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61 lines
1.8 KiB
C
61 lines
1.8 KiB
C
#include "fe25519.h"
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#include "ge25519.h"
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/* d */
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static const fe25519 ecd = {{0x75EB4DCA135978A3, 0x00700A4D4141D8AB, 0x8CC740797779E898, 0x52036CEE2B6FFE73}};
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/* sqrt(-1) */
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static const fe25519 sqrtm1 = {{0xC4EE1B274A0EA0B0, 0x2F431806AD2FE478, 0x2B4D00993DFBD7A7, 0x2B8324804FC1DF0B}};
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/* return 0 on success, -1 otherwise */
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int ge25519_unpackneg_vartime(ge25519_p3 *r, const unsigned char p[32])
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{
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fe25519 t, chk, num, den, den2, den4, den6;
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unsigned char par = p[31] >> 7;
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fe25519_setint(&r->z,1);
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fe25519_unpack(&r->y, p);
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fe25519_square(&num, &r->y); /* x = y^2 */
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fe25519_mul(&den, &num, &ecd); /* den = dy^2 */
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fe25519_sub(&num, &num, &r->z); /* x = y^2-1 */
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fe25519_add(&den, &r->z, &den); /* den = dy^2+1 */
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/* Computation of sqrt(num/den)
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1.: computation of num^((p-5)/8)*den^((7p-35)/8) = (num*den^7)^((p-5)/8)
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*/
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fe25519_square(&den2, &den);
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fe25519_square(&den4, &den2);
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fe25519_mul(&den6, &den4, &den2);
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fe25519_mul(&t, &den6, &num);
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fe25519_mul(&t, &t, &den);
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fe25519_pow2523(&t, &t);
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/* 2. computation of r->x = t * num * den^3
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*/
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fe25519_mul(&t, &t, &num);
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fe25519_mul(&t, &t, &den);
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fe25519_mul(&t, &t, &den);
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fe25519_mul(&r->x, &t, &den);
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/* 3. Check whether sqrt computation gave correct result, multiply by sqrt(-1) if not:
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*/
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fe25519_square(&chk, &r->x);
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fe25519_mul(&chk, &chk, &den);
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if (!fe25519_iseq_vartime(&chk, &num))
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fe25519_mul(&r->x, &r->x, &sqrtm1);
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/* 4. Now we have one of the two square roots, except if input was not a square
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*/
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fe25519_square(&chk, &r->x);
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fe25519_mul(&chk, &chk, &den);
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if (!fe25519_iseq_vartime(&chk, &num))
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return -1;
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/* 5. Choose the desired square root according to parity:
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*/
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if(fe25519_getparity(&r->x) != (1-par))
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fe25519_neg(&r->x, &r->x);
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fe25519_mul(&r->t, &r->x, &r->y);
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return 0;
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}
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