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Change constant time and cache resistant ECC mulmod

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Ensure points being operated on change to make constant time.
This commit is contained in:
Sean Parkinson
2020-04-08 23:23:46 +10:00
parent 9e08efe8e0
commit ffd06e359f
3 changed files with 93 additions and 57 deletions
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40
wolfcrypt/src/wolfmath.c
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@@ -89,6 +89,45 @@ mp_digit get_digit(mp_int* a, int n)
return (n >= a->used || n < 0) ? 0 : a->dp[n];
}
/* Conditionally copy a into b. Performed in constant time.
*
* a MP integer to copy.
* copy On 1, copy a into b. on 0 leave b unchanged.
* b MP integer to copy into.
* returns BAD_FUNC_ARG when a or b is NULL, MEMORY_E when growing b fails and
* MP_OKAY otherwise.
*/
int mp_cond_copy(mp_int* a, int copy, mp_int* b)
{
int err = MP_OKAY;
int i;
mp_digit mask = (mp_digit)0 - copy;
if (a == NULL || b == NULL)
err = BAD_FUNC_ARG;
/* Ensure b has enough space to copy a into */
if (err == MP_OKAY)
err = mp_grow(b, a->used + 1);
if (err == MP_OKAY) {
/* When mask 0, b is unchanged2
* When mask all set, b ^ b ^ a = a
*/
/* Conditionaly copy all digits and then number of used diigits.
* get_digit() returns 0 when index greater than available digit.
*/
for (i = 0; i < a->used; i++) {
b->dp[i] ^= (get_digit(a, i) ^ get_digit(b, i)) & mask;
}
for (; i < b->used; i++) {
b->dp[i] ^= (get_digit(a, i) ^ get_digit(b, i)) & mask;
}
b->used ^= (a->used ^ b->used) & (int)mask;
}
return err;
}
#ifndef WC_NO_RNG
int get_rand_digit(WC_RNG* rng, mp_digit* d)
{
@@ -337,7 +376,6 @@ int wc_bigint_to_mp(WC_BIGINT* src, mp_int* dst)
return err;
}
#endif /* HAVE_WOLF_BIGINT */
#endif /* USE_FAST_MATH || !NO_BIG_INT */