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Optimize ecc_25519_scalarmult_base

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We can elide one multiplication assuming that Z == 1 for the default base.
This commit is contained in:
Matthias Schiffer 2016-03-19 00:47:14 +01:00
parent 26cbc55f78
commit bb87f7b0e8
1 changed files with 50 additions and 2 deletions
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52
src/ec25519.c
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@ -842,6 +842,38 @@ void ecc_25519_add(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc
mult(out->Z, F, G);
}
/** Adds two points of the Elliptic Curve, assuming that in2->Z == 1 */
static void ecc_25519_add1(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc_25519_work_t *in2) {
const uint32_t j = UINT32_C(60833);
const uint32_t k = UINT32_C(121665);
uint32_t A[32], B[32], C[32], D[32], E[32], F[32], G[32], H[32], t0[32], t1[32];
sub(t0, in1->Y, in1->X);
mult_int(t1, j, t0);
sub(t0, in2->Y, in2->X);
mult(A, t0, t1);
add(t0, in1->Y, in1->X);
mult_int(t1, j, t0);
add(t0, in2->Y, in2->X);
mult(B, t0, t1);
mult_int(t0, k, in2->T);
mult(C, in1->T, t0);
mult_int(D, 2*j, in1->Z);
sub(E, B, A);
add(F, D, C);
sub(G, D, C);
add(H, B, A);
mult(out->X, E, F);
mult(out->Y, G, H);
mult(out->T, E, H);
mult(out->Z, F, G);
}
void ecc_25519_sub(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc_25519_work_t *in2) {
ecc_25519_work_t in2_neg;
@ -874,9 +906,25 @@ void ecc_25519_scalarmult(ecc_25519_work_t *out, const ecc_int256_t *n, const ec
}
void ecc_25519_scalarmult_base_bits(ecc_25519_work_t *out, const ecc_int256_t *n, unsigned bits) {
ecc_25519_scalarmult_bits(out, n, &ecc_25519_work_default_base, bits);
ecc_25519_work_t Q2, Q2p;
ecc_25519_work_t cur = ecc_25519_work_identity;
int b, pos;
if (bits > 256)
bits = 256;
for (pos = bits - 1; pos >= 0; --pos) {
b = n->p[pos / 8] >> (pos & 7);
b &= 1;
ecc_25519_double(&Q2, &cur);
ecc_25519_add1(&Q2p, &Q2, &ecc_25519_work_default_base);
selectw(&cur, &Q2, &Q2p, b);
}
*out = cur;
}
void ecc_25519_scalarmult_base(ecc_25519_work_t *out, const ecc_int256_t *n) {
ecc_25519_scalarmult(out, n, &ecc_25519_work_default_base);
ecc_25519_scalarmult_base_bits(out, n, 256);
}