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ecc: added Y^2 and legendre API's

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SAE requires checking a values legendre symbol. Also added a helper
for computing Y^2, or, calculating the sum for the X side of the
equation.
This commit is contained in:
James Prestwood 2018-08-07 14:29:08 -07:00 committed by Denis Kenzior
parent 35278ae108
commit 0b452b9200
2 changed files with 40 additions and 0 deletions
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36
src/ecc.c
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@ -900,3 +900,39 @@ bool ecc_compute_y(uint64_t *y, uint64_t *x)
return true;
}
void ecc_compute_y_sqr(uint64_t *y_sqr, uint64_t *x)
{
uint64_t sum[NUM_ECC_DIGITS] = { 0 };
uint64_t tmp[NUM_ECC_DIGITS] = { 0 };
uint64_t _3[NUM_ECC_DIGITS] = { 3ull }; /* -a = 3 */
vli_mod_square_fast(sum, x);
vli_mod_mult_fast(sum, sum, x); /* x^3 */
vli_mod_mult_fast(tmp, _3, x);
vli_mod_sub(sum, sum, tmp, curve_p); /* x^3 - ax */
vli_mod_add(sum, sum, curve_b, curve_p); /* x^3 - ax + b */
memcpy(y_sqr, sum, 32);
}
int vli_legendre(uint64_t *val, const uint64_t *p)
{
uint64_t tmp[NUM_ECC_DIGITS];
uint64_t exp[NUM_ECC_DIGITS];
uint64_t _1[NUM_ECC_DIGITS] = { 1ull };
uint64_t _0[NUM_ECC_DIGITS] = { 0 };
/* check that val ^ ((p - 1) / 2) == [1, 0 or -1] */
vli_sub(exp, p, _1);
vli_rshift1(exp);
vli_mod_exp(tmp, val, exp, p);
if (vli_cmp(tmp, _1) == 0)
return 1;
else if (vli_cmp(tmp, _0) == 0)
return 0;
else
return -1;
}

4
src/ecc.h
View File

@ -87,3 +87,7 @@ void vli_mod_exp(uint64_t *result, uint64_t *base, uint64_t *exp,
int vli_cmp(const uint64_t *left, const uint64_t *right);
unsigned int vli_num_bits(const uint64_t *vli);
int vli_legendre(uint64_t *val, const uint64_t *p);
void ecc_compute_y_sqr(uint64_t *y_sqr, uint64_t *x);