void ge25519_pack(unsigned char r[32], const ge25519_p3 *p)
{
fe25519 tx, ty, zi;
fe25519_invert(&zi, &p->z);
fe25519_mul(&tx, &p->x, &zi);
fe25519_mul(&ty, &p->y, &zi);
fe25519_pack(r, &ty);
r[31] ^= fe25519_getparity(&tx) << 7;
}
/* return 0 on success, -1 otherwise */
int ge25519_unpackneg_vartime(ge25519_p3 *r, const unsigned char p[32])
{
unsigned char par;
fe25519 t, chk, num, den, den2, den4, den6;
fe25519_setone(&r->z);
par = p[31] >> 7;
fe25519_unpack(&r->y, p);
fe25519_square(&num, &r->y); /* x = y^2 */
fe25519_mul(&den, &num, &ge25519_ecd); /* den = dy^2 */
fe25519_sub(&num, &num, &r->z); /* x = y^2-1 */
fe25519_add(&den, &r->z, &den); /* 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) */
fe25519_square(&den2, &den);
fe25519_square(&den4, &den2);
fe25519_mul(&den6, &den4, &den2);
fe25519_mul(&t, &den6, &num);
fe25519_mul(&t, &t, &den);
fe25519_pow2523(&t, &t);
/* 2. computation of r->x = t * num * den^3 */
fe25519_mul(&t, &t, &num);
fe25519_mul(&t, &t, &den);
fe25519_mul(&t, &t, &den);
fe25519_mul(&r->x, &t, &den);
/* 3. Check whether sqrt computation gave correct result, multiply by sqrt(-1) if not: */
fe25519_square(&chk, &r->x);
fe25519_mul(&chk, &chk, &den);
if (!fe25519_iseq_vartime(&chk, &num)) {
fe25519_mul(&r->x, &r->x, &ge25519_sqrtm1);
}
/* 4. Now we have one of the two square roots, except if input was not a square */
fe25519_square(&chk, &r->x);
fe25519_mul(&chk, &chk, &den);
if (!fe25519_iseq_vartime(&chk, &num)) {
return -1;
}
/* 5. Choose the desired square root according to parity: */
if(fe25519_getparity(&r->x) != (1-par)) {
fe25519_neg(&r->x, &r->x);
}
fe25519_mul(&r->t, &r->x, &r->y);
return 0;
}