-rw-r--r-- 1871 lib25519-20220726/crypto_sign/ed25519/amd64-51-30k/ge25519_unpackneg.c raw
#include "fe25519.h" #include "ge25519.h" /* d */ static const fe25519 ecd = {{929955233495203, 466365720129213, 1662059464998953, 2033849074728123, 1442794654840575}}; /* sqrt(-1) */ static const fe25519 sqrtm1 = {{1718705420411056, 234908883556509, 2233514472574048, 2117202627021982, 765476049583133}}; /* return 0 on success, -1 otherwise */ int ge25519_unpackneg_vartime(ge25519_p3 *r, const unsigned char p[32]) { fe25519 t, chk, num, den, den2, den4, den6; unsigned char par = p[31] >> 7; fe25519_setint(&r->z,1); fe25519_unpack(&r->y, p); fe25519_square(&num, &r->y); /* x = y^2 */ fe25519_mul(&den, &num, &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, &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; }