x, &a->x, &z2);
    secp256k1_fe_mul(&a->y, &a->y, &z3);
    secp256k1_fe_set_int(&a->z, 1);
    r->x = a->x;
    r->y = a->y;
}
 
static void secp256k1_ge_set_gej_var(secp256k1_ge *r, secp256k1_gej *a) {
    secp256k1_fe z2, z3;
    if (a->infinity) {
        secp256k1_ge_set_infinity(r);
        return;
    }
    secp256k1_fe_inv_var(&a->z, &a->z);
    secp256k1_fe_sqr(&z2, &a->z);
    secp256k1_fe_mul(&z3, &a->z, &z2);
    secp256k1_fe_mul(&a->x, &a->x, &z2);
    secp256k1_fe_mul(&a->y, &a->y, &z3);
    secp256k1_fe_set_int(&a->z, 1);
    secp256k1_ge_set_xy(r, &a->x, &a->y);
}
 
static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a, size_t len) {
    secp256k1_fe u;
    size_t i;
    size_t last_i = SIZE_MAX;
 
    for (i = 0; i < len; i++) {
        if (a[i].infinity) {
            secp256k1_ge_set_infinity(&r[i]);
        } else {
            /* Use destination's x coordinates as scratch space */
            if (last_i == SIZE_MAX) {
                r[i].x = a[i].z;
            } else {
                secp256k1_fe_mul(&r[i].x, &r[last_i].x, &a[i].z);
            }
            last_i = i;
        }
    }
    if (last_i == SIZE_MAX) {
        return;
    }
    secp256k1_fe_inv_var(&u, &r[last_i].x);
 
    i = last_i;
    while (i > 0) {
        i--;
        if (!a[i].infinity) {
            secp256k1_fe_mul(&r[last_i].x, &r[i].x, &u);
            secp256k1_fe_mul(&u, &u, &a[last_i].z);
            last_i = i;
        }
    }
    VERIFY_CHECK(!a[last_i].infinity);
    r[last_i].x = u;
 
    for (i = 0; i < len; i++) {
        if (!a[i].infinity) {
            secp256k1_ge_set_gej_zinv(&r[i], &a[i], &r[i].x);
        }
    }
}
 
static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const secp256k1_fe *zr) {
    size_t i = len - 1;
    secp256k1_fe zs;
 
    if (len > 0) {
        /* Ensure all y values are in weak normal form for fast negation of points */
        secp256k1_fe_normalize_weak(&a[i].y);
        zs = zr[i];
 
        /* Work our way backwards, using the z-ratios to scale the x/y values. */
        while (i > 0) {
            secp256k1_gej tmpa;
            if (i != len - 1) {
                secp256k1_fe_mul(&zs, &zs, &zr[i]);
            }
            i--;
            tmpa.x = a[i].x;
            tmpa.y = a[i].y;
            tmpa.infinity = 0;
            secp256k1_ge_set_gej_zinv(&a[i], &tmpa, &zs);
        }
    }
}
 
static void secp256k1_gej_set_infinity(secp256k1_gej *r) {
    r->infinity = 1;
    secp256k1_fe_clear(&r->x);
    secp256k1_fe_clear(&r->y);
    secp256k1_fe_clear(&r->z);
}
 
static void secp256k1_ge_set_infinity(secp256k1_ge *r) {
    r->infinity = 1;
    secp256k1_fe_clear(&r->x);
    secp256k1_fe_clear(&r->y);
}
 
static void secp256k1_gej_clear(secp256k1_gej *r) {
    r->infinity = 0;
    secp256k1_fe_clear(&r->x);
    secp256k1_fe_clear(&r->y);
    secp256k1_fe_clear(&r->z);
}
 
static void secp256k1_ge_clear(secp256k1_ge *r) {
    r->infinity = 0;
    secp256k1_fe_clear(&r->x);
    secp256k1_fe_clear(&r->y);
}
 
static int secp256k1_ge_set_xquad(secp256k1_ge *r, const secp256k1_fe *x) {
    secp256k1_fe x2, x3;
    r->x = *x;
    secp256k1_fe_sqr(&x2, x);
    secp256k1_fe_mul(&x3, x, &x2);
    r->infinity = 0;
    secp256k1_fe_add(&x3, &secp256k1_fe_const_b);
    return secp256k1_fe_sqrt(&r->y, &x3);
}
 
static int secp256k1_ge_set_xo_var(secp256k1_ge *r, const secp256k1_fe *x, int odd) {
    if (!secp256k1_ge_set_xquad(r, x)) {
        return 0;
    }
    secp256k1_fe_normalize_var(&r->y);
    if (secp256k1_fe_is_odd(&r->y) != odd) {
        secp256k1_fe_negate(&r->y, &r->y, 1);
    }
    return 1;
 
}
 
static void secp256k1_gej_set_ge(secp256k1_gej *r, const secp256k1_ge *a) {
   r->infinity = a->infinity;
   r->x = a->x;
   r->y = a->y;
   secp256k1_fe_set_int(&r->z, 1);
}
 
static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a) {
    secp256k1_fe r, r2;
    VERIFY_CHECK(!a->infinity);
    secp256k1_fe_sqr(&r, &a->z); secp256k1_fe_mul(&r, &r, x);
    r2 = a->x; secp256k1_fe_normalize_weak(&r2);
    return secp256k1_fe_equal_var(&r, &r2);
}
 
static void secp256k1_gej_neg(secp256k1_gej *r, const secp256k1_gej *a) {
    r->infinity = a->infinity;
    r->x = a->x;
    r->y = a->y;
    r->z = a->z;
    secp256k1_fe_normalize_weak(&r->y);
    secp256k1_fe_negate(&r->y, &r->y, 1);
}
 
static int secp256k1_gej_is_infinity(const secp256k1_gej *a) {
    return a->infinity;
}
 
static int secp256k1_ge_is_valid_var(const secp256k1_ge *a) {
    secp256k1_fe y2, x3;
    if (a->infinity) {
        return 0;
    }
    /* y^2 = x^3 + 7 */
    secp256k1_fe_sqr(&y2, &a->y);
    secp256k1_fe_sqr(&x3, &a->x); secp256k1_fe_mul(&x3, &x3, &a->x);
    secp256k1_fe_add(&x3, &secp256k1_fe_const_b);
    secp256k1_fe_normalize_weak(&x3);
    return secp256k1_fe_equal_var(&y2, &x3);
}
 
static SECP256K1_INLINE void secp256k1_gej_double(secp256k1_gej *r, const secp256k1_gej *a) {
    /* Operations: 3 mul, 4 sqr, 8 add/half/mul_int/negate */
    secp256k1_fe l, s, t;
 
    r->infinity = a->infinity;
 
    /* Formula used:
     * L = (3/2) * X1^2
     * S = Y1^2
     * T = -X1*S
     * X3 = L^2 + 2*T
     * Y3 = -(L*(X3 + T) + S^2)
     * Z3 = Y1*Z1
     */
 
    secp256k1_fe_mul(&r->z, &a->z, &a->y); /* Z3 = Y1*Z1 (1) */
    secp256k1_fe_sqr(&s, &a->y);           /* S = Y1^2 (1) */
    secp256k1_fe_sqr(&l, &a->x);           /* L = X1^2 (1) */
    secp256k1_fe_mul_int(&l, 3);           /* L = 3*X1^2 (3) */
    secp256k1_fe_half(&l);                 /* L = 3/2*X1^2 (2) */
    secp256k1_fe_negate(&t, &s, 1);        /* T = -S (2) */
    secp256k1_fe_mul(&t, &t, &a->x);       /* T = -X1*S (1) */
    secp256k1_fe_sqr(&r->x, &l);           /* X3 = L^2 (1) */
    secp256k1_fe_add(&r->x, &t);           /* X3 = L^2 + T (2) */
    secp256k1_fe_add(&r->x, &t);           /* X3 = L^2 + 2*T (3) */
    secp256k1_fe_sqr(&s, &s);              /* S' = S^2 (1) */
    secp256k1_fe_add(&t, &r->x);           /* T' = X3 + T (4) */
    secp256k1_fe_mul(&r->y, &t, &l);       /* Y3 = L*(X3 + T) (1) */
    secp256k1_fe_add(&r->y, &s);           /* Y3 = L*(X3 + T) + S^2 (2) */
    secp256k1_fe_negate(&r->y, &r->y, 2);  /* Y3 = -(L*(X3 + T) + S^2) (3) */
}
 
static void secp256k1_gej_double_var(secp256k1_gej *r, const secp256k1_gej *a, secp256k1_fe *rzr) {
    if (a->infinity) {
        secp256k1_gej_set_infinity(r);
        if (rzr != NULL) {
            secp256k1_fe_set_int(rzr, 1);
        }
        return;
    }
 
    if (rzr != NULL) {
        *rzr = a->y;
        secp256k1_fe_normalize_weak(rzr);
    }
 
    secp256k1_gej_double(r, a);
}
 
static void secp256k1_gej_add_var(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_gej *b, secp256k1_fe *rzr) {
    /* 12 mul, 4 sqr, 11 add/negate/normalizes_to_zero (ignoring special cases) */
    secp256k1_fe z22, z12, u1, u2, s1, s2, h, i, h2, h3, t;
 
    if (a->infinity) {
        VERIFY_CHECK(rzr == NULL);
        *r = *b;
        return;
    }
    if (b->infinity) {
        if (rzr != NULL) {
            secp256k1_fe_set_int(rzr, 1);
        }
        *r = *a;
        return;
    }
 
    secp256k1_fe_sqr(&z22, &b->z);
    secp256k1_fe_sqr(&z12, &a->z);
    secp256k1_fe_mul(&u1, &a->x, &z22);
    secp256k1_fe_mul(&u2, &b->x, &z12);
    secp256k1_fe_mul(&s1, &a->y, &z22); secp256k1_fe_mul(&s1, &s1, &b->z);
    secp256k1_fe_mul(&s2, &b->y, &z12); secp256k1_fe_mul(&s2, &s2, &a->z);
    secp256k1_fe_negate(&h, &u1, 1); secp256k1_fe_add(&h, &u2);
    secp256k1_fe_negate(&i, &s2, 1); secp256k1_fe_add(&i, &s1);
    if (secp256k1_fe_normalizes_to_zero_var(&h)) {
        if (secp256k1_fe_normalizes_to_zero_var(&i)) {
            secp256k1_gej_double_var(r, a, rzr);
        } else {
            if (rzr != NULL) {
                secp256k1_fe_set_int(rzr, 0);
            }
            secp256k1_gej_set_infinity(r);
        }
        return;
    }
 
    r->infinity = 0;
    secp256k1_fe_mul(&t, &h, &b->z);
    if (rzr != NULL) {
        *rzr = t;
    }
    secp256k1_fe_mul(&r->z, &a->z, &t);
 
    secp256k1_fe_sqr(&h2, &h);
    secp256k1_fe_negate(&h2, &h2, 1);
    secp256k1_fe_mul(&h3, &h2, &h);
    secp256k1_fe_mul(&t, &u1, &h2);
 
    secp256k1_fe_sqr(&r->x, &i);
    secp256k1_fe_add(&r->x, &h3);
    secp256k1_fe_add(&r->x, &t);
    secp256k1_fe_add(&r->x, &t);
 
    secp256k1_fe_add(&t, &r->x);
    secp256k1_fe_mul(&r->y, &t, &i);
    secp256k1_fe_mul(&h3, &h3, &s1);
    secp256k1_fe_add(&r->y, &h3);
}
 
static void secp256k1_gej_add_ge_var(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_ge *b, secp256k1_fe *rzr) {
    /* 8 mul, 3 sqr, 13 add/negate/normalize_weak/normalizes_to_zero (ignoring special cases) */
    secp256k1_fe z12, u1, u2, s1, s2, h, i, h2, h3, t;
    if (a->infinity) {
        VERIFY_CHECK(rzr == NULL);
        secp256k1_gej_set_ge(r, b);
        return;
    }
    if (b->infinity) {
        if (rzr != NULL) {
            secp256k1_fe_set_int(rzr, 1);
        }
        *r = *a;
        return;
    }
 
    secp256k1_fe_sqr(&z12, &a->z);
    u1 = a->x; secp256k1_fe_normalize_weak(&u1);
    secp256k1_fe_mul(&u2, &b->x, &z12);
    s1 = a->y; secp256k1_fe_normalize_weak(&s1);
    secp256k1_fe_mul(&s2, &b->y, &z12); secp256k1_fe_mul(&s2, &s2, &a->z);
    secp256k1_fe_negate(&h, &u1, 1); secp256k1_fe_add(&h, &u2);
    secp256k1_fe_negate(&i, &s2, 1); secp256k1_fe_add(&i, &s1);
    if (secp256k1_fe_normalizes_to_zero_var(&h)) {
        if (secp256k1_fe_normalizes_to_zero_var(&i)) {
            secp256k1_gej_double_var(r, a, rzr);
        } else {
            if (rzr != NULL) {
                secp256k1_fe_set_int(rzr, 0);
            }
            secp256k1_gej_set_infinity(r);
        }
        return;
    }
 
    r->infinity = 0;
    if (rzr != NULL) {
        *rzr = h;
    }
    secp256k1_fe_mul(&r->z, &a->z, &h);
 
    secp256k1_fe_sqr(&h2, &h);
    secp256k1_fe_negate(&h2, &h2, 1);
    secp256k1_fe_mul(&h3, &h2, &h);
    secp256k1_fe_mul(&t, &u1, &h2);
 
    secp256k1_fe_sqr(&r->x, &i);
    secp256k1_fe_add(&r->x, &h3);
    secp256k1_fe_add(&r->x, &t);
    secp256k1_fe_add(&r->x, &t);
 
    secp256k1_fe_add(&t, &r->x);
    secp256k1_fe_mul(&r->y, &t, &i);
    secp256k1_fe_mul(&h3, &h3, &s1);
    secp256k1_fe_add(&r->y, &h3);
}
 
static void secp256k1_gej_add_zinv_var(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_ge *b, const secp256k1_fe *bzinv) {
    /* 9 mul, 3 sqr, 13 add/negate/normalize_weak/normalizes_to_zero (ignoring special cases) */
    secp256k1_fe az, z12, u1, u2, s1, s2, h, i, h2, h3, t;
 
    if (a->infinity) {
        secp256k1_fe bzinv2, bzinv3;
        r->infinity = b->infinity;
        secp256k1_fe_sqr(&bzinv2, bzinv);
        secp256k1_fe_mul(&bzinv3, &bzinv2, bzinv);
        secp256k1_fe_mul(&r->x, &b->x, &bzinv2);
        secp256k1_fe_mul(&r->y, &b->y, &bzinv3);
        secp256k1_fe_set_int(&r->z, 1);
        return;
    }
    if (b->infinity) {
        *r = *a;
        return;
    }
 
    secp256k1_fe_mul(&az, &a->z, bzinv);
 
    secp256k1_fe_sqr(&z12, &az);
    u1 = a->x; secp256k1_fe_normalize_weak(&u1);
    secp256k1_fe_mul(&u2, &b->x, &z12);
    s1 = a->y; secp256k1_fe_normalize_weak(&s1);
    secp256k1_fe_mul(&s2, &b->y, &z12); secp256k1_fe_mul(&s2, &s2, &az);
    secp256k1_fe_negate(&h, &u1, 1); secp256k1_fe_add(&h, &u2);
    secp256k1_fe_negate(&i, &s2, 1); secp256k1_fe_add(&i, &s1);
    if (secp256k1_fe_normalizes_to_zero_var(&h)) {
        if (secp256k1_fe_normalizes_to_zero_var(&i)) {
            secp256k1_gej_double_var(r, a, NULL);
        } else {
            secp256k1_gej_set_infinity(r);
        }
        return;
    }
 
    r->infinity = 0;
    secp256k1_fe_mul(&r->z, &a->z, &h);
 
    secp256k1_fe_sqr(&h2, &h);
    secp256k1_fe_negate(&h2, &h2, 1);
    secp256k1_fe_mul(&h3, &h2, &h);
    secp256k1_fe_mul(&t, &u1, &h2);
 
    secp256k1_fe_sqr(&r->x, &i);
    secp256k1_fe_add(&r->x, &h3);
    secp256k1_fe_add(&r->x, &t);
    secp256k1_fe_add(&r->x, &t);
 
    secp256k1_fe_add(&t, &r->x);
    secp256k1_fe_mul(&r->y, &t, &i);
    secp256k1_fe_mul(&h3, &h3, &s1);
    secp256k1_fe_add(&r->y, &h3);
}
 
 
static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_ge *b) {
    /* Operations: 7 mul, 5 sqr, 24 add/cmov/half/mul_int/negate/normalize_weak/normalizes_to_zero */
    secp256k1_fe zz, u1, u2, s1, s2, t, tt, m, n, q, rr;
    secp256k1_fe m_alt, rr_alt;
    int infinity, degenerate;
    VERIFY_CHECK(!b->infinity);
    VERIFY_CHECK(a->infinity == 0 || a->infinity == 1);
 
    secp256k1_fe_sqr(&zz, &a->z);                       /* z = Z1^2 */
    u1 = a->x; secp256k1_fe_normalize_weak(&u1);        /* u1 = U1 = X1*Z2^2 (1) */
    secp256k1_fe_mul(&u2, &b->x, &zz);                  /* u2 = U2 = X2*Z1^2 (1) */
    s1 = a->y; secp256k1_fe_normalize_weak(&s1);        /* s1 = S1 = Y1*Z2^3 (1) */
    secp256k1_fe_mul(&s2, &b->y, &zz);                  /* s2 = Y2*Z1^2 (1) */
    secp256k1_fe_mul(&s2, &s2, &a->z);                  /* s2 = S2 = Y2*Z1^3 (1) */
    t = u1; secp256k1_fe_add(&t, &u2);                  /* t = T = U1+U2 (2) */
    m = s1; secp256k1_fe_add(&m, &s2);                  /* m = M = S1+S2 (2) */
    secp256k1_fe_sqr(&rr, &t);                          /* rr = T^2 (1) */
    secp256k1_fe_negate(&m_alt, &u2, 1);                /* Malt = -X2*Z1^2 */
    secp256k1_fe_mul(&tt, &u1, &m_alt);                 /* tt = -U1*U2 (2) */
    secp256k1_fe_add(&rr, &tt);                         /* rr = R = T^2-U1*U2 (3) */
    degenerate = secp256k1_fe_normalizes_to_zero(&m) &
                 secp256k1_fe_normalizes_to_zero(&rr);
    /* This only occurs when y1 == -y2 and x1^3 == x2^3, but x1 != x2.
     * This means either x1 == beta*x2 or beta*x1 == x2, where beta is
     * a nontrivial cube root of one. In either case, an alternate
     * non-indeterminate expression for lambda is (y1 - y2)/(x1 - x2),
     * so we set R/M equal to this. */
    rr_alt = s1;
    secp256k1_fe_mul_int(&rr_alt, 2);       /* rr = Y1*Z2^3 - Y2*Z1^3 (2) */
    secp256k1_fe_add(&m_alt, &u1);          /* Malt = X1*Z2^2 - X2*Z1^2 */
 
    secp256k1_fe_cmov(&rr_alt, &rr, !degenerate);
    secp256k1_fe_cmov(&m_alt, &m, !degenerate);
    /* Now Ralt / Malt = lambda and is guaranteed not to be 0/0.
     * From here on out Ralt and Malt represent the numerator
     * and denominator of lambda; R and M represent the explicit
     * expressions x1^2 + x2^2 + x1x2 and y1 + y2. */
    secp256k1_fe_sqr(&n, &m_alt);                       /* n = Malt^2 (1) */
    secp256k1_fe_negate(&q, &t, 2);                     /* q = -T (3) */
    secp256k1_fe_mul(&q, &q, &n);                       /* q = Q = -T*Malt^2 (1) */
    /* These two lines use the observation that either M == Malt or M == 0,
     * so M^3 * Malt is either Malt^4 (which is computed by squaring), or
     * zero (which is "computed" by cmov). So the cost is one squaring
     * versus two multiplications. */
    secp256k1_fe_sqr(&n, &n);
    secp256k1_fe_cmov(&n, &m, degenerate);              /* n = M^3 * Malt (2) */
    secp256k1_fe_sqr(&t, &rr_alt);                      /* t = Ralt^2 (1) */
    secp256k1_fe_mul(&r->z, &a->z, &m_alt);             /* r->z = Z3 = Malt*Z (1) */
    infinity = secp256k1_fe_normalizes_to_zero(&r->z) & ~a->infinity;
    secp256k1_fe_add(&t, &q);                           /* t = Ralt^2 + Q (2) */
    r->x = t;                                           /* r->x = X3 = Ralt^2 + Q (2) */
    secp256k1_fe_mul_int(&t, 2);                        /* t = 2*X3 (4) */
    secp256k1_fe_add(&t, &q);                           /* t = 2*X3 + Q (5) */
    secp256k1_fe_mul(&t, &t, &rr_alt);                  /* t = Ralt*(2*X3 + Q) (1) */
    secp256k1_fe_add(&t, &n);                           /* t = Ralt*(2*X3 + Q) + M^3*Malt (3) */
    secp256k1_fe_negate(&r->y, &t, 3);                  /* r->y = -(Ralt*(2*X3 + Q) + M^3*Malt) (4) */
    secp256k1_fe_half(&r->y);                           /* r->y = Y3 = -(Ralt*(2*X3 + Q) + M^3*Malt)/2 (3) */
 
    secp256k1_fe_cmov(&r->x, &b->x, a->infinity);
    secp256k1_fe_cmov(&r->y, &b->y, a->infinity);
    secp256k1_fe_cmov(&r->z, &secp256k1_fe_one, a->infinity);
    r->infinity = infinity;
}
 
static void secp256k1_gej_rescale(secp256k1_gej *r, const secp256k1_fe *s) {
    /* Operations: 4 mul, 1 sqr */
    secp256k1_fe zz;
    VERIFY_CHECK(!secp256k1_fe_is_zero(s));
    secp256k1_fe_sqr(&zz, s);
    secp256k1_fe_mul(&r->x, &r->x, &zz);                /* r->x *= s^2 */
    secp256k1_fe_mul(&r->y, &r->y, &zz);
    secp256k1_fe_mul(&r->y, &r->y, s);                  /* r->y *= s^3 */
    secp256k1_fe_mul(&r->z, &r->z, s);                  /* r->z *= s   */
}
 
static void secp256k1_ge_to_storage(secp256k1_ge_storage *r, const secp256k1_ge *a) {
    secp256k1_fe x, y;
    VERIFY_CHECK(!a->infinity);
    x = a->x;
    secp256k1_fe_normalize(&x);
    y = a->y;
    secp256k1_fe_normalize(&y);
    secp256k1_fe_to_storage(&r->x, &x);
    secp256k1_fe_to_storage(&r->y, &y);
}
 
static void secp256k1_ge_from_storage(secp256k1_ge *r, const secp256k1_ge_storage *a) {
    secp256k1_fe_from_storage(&r->x, &a->x);
    secp256k1_fe_from_storage(&r->y, &a->y);
    r->infinity = 0;
}
 
static SECP256K1_INLINE void secp256k1_gej_cmov(secp256k1_gej *r, const secp256k1_gej *a, int flag) {
    secp256k1_fe_cmov(&r->x, &a->x, flag);
    secp256k1_fe_cmov(&r->y, &a->y, flag);
    secp256k1_fe_cmov(&r->z, &a->z, flag);
 
    r->infinity ^= (r->infinity ^ a->infinity) & flag;
}
 
static SECP256K1_INLINE void secp256k1_ge_storage_cmov(secp256k1_ge_storage *r, const secp256k1_ge_storage *a, int flag) {
    secp256k1_fe_storage_cmov(&r->x, &a->x, flag);
    secp256k1_fe_storage_cmov(&r->y, &a->y, flag);
}
 
static void secp256k1_ge_mul_lambda(secp256k1_ge *r, const secp256k1_ge *a) {
    *r = *a;
    secp256k1_fe_mul(&r->x, &r->x, &secp256k1_const_beta);
}
 
static int secp256k1_gej_has_quad_y_var(const secp256k1_gej *a) {
    secp256k1_fe yz;
 
    if (a->infinity) {
        return 0;
    }
 
    /* We rely on the fact that the Jacobi symbol of 1 / a->z^3 is the same as
     * that of a->z. Thus a->y / a->z^3 is a quadratic residue iff a->y * a->z
       is */
    secp256k1_fe_mul(&yz, &a->y, &a->z);
    return secp256k1_fe_is_quad_var(&yz);
}
 
static int secp256k1_ge_is_in_correct_subgroup(const secp256k1_ge* ge) {
#ifdef EXHAUSTIVE_TEST_ORDER
    secp256k1_gej out;
    int i;
 
    /* A very simple EC multiplication ladder that avoids a dependency on ecmult. */
    secp256k1_gej_set_infinity(&out);
    for (i = 0; i < 32; ++i) {
        secp256k1_gej_double_var(&out, &out, NULL);
        if ((((uint32_t)EXHAUSTIVE_TEST_ORDER) >> (31 - i)) & 1) {
            secp256k1_gej_add_ge_var(&out, &out, ge, NULL);
        }
    }
    return secp256k1_gej_is_infinity(&out);
#else
    (void)ge;
    /* The real secp256k1 group has cofactor 1, so the subgroup is the entire curve. */
    return 1;
#endif
}
 
#endif /* SECP256K1_GROUP_IMPL_H */