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Add comments clarifying when subtractions without squeeze are valid

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This commit is contained in:
Matthias Schiffer 2015-10-03 13:35:59 +02:00
parent 962888f03f
commit 16636d4f90
1 changed files with 2 additions and 0 deletions
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2
src/ec25519.c
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@ -473,6 +473,7 @@ int ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in) {
if (!square_root(Y, Y2)) if (!square_root(Y, Y2))
return 0; return 0;
/* No squeeze is necessary for subtractions from zero */
sub(Yt, zero, Y); sub(Yt, zero, Y);
select(out->Y, Y, Yt, (in->p[31] >> 7) ^ (Y[0] & 1)); select(out->Y, Y, Yt, (in->p[31] >> 7) ^ (Y[0] & 1));
@ -506,6 +507,7 @@ void ecc_25519_negate(ecc_25519_work_t *out, const ecc_25519_work_t *in) {
out->Z[i] = in->Z[i]; out->Z[i] = in->Z[i];
} }
/* No squeeze is necessary for subtractions from zero */
sub(out->X, zero, in->X); sub(out->X, zero, in->X);
sub(out->T, zero, in->T); sub(out->T, zero, in->T);
} }