Momentum-Apps/flipbip/lib/crypto/ed25519_donna/ed25519_donna_impl_base.c

#include <assert.h>
#include "ed25519_donna.h"
#include "../memzero.h"
#include "options.h"

/* sqrt(x) is such an integer y that 0 <= y <= p - 1, y % 2 = 0, and y^2 = x (mod p). */
/* d = -121665 / 121666 */
#if !defined(NDEBUG)
static const bignum25519 ALIGN(16) fe_d = {
    0x35978a3,
    0x0d37284,
    0x3156ebd,
    0x06a0a0e,
    0x001c029,
    0x179e898,
    0x3a03cbb,
    0x1ce7198,
    0x2e2b6ff,
    0x1480db3}; /* d */
#endif
static const bignum25519 ALIGN(16) fe_sqrtm1 = {
    0x20ea0b0,
    0x186c9d2,
    0x08f189d,
    0x035697f,
    0x0bd0c60,
    0x1fbd7a7,
    0x2804c9e,
    0x1e16569,
    0x004fc1d,
    0x0ae0c92}; /* sqrt(-1) */
//static const bignum25519 ALIGN(16) fe_d2 = {
//		0x2b2f159, 0x1a6e509, 0x22add7a, 0x0d4141d, 0x0038052, 0x0f3d130, 0x3407977, 0x19ce331, 0x1c56dff, 0x0901b67}; /* 2 * d */

/* A = 2 * (1 - d) / (1 + d) = 486662 */
static const bignum25519 ALIGN(16) fe_ma2 = {
    0x33de3c9,
    0x1fff236,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff}; /* -A^2 */
static const bignum25519 ALIGN(16) fe_ma = {
    0x3f892e7,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff,
    0x3ffffff,
    0x1ffffff}; /* -A */
static const bignum25519 ALIGN(16) fe_fffb1 = {
    0x1e3bdff,
    0x025a2b3,
    0x18e5bab,
    0x0ba36ac,
    0x0b9afed,
    0x004e61c,
    0x31d645f,
    0x09d1bea,
    0x102529e,
    0x0063810}; /* sqrt(-2 * A * (A + 2)) */
static const bignum25519 ALIGN(16) fe_fffb2 = {
    0x383650d,
    0x066df27,
    0x10405a4,
    0x1cfdd48,
    0x2b887f2,
    0x1e9a041,
    0x1d7241f,
    0x0612dc5,
    0x35fba5d,
    0x0cbe787}; /* sqrt(2 * A * (A + 2)) */
static const bignum25519 ALIGN(16) fe_fffb3 = {
    0x0cfd387,
    0x1209e3a,
    0x3bad4fc,
    0x18ad34d,
    0x2ff6c02,
    0x0f25d12,
    0x15cdfe0,
    0x0e208ed,
    0x32eb3df,
    0x062d7bb}; /* sqrt(-sqrt(-1) * A * (A + 2)) */
static const bignum25519 ALIGN(16) fe_fffb4 = {
    0x2b39186,
    0x14640ed,
    0x14930a7,
    0x04509fa,
    0x3b91bf0,
    0x0f7432e,
    0x07a443f,
    0x17f24d8,
    0x031067d,
    0x0690fcc}; /* sqrt(sqrt(-1) * A * (A + 2)) */

/*
	Timing safe memory compare
*/
int ed25519_verify(const unsigned char* x, const unsigned char* y, size_t len) {
    size_t differentbits = 0;
    while(len--) differentbits |= (*x++ ^ *y++);
    return (int)(1 & ((differentbits - 1) >> 8));
}

/*
	conversions
*/

void ge25519_p1p1_to_partial(ge25519* r, const ge25519_p1p1* p) {
    curve25519_mul(r->x, p->x, p->t);
    curve25519_mul(r->y, p->y, p->z);
    curve25519_mul(r->z, p->z, p->t);
}

void ge25519_p1p1_to_full(ge25519* r, const ge25519_p1p1* p) {
    curve25519_mul(r->x, p->x, p->t);
    curve25519_mul(r->y, p->y, p->z);
    curve25519_mul(r->z, p->z, p->t);
    curve25519_mul(r->t, p->x, p->y);
}

void ge25519_full_to_pniels(ge25519_pniels* p, const ge25519* r) {
    curve25519_sub(p->ysubx, r->y, r->x);
    curve25519_add(p->xaddy, r->y, r->x);
    curve25519_copy(p->z, r->z);
    curve25519_mul(p->t2d, r->t, ge25519_ec2d);
}

/*
	adding & doubling
*/

void ge25519_double_p1p1(ge25519_p1p1* r, const ge25519* p) {
    bignum25519 a = {0}, b = {0}, c = {0};

    curve25519_square(a, p->x);
    curve25519_square(b, p->y);
    curve25519_square(c, p->z);
    curve25519_add_reduce(c, c, c);
    curve25519_add(r->x, p->x, p->y);
    curve25519_square(r->x, r->x);
    curve25519_add(r->y, b, a);
    curve25519_sub(r->z, b, a);
    curve25519_sub_after_basic(r->x, r->x, r->y);
    curve25519_sub_after_basic(r->t, c, r->z);
}

#ifndef ED25519_NO_PRECOMP
void ge25519_nielsadd2_p1p1(
    ge25519_p1p1* r,
    const ge25519* p,
    const ge25519_niels* q,
    unsigned char signbit) {
    const bignum25519* qb = (const bignum25519*)q;
    bignum25519* rb = (bignum25519*)r;
    bignum25519 a = {0}, b = {0}, c = {0};

    curve25519_sub(a, p->y, p->x);
    curve25519_add(b, p->y, p->x);
    curve25519_mul(a, a, qb[signbit]); /* x for +, y for - */
    curve25519_mul(r->x, b, qb[signbit ^ 1]); /* y for +, x for - */
    curve25519_add(r->y, r->x, a);
    curve25519_sub(r->x, r->x, a);
    curve25519_mul(c, p->t, q->t2d);
    curve25519_add_reduce(r->t, p->z, p->z);
    curve25519_copy(r->z, r->t);
    curve25519_add(rb[2 + signbit], rb[2 + signbit], c); /* z for +, t for - */
    curve25519_sub(rb[2 + (signbit ^ 1)], rb[2 + (signbit ^ 1)], c); /* t for +, z for - */
}
#endif

void ge25519_pnielsadd_p1p1(
    ge25519_p1p1* r,
    const ge25519* p,
    const ge25519_pniels* q,
    unsigned char signbit) {
    const bignum25519* qb = (const bignum25519*)q;
    bignum25519* rb = (bignum25519*)r;
    bignum25519 a = {0}, b = {0}, c = {0};

    curve25519_sub(a, p->y, p->x);
    curve25519_add(b, p->y, p->x);
    curve25519_mul(a, a, qb[signbit]); /* ysubx for +, xaddy for - */
    curve25519_mul(r->x, b, qb[signbit ^ 1]); /* xaddy for +, ysubx for - */
    curve25519_add(r->y, r->x, a);
    curve25519_sub(r->x, r->x, a);
    curve25519_mul(c, p->t, q->t2d);
    curve25519_mul(r->t, p->z, q->z);
    curve25519_add_reduce(r->t, r->t, r->t);
    curve25519_copy(r->z, r->t);
    curve25519_add(rb[2 + signbit], rb[2 + signbit], c); /* z for +, t for - */
    curve25519_sub(rb[2 + (signbit ^ 1)], rb[2 + (signbit ^ 1)], c); /* t for +, z for - */
}

void ge25519_double_partial(ge25519* r, const ge25519* p) {
    ge25519_p1p1 t = {0};
    ge25519_double_p1p1(&t, p);
    ge25519_p1p1_to_partial(r, &t);
}

void ge25519_double(ge25519* r, const ge25519* p) {
    ge25519_p1p1 t = {0};
    ge25519_double_p1p1(&t, p);
    ge25519_p1p1_to_full(r, &t);
}

void ge25519_nielsadd2(ge25519* r, const ge25519_niels* q) {
    bignum25519 a = {0}, b = {0}, c = {0}, e = {0}, f = {0}, g = {0}, h = {0};

    curve25519_sub(a, r->y, r->x);
    curve25519_add(b, r->y, r->x);
    curve25519_mul(a, a, q->ysubx);
    curve25519_mul(e, b, q->xaddy);
    curve25519_add(h, e, a);
    curve25519_sub(e, e, a);
    curve25519_mul(c, r->t, q->t2d);
    curve25519_add(f, r->z, r->z);
    curve25519_add_after_basic(g, f, c);
    curve25519_sub_after_basic(f, f, c);
    curve25519_mul(r->x, e, f);
    curve25519_mul(r->y, h, g);
    curve25519_mul(r->z, g, f);
    curve25519_mul(r->t, e, h);
}

void ge25519_pnielsadd(ge25519_pniels* r, const ge25519* p, const ge25519_pniels* q) {
    bignum25519 a = {0}, b = {0}, c = {0}, x = {0}, y = {0}, z = {0}, t = {0};

    curve25519_sub(a, p->y, p->x);
    curve25519_add(b, p->y, p->x);
    curve25519_mul(a, a, q->ysubx);
    curve25519_mul(x, b, q->xaddy);
    curve25519_add(y, x, a);
    curve25519_sub(x, x, a);
    curve25519_mul(c, p->t, q->t2d);
    curve25519_mul(t, p->z, q->z);
    curve25519_add(t, t, t);
    curve25519_add_after_basic(z, t, c);
    curve25519_sub_after_basic(t, t, c);
    curve25519_mul(r->xaddy, x, t);
    curve25519_mul(r->ysubx, y, z);
    curve25519_mul(r->z, z, t);
    curve25519_mul(r->t2d, x, y);
    curve25519_copy(y, r->ysubx);
    curve25519_sub(r->ysubx, r->ysubx, r->xaddy);
    curve25519_add(r->xaddy, r->xaddy, y);
    curve25519_mul(r->t2d, r->t2d, ge25519_ec2d);
}

/*
	pack & unpack
*/

void ge25519_pack(unsigned char r[32], const ge25519* p) {
    bignum25519 tx = {0}, ty = {0}, zi = {0};
    unsigned char parity[32] = {0};
    curve25519_recip(zi, p->z);
    curve25519_mul(tx, p->x, zi);
    curve25519_mul(ty, p->y, zi);
    curve25519_contract(r, ty);
    curve25519_contract(parity, tx);
    r[31] ^= ((parity[0] & 1) << 7);
}

int ge25519_unpack_negative_vartime(ge25519* r, const unsigned char p[32]) {
    const unsigned char zero[32] = {0};
    const bignum25519 one = {1};
    unsigned char parity = p[31] >> 7;
    unsigned char check[32] = {0};
    bignum25519 t = {0}, root = {0}, num = {0}, den = {0}, d3 = {0};

    curve25519_expand(r->y, p);
    curve25519_copy(r->z, one);
    curve25519_square(num, r->y); /* x = y^2 */
    curve25519_mul(den, num, ge25519_ecd); /* den = dy^2 */
    curve25519_sub_reduce(num, num, r->z); /* x = y^1 - 1 */
    curve25519_add(den, den, r->z); /* den = dy^2 + 1 */

    /* Computation of sqrt(num/den) */
    /* 1.: computation of num^((p-5)/8)*den^((7p-35)/8) = (num*den^7)^((p-5)/8) */
    curve25519_square(t, den);
    curve25519_mul(d3, t, den);
    curve25519_square(r->x, d3);
    curve25519_mul(r->x, r->x, den);
    curve25519_mul(r->x, r->x, num);
    curve25519_pow_two252m3(r->x, r->x);

    /* 2. computation of r->x = num * den^3 * (num*den^7)^((p-5)/8) */
    curve25519_mul(r->x, r->x, d3);
    curve25519_mul(r->x, r->x, num);

    /* 3. Check if either of the roots works: */
    curve25519_square(t, r->x);
    curve25519_mul(t, t, den);
    curve25519_sub_reduce(root, t, num);
    curve25519_contract(check, root);
    if(!ed25519_verify(check, zero, 32)) {
        curve25519_add_reduce(t, t, num);
        curve25519_contract(check, t);
        if(!ed25519_verify(check, zero, 32)) return 0;
        curve25519_mul(r->x, r->x, ge25519_sqrtneg1);
    }

    curve25519_contract(check, r->x);
    if((check[0] & 1) == parity) {
        curve25519_copy(t, r->x);
        curve25519_neg(r->x, t);
    }
    curve25519_mul(r->t, r->x, r->y);
    return 1;
}

/*
	scalarmults
*/

void ge25519_set_neutral(ge25519* r) {
    memzero(r, sizeof(ge25519));
    r->y[0] = 1;
    r->z[0] = 1;
}

#define S1_SWINDOWSIZE 5
#define S1_TABLE_SIZE (1 << (S1_SWINDOWSIZE - 2))
#ifdef ED25519_NO_PRECOMP
#define S2_SWINDOWSIZE 5
#else
#define S2_SWINDOWSIZE 7
#endif
#define S2_TABLE_SIZE (1 << (S2_SWINDOWSIZE - 2))

/* computes [s1]p1 + [s2]base */
void ge25519_double_scalarmult_vartime(
    ge25519* r,
    const ge25519* p1,
    const bignum256modm s1,
    const bignum256modm s2) {
    signed char slide1[256] = {0}, slide2[256] = {0};
    ge25519_pniels pre1[S1_TABLE_SIZE] = {0};
#ifdef ED25519_NO_PRECOMP
    ge25519_pniels pre2[S2_TABLE_SIZE] = {0};
#endif
    ge25519 dp = {0};
    ge25519_p1p1 t = {0};
    int32_t i = 0;

    memzero(&t, sizeof(ge25519_p1p1));
    contract256_slidingwindow_modm(slide1, s1, S1_SWINDOWSIZE);
    contract256_slidingwindow_modm(slide2, s2, S2_SWINDOWSIZE);

    ge25519_double(&dp, p1);
    ge25519_full_to_pniels(pre1, p1);
    for(i = 0; i < S1_TABLE_SIZE - 1; i++) ge25519_pnielsadd(&pre1[i + 1], &dp, &pre1[i]);

#ifdef ED25519_NO_PRECOMP
    ge25519_double(&dp, &ge25519_basepoint);
    ge25519_full_to_pniels(pre2, &ge25519_basepoint);
    for(i = 0; i < S2_TABLE_SIZE - 1; i++) ge25519_pnielsadd(&pre2[i + 1], &dp, &pre2[i]);
#endif

    ge25519_set_neutral(r);

    i = 255;
    while((i >= 0) && !(slide1[i] | slide2[i])) i--;

    for(; i >= 0; i--) {
        ge25519_double_p1p1(&t, r);

        if(slide1[i]) {
            ge25519_p1p1_to_full(r, &t);
            ge25519_pnielsadd_p1p1(
                &t, r, &pre1[abs(slide1[i]) / 2], (unsigned char)slide1[i] >> 7);
        }

        if(slide2[i]) {
            ge25519_p1p1_to_full(r, &t);
#ifdef ED25519_NO_PRECOMP
            ge25519_pnielsadd_p1p1(
                &t, r, &pre2[abs(slide2[i]) / 2], (unsigned char)slide2[i] >> 7);
#else
            ge25519_nielsadd2_p1p1(
                &t,
                r,
                &ge25519_niels_sliding_multiples[abs(slide2[i]) / 2],
                (unsigned char)slide2[i] >> 7);
#endif
        }

        ge25519_p1p1_to_partial(r, &t);
    }
    curve25519_mul(r->t, t.x, t.y);
    memzero(slide1, sizeof(slide1));
    memzero(slide2, sizeof(slide2));
}

/* computes [s1]p1 + [s2]p2 */
#if USE_MONERO
void ge25519_double_scalarmult_vartime2(
    ge25519* r,
    const ge25519* p1,
    const bignum256modm s1,
    const ge25519* p2,
    const bignum256modm s2) {
    signed char slide1[256] = {0}, slide2[256] = {0};
    ge25519_pniels pre1[S1_TABLE_SIZE] = {0};
    ge25519_pniels pre2[S1_TABLE_SIZE] = {0};
    ge25519 dp = {0};
    ge25519_p1p1 t = {0};
    int32_t i = 0;

    memzero(&t, sizeof(ge25519_p1p1));
    contract256_slidingwindow_modm(slide1, s1, S1_SWINDOWSIZE);
    contract256_slidingwindow_modm(slide2, s2, S1_SWINDOWSIZE);

    ge25519_double(&dp, p1);
    ge25519_full_to_pniels(pre1, p1);
    for(i = 0; i < S1_TABLE_SIZE - 1; i++) ge25519_pnielsadd(&pre1[i + 1], &dp, &pre1[i]);

    ge25519_double(&dp, p2);
    ge25519_full_to_pniels(pre2, p2);
    for(i = 0; i < S1_TABLE_SIZE - 1; i++) ge25519_pnielsadd(&pre2[i + 1], &dp, &pre2[i]);

    ge25519_set_neutral(r);

    i = 255;
    while((i >= 0) && !(slide1[i] | slide2[i])) i--;

    for(; i >= 0; i--) {
        ge25519_double_p1p1(&t, r);

        if(slide1[i]) {
            ge25519_p1p1_to_full(r, &t);
            ge25519_pnielsadd_p1p1(
                &t, r, &pre1[abs(slide1[i]) / 2], (unsigned char)slide1[i] >> 7);
        }

        if(slide2[i]) {
            ge25519_p1p1_to_full(r, &t);
            ge25519_pnielsadd_p1p1(
                &t, r, &pre2[abs(slide2[i]) / 2], (unsigned char)slide2[i] >> 7);
        }

        ge25519_p1p1_to_partial(r, &t);
    }
    curve25519_mul(r->t, t.x, t.y);
    memzero(slide1, sizeof(slide1));
    memzero(slide2, sizeof(slide2));
}
#endif

/*
 * The following conditional move stuff uses conditional moves.
 * I will check on which compilers this works, and provide suitable
 * workarounds for those where it doesn't.
 *
 * This works on gcc 4.x and above with -O3.  Don't use -O2, this will
 * cause the code to not generate conditional moves.  Don't use any -march=
 * with less than i686 on x86
 */
static void ge25519_cmove_stride4(long* r, long* p, long* pos, long* n, int stride) {
    long x0 = r[0], x1 = r[1], x2 = r[2], x3 = r[3], y0 = 0, y1 = 0, y2 = 0, y3 = 0;
    for(; p < n; p += stride) {
        volatile int flag = (p == pos);
        y0 = p[0];
        y1 = p[1];
        y2 = p[2];
        y3 = p[3];
        x0 = flag ? y0 : x0;
        x1 = flag ? y1 : x1;
        x2 = flag ? y2 : x2;
        x3 = flag ? y3 : x3;
    }
    r[0] = x0;
    r[1] = x1;
    r[2] = x2;
    r[3] = x3;
}
#define HAS_CMOVE_STRIDE4

static void ge25519_cmove_stride4b(long* r, long* p, long* pos, long* n, int stride) {
    long x0 = p[0], x1 = p[1], x2 = p[2], x3 = p[3], y0 = 0, y1 = 0, y2 = 0, y3 = 0;
    for(p += stride; p < n; p += stride) {
        volatile int flag = (p == pos);
        y0 = p[0];
        y1 = p[1];
        y2 = p[2];
        y3 = p[3];
        x0 = flag ? y0 : x0;
        x1 = flag ? y1 : x1;
        x2 = flag ? y2 : x2;
        x3 = flag ? y3 : x3;
    }
    r[0] = x0;
    r[1] = x1;
    r[2] = x2;
    r[3] = x3;
}
#define HAS_CMOVE_STRIDE4B

void ge25519_move_conditional_pniels_array(
    ge25519_pniels* r,
    const ge25519_pniels* p,
    int pos,
    int n) {
#ifdef HAS_CMOVE_STRIDE4B
    size_t i = 0;
    for(i = 0; i < sizeof(ge25519_pniels) / sizeof(long); i += 4) {
        ge25519_cmove_stride4b(
            ((long*)r) + i,
            ((long*)p) + i,
            ((long*)(p + pos)) + i,
            ((long*)(p + n)) + i,
            sizeof(ge25519_pniels) / sizeof(long));
    }
#else
    size_t i = 0;
    for(i = 0; i < n; i++) {
        ge25519_move_conditional_pniels(r, p + i, pos == i);
    }
#endif
}

void ge25519_move_conditional_niels_array(ge25519_niels* r, const uint8_t p[8][96], int pos, int n) {
    size_t i = 0;
    for(i = 0; i < 96 / sizeof(long); i += 4) {
        ge25519_cmove_stride4(
            ((long*)r) + i,
            ((long*)p) + i,
            ((long*)(p + pos)) + i,
            ((long*)(p + n)) + i,
            96 / sizeof(long));
    }
}

/* computes [s1]p1, constant time */
void ge25519_scalarmult(ge25519* r, const ge25519* p1, const bignum256modm s1) {
    signed char slide1[64] = {0};
    ge25519_pniels pre1[9] = {0};
    ge25519_pniels pre = {0};
    ge25519 d1 = {0};
    ge25519_p1p1 t = {0};
    int32_t i = 0;

    contract256_window4_modm(slide1, s1);

    ge25519_full_to_pniels(pre1 + 1, p1);
    ge25519_double(&d1, p1);

    ge25519_set_neutral(r);
    ge25519_full_to_pniels(pre1, r);

    ge25519_full_to_pniels(pre1 + 2, &d1);
    for(i = 1; i < 7; i++) {
        ge25519_pnielsadd(&pre1[i + 2], &d1, &pre1[i]);
    }

    for(i = 63; i >= 0; i--) {
        int k = abs(slide1[i]);
        ge25519_double_partial(r, r);
        ge25519_double_partial(r, r);
        ge25519_double_partial(r, r);
        ge25519_double_p1p1(&t, r);
        ge25519_move_conditional_pniels_array(&pre, pre1, k, 9);
        ge25519_p1p1_to_full(r, &t);
        ge25519_pnielsadd_p1p1(&t, r, &pre, (unsigned char)slide1[i] >> 7);
        ge25519_p1p1_to_partial(r, &t);
    }
    curve25519_mul(r->t, t.x, t.y);
    memzero(slide1, sizeof(slide1));
}

void ge25519_scalarmult_base_choose_niels(
    ge25519_niels* t,
    const uint8_t table[256][96],
    uint32_t pos,
    signed char b) {
    bignum25519 neg = {0};
    uint32_t sign = (uint32_t)((unsigned char)b >> 7);
    uint32_t mask = ~(sign - 1);
    uint32_t u = (b + mask) ^ mask;

    /* ysubx, xaddy, t2d in packed form. initialize to ysubx = 1, xaddy = 1, t2d = 0 */
    uint8_t packed[96] = {0};
    packed[0] = 1;
    packed[32] = 1;

    ge25519_move_conditional_niels_array((ge25519_niels*)packed, &table[pos * 8], u - 1, 8);

    /* expand in to t */
    curve25519_expand(t->ysubx, packed + 0);
    curve25519_expand(t->xaddy, packed + 32);
    curve25519_expand(t->t2d, packed + 64);

    /* adjust for sign */
    curve25519_swap_conditional(t->ysubx, t->xaddy, sign);
    curve25519_neg(neg, t->t2d);
    curve25519_swap_conditional(t->t2d, neg, sign);
}

/* computes [s]basepoint */
void ge25519_scalarmult_base_niels(
    ge25519* r,
    const uint8_t basepoint_table[256][96],
    const bignum256modm s) {
    signed char b[64] = {0};
    uint32_t i = 0;
    ge25519_niels t = {0};

    contract256_window4_modm(b, s);

    ge25519_scalarmult_base_choose_niels(&t, basepoint_table, 0, b[1]);
    curve25519_sub_reduce(r->x, t.xaddy, t.ysubx);
    curve25519_add_reduce(r->y, t.xaddy, t.ysubx);
    memzero(r->z, sizeof(bignum25519));
    curve25519_copy(r->t, t.t2d);
    r->z[0] = 2;
    for(i = 3; i < 64; i += 2) {
        ge25519_scalarmult_base_choose_niels(&t, basepoint_table, i / 2, b[i]);
        ge25519_nielsadd2(r, &t);
    }
    ge25519_double_partial(r, r);
    ge25519_double_partial(r, r);
    ge25519_double_partial(r, r);
    ge25519_double(r, r);
    ge25519_scalarmult_base_choose_niels(&t, basepoint_table, 0, b[0]);
    curve25519_mul(t.t2d, t.t2d, ge25519_ecd);
    ge25519_nielsadd2(r, &t);
    for(i = 2; i < 64; i += 2) {
        ge25519_scalarmult_base_choose_niels(&t, basepoint_table, i / 2, b[i]);
        ge25519_nielsadd2(r, &t);
    }
}

int ge25519_check(const ge25519* r) {
    /* return (z % q != 0 and
						 x * y % q == z * t % q and
						(y * y - x * x - z * z - ed25519.d * t * t) % q == 0)
	 */

    bignum25519 z = {0}, lhs = {0}, rhs = {0}, tmp = {0}, res = {0};
    curve25519_reduce(z, r->z);

    curve25519_mul(lhs, r->x, r->y);
    curve25519_mul(rhs, r->z, r->t);
    curve25519_sub_reduce(lhs, lhs, rhs);

    curve25519_square(res, r->y);
    curve25519_square(tmp, r->x);
    curve25519_sub_reduce(res, res, tmp);
    curve25519_square(tmp, r->z);
    curve25519_sub_reduce(res, res, tmp);
    curve25519_square(tmp, r->t);
    curve25519_mul(tmp, tmp, ge25519_ecd);
    curve25519_sub_reduce(res, res, tmp);

    const int c1 = curve25519_isnonzero(z);
    const int c2 = curve25519_isnonzero(lhs);
    const int c3 = curve25519_isnonzero(res);
    return c1 & (c2 ^ 0x1) & (c3 ^ 0x1);
}

int ge25519_eq(const ge25519* a, const ge25519* b) {
    int eq = 1;
    bignum25519 t1 = {0}, t2 = {0};

    eq &= ge25519_check(a);
    eq &= ge25519_check(b);

    curve25519_mul(t1, a->x, b->z);
    curve25519_mul(t2, b->x, a->z);
    curve25519_sub_reduce(t1, t1, t2);
    eq &= curve25519_isnonzero(t1) ^ 1;

    curve25519_mul(t1, a->y, b->z);
    curve25519_mul(t2, b->y, a->z);
    curve25519_sub_reduce(t1, t1, t2);
    eq &= curve25519_isnonzero(t1) ^ 1;

    return eq;
}

void ge25519_copy(ge25519* dst, const ge25519* src) {
    curve25519_copy(dst->x, src->x);
    curve25519_copy(dst->y, src->y);
    curve25519_copy(dst->z, src->z);
    curve25519_copy(dst->t, src->t);
}

void ge25519_set_base(ge25519* r) {
    ge25519_copy(r, &ge25519_basepoint);
}

void ge25519_mul8(ge25519* r, const ge25519* t) {
    ge25519_double_partial(r, t);
    ge25519_double_partial(r, r);
    ge25519_double(r, r);
}

void ge25519_neg_partial(ge25519* r) {
    curve25519_neg(r->x, r->x);
}

void ge25519_neg_full(ge25519* r) {
    curve25519_neg(r->x, r->x);
    curve25519_neg(r->t, r->t);
}

void ge25519_reduce(ge25519* r, const ge25519* t) {
    curve25519_reduce(r->x, t->x);
    curve25519_reduce(r->y, t->y);
    curve25519_reduce(r->z, t->z);
    curve25519_reduce(r->t, t->t);
}

void ge25519_norm(ge25519* r, const ge25519* t) {
    bignum25519 zinv = {0};
    curve25519_recip(zinv, t->z);
    curve25519_mul(r->x, t->x, zinv);
    curve25519_mul(r->y, t->y, zinv);
    curve25519_mul(r->t, r->x, r->y);
    curve25519_set(r->z, 1);
}

void ge25519_add(ge25519* r, const ge25519* p, const ge25519* q, unsigned char signbit) {
    ge25519_pniels P_ni = {0};
    ge25519_p1p1 P_11 = {0};

    ge25519_full_to_pniels(&P_ni, q);
    ge25519_pnielsadd_p1p1(&P_11, p, &P_ni, signbit);
    ge25519_p1p1_to_full(r, &P_11);
}

void ge25519_fromfe_frombytes_vartime(ge25519* r, const unsigned char* s) {
    bignum25519 u = {0}, v = {0}, w = {0}, x = {0}, y = {0}, z = {0};
    unsigned char sign = 0;

    curve25519_expand_reduce(u, s);

    curve25519_square(v, u);
    curve25519_add_reduce(v, v, v); /* 2 * u^2 */
    curve25519_set(w, 1);
    curve25519_add_reduce(w, v, w); /* w = 2 * u^2 + 1 */

    curve25519_square(x, w); /* w^2 */
    curve25519_mul(y, fe_ma2, v); /* -2 * A^2 * u^2 */
    curve25519_add_reduce(x, x, y); /* x = w^2 - 2 * A^2 * u^2 */

    curve25519_divpowm1(r->x, w, x); /* (w / x)^(m + 1) */
    curve25519_square(y, r->x);
    curve25519_mul(x, y, x);
    curve25519_sub_reduce(y, w, x);
    curve25519_copy(z, fe_ma);

    if(curve25519_isnonzero(y)) {
        curve25519_add_reduce(y, w, x);
        if(curve25519_isnonzero(y)) {
            goto negative;
        } else {
            curve25519_mul(r->x, r->x, fe_fffb1);
        }
    } else {
        curve25519_mul(r->x, r->x, fe_fffb2);
    }
    curve25519_mul(r->x, r->x, u); /* u * sqrt(2 * A * (A + 2) * w / x) */
    curve25519_mul(z, z, v); /* -2 * A * u^2 */
    sign = 0;
    goto setsign;
negative:
    curve25519_mul(x, x, fe_sqrtm1);
    curve25519_sub_reduce(y, w, x);
    if(curve25519_isnonzero(y)) {
        assert((curve25519_add_reduce(y, w, x), !curve25519_isnonzero(y)));
        curve25519_mul(r->x, r->x, fe_fffb3);
    } else {
        curve25519_mul(r->x, r->x, fe_fffb4);
    }
    /* r->x = sqrt(A * (A + 2) * w / x) */
    /* z = -A */
    sign = 1;
setsign:
    if(curve25519_isnegative(r->x) != sign) {
        assert(curve25519_isnonzero(r->x));
        curve25519_neg(r->x, r->x);
    }
    curve25519_add_reduce(r->z, z, w);
    curve25519_sub_reduce(r->y, z, w);
    curve25519_mul(r->x, r->x, r->z);

    // Partial form, saving from T coord computation .
    // Later is mul8 discarding T anyway.
    // rt = ((rx * ry % q) * inv(rz)) % q
    // curve25519_mul(x, r->x, r->y);
    // curve25519_recip(z, r->z);
    // curve25519_mul(r->t, x, z);

#if !defined(NDEBUG)
    {
        bignum25519 check_x = {0}, check_y = {0}, check_iz = {0}, check_v = {0};
        curve25519_recip(check_iz, r->z);
        curve25519_mul(check_x, r->x, check_iz);
        curve25519_mul(check_y, r->y, check_iz);
        curve25519_square(check_x, check_x);
        curve25519_square(check_y, check_y);
        curve25519_mul(check_v, check_x, check_y);
        curve25519_mul(check_v, fe_d, check_v);
        curve25519_add_reduce(check_v, check_v, check_x);
        curve25519_sub_reduce(check_v, check_v, check_y);
        curve25519_set(check_x, 1);
        curve25519_add_reduce(check_v, check_v, check_x);
        assert(!curve25519_isnonzero(check_v));
    }
#endif
}

int ge25519_unpack_vartime(ge25519* r, const unsigned char* s) {
    int res = ge25519_unpack_negative_vartime(r, s);
    ge25519_neg_full(r);
    return res;
}

void ge25519_scalarmult_base_wrapper(ge25519* r, const bignum256modm s) {
    ge25519_scalarmult_base_niels(r, ge25519_niels_base_multiples, s);
}