void ge_montx_to_p2(ge_p2* p, const fe u, const unsigned char ed_sign_bit)
{
fe x, y, A, v, v2, iv, nx;
fe_frombytes(A, A_bytes);
/* given u, recover edwards y */
/* given u, recover v */
/* given u and v, recover edwards x */
fe_montx_to_edy(y, u); /* y = (u - 1) / (u + 1) */
fe_mont_rhs(v2, u); /* v^2 = u(u^2 + Au + 1) */
fe_sqrt(v, v2); /* v = sqrt(v^2) */
fe_mul(x, u, A); /* x = u * sqrt(-(A+2)) */
fe_invert(iv, v); /* 1/v */
fe_mul(x, x, iv); /* x = (u/v) * sqrt(-(A+2)) */
fe_neg(nx, x); /* negate x to match sign bit */
fe_cmov(x, nx, fe_isnegative(x) ^ ed_sign_bit);
fe_copy(p->X, x);
fe_copy(p->Y, y);
fe_1(p->Z);
/* POSTCONDITION: check that p->X and p->Y satisfy the Ed curve equation */
/* -x^2 + y^2 = 1 + dx^2y^2 */
#ifndef NDEBUG
{
fe one, d, x2, y2, x2y2, dx2y2;
unsigned char dbytes[32] = {
0xa3, 0x78, 0x59, 0x13, 0xca, 0x4d, 0xeb, 0x75,
0xab, 0xd8, 0x41, 0x41, 0x4d, 0x0a, 0x70, 0x00,
0x98, 0xe8, 0x79, 0x77, 0x79, 0x40, 0xc7, 0x8c,
0x73, 0xfe, 0x6f, 0x2b, 0xee, 0x6c, 0x03, 0x52
};
fe_frombytes(d, dbytes);
fe_1(one);
fe_sq(x2, p->X); /* x^2 */
fe_sq(y2, p->Y); /* y^2 */
fe_mul(dx2y2, x2, y2); /* x^2y^2 */
fe_mul(dx2y2, dx2y2, d); /* dx^2y^2 */
fe_add(dx2y2, dx2y2, one); /* dx^2y^2 + 1 */
fe_neg(x2y2, x2); /* -x^2 */
fe_add(x2y2, x2y2, y2); /* -x^2 + y^2 */
assert(fe_isequal(x2y2, dx2y2));
}
#endif
}
/*
Test if the public key can be uncommpressed and negate it (-X,Y,Z,-T)
return 0 on success
*/
int ge_frombytes_negate_vartime(ge_p3 *p,const unsigned char *s)
{
byte parity;
byte x[F25519_SIZE];
byte y[F25519_SIZE];
byte a[F25519_SIZE];
byte b[F25519_SIZE];
byte c[F25519_SIZE];
int ret = 0;
/* unpack the key s */
parity = s[31] >> 7;
fe_copy(y, s);
y[31] &= 127;
fe_mul__distinct(c, y, y);
fe_mul__distinct(b, c, ed25519_d);
fe_add(a, b, f25519_one);
fe_inv__distinct(b, a);
fe_sub(a, c, f25519_one);
fe_mul__distinct(c, a, b);
fe_sqrt(a, c);
fe_neg(b, a);
fe_select(x, a, b, (a[0] ^ parity) & 1);
/* test that x^2 is equal to c */
fe_mul__distinct(a, x, x);
fe_normalize(a);
fe_normalize(c);
ret |= ConstantCompare(a, c, F25519_SIZE);
/* project the key s onto p */
fe_copy(p->X, x);
fe_copy(p->Y, y);
fe_load(p->Z, 1);
fe_mul__distinct(p->T, x, y);
/* negate, the point becomes (-X,Y,Z,-T) */
fe_neg(p->X,p->X);
fe_neg(p->T,p->T);
return ret;
}
