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Start comparison from a random location in the uECC_vli_equal.

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This increases security and increases resistance to the side channel leakage.

Signed-off-by: Piotr Nowicki <piotr.nowicki@arm.com>
This commit is contained in:
Piotr Nowicki 2020-05-20 22:10:14 +02:00
parent 4aaa34c03f
commit 1a9d33e8c8
1 changed files with 16 additions and 6 deletions
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22
tinycrypt/ecc.c
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@ -286,20 +286,30 @@ uECC_word_t uECC_vli_equal(const uECC_word_t *left, const uECC_word_t *right)
{
uECC_word_t diff = 0;
uECC_word_t flow_monitor = 0;
uECC_word_t tmp1, tmp2;
volatile int i;
for (i = NUM_ECC_WORDS - 1; i >= 0; --i) {
int start_offset = mbedtls_platform_random_in_range(NUM_ECC_WORDS);
for (i = start_offset; i < NUM_ECC_WORDS; ++i) {
tmp1 = left[i];
tmp2 = right[i];
flow_monitor++;
diff |= (tmp1 ^ tmp2);
}
/* i should be -1 now */
mbedtls_platform_random_delay();
diff |= i ^ -1;
for (i = 0; i < start_offset; ++i) {
tmp1 = left[i];
tmp2 = right[i];
flow_monitor++;
diff |= (tmp1 ^ tmp2);
}
return diff;
mbedtls_platform_random_delay();
/* Return 0 only when diff is 0 and flow_counter is equal to NUM_ECC_WORDS */
return (diff | (flow_monitor ^ NUM_ECC_WORDS));
}
uECC_word_t cond_set(uECC_word_t p_true, uECC_word_t p_false, unsigned int cond)
@ -848,7 +858,7 @@ void vli_mmod_fast_secp256r1(unsigned int *result, unsigned int*product)
}
while (carry < 0);
} else {
while (carry ||
while (carry ||
uECC_vli_cmp_unsafe(curve_p, result) != 1) {
carry -= uECC_vli_sub(result, result, curve_p);
}