Random initial Z when calculating the public key
diff --git a/uECC.c b/uECC.c
index 3cf2b1f..b24dc59 100644
--- a/uECC.c
+++ b/uECC.c
@@ -917,19 +917,54 @@
return carry;
}
+/* Generates a random integer in the range 0 < random < top.
+ Both random and top have num_words words. */
+uECC_VLI_API int uECC_generate_random_int(uECC_word_t *random,
+ const uECC_word_t *top,
+ wordcount_t num_words) {
+ uECC_word_t mask = (uECC_word_t)-1;
+ uECC_word_t tries;
+ bitcount_t num_bits = uECC_vli_numBits(top, num_words);
+
+ if (!g_rng_function) {
+ return 0;
+ }
+
+ for (tries = 0; tries < uECC_RNG_MAX_TRIES; ++tries) {
+ if (!g_rng_function((uint8_t *)random, num_words * uECC_WORD_SIZE)) {
+ return 0;
+ }
+ random[num_words - 1] &= mask >> ((bitcount_t)(num_words * uECC_WORD_SIZE * 8 - num_bits));
+ if (!uECC_vli_isZero(random, num_words) &&
+ uECC_vli_cmp(top, random, num_words) == 1) {
+ return 1;
+ }
+ }
+ return 0;
+}
+
static uECC_word_t EccPoint_compute_public_key(uECC_word_t *result,
uECC_word_t *private_key,
uECC_Curve curve) {
uECC_word_t tmp1[uECC_MAX_WORDS];
uECC_word_t tmp2[uECC_MAX_WORDS];
uECC_word_t *p2[2] = {tmp1, tmp2};
+ uECC_word_t *initial_Z = 0;
uECC_word_t carry;
/* Regularize the bitcount for the private key so that attackers cannot use a side channel
attack to learn the number of leading zeros. */
carry = regularize_k(private_key, tmp1, tmp2, curve);
- EccPoint_mult(result, curve->G, p2[!carry], 0, curve->num_n_bits + 1, curve);
+ /* If an RNG function was specified, try to get a random initial Z value to improve
+ protection against side-channel attacks. */
+ if (g_rng_function) {
+ if (!uECC_generate_random_int(p2[carry], curve->p, curve->num_words)) {
+ return 0;
+ }
+ initial_Z = p2[carry];
+ }
+ EccPoint_mult(result, curve->G, p2[!carry], initial_Z, curve->num_n_bits + 1, curve);
if (EccPoint_isZero(result, curve)) {
return 0;
@@ -980,32 +1015,6 @@
#endif /* uECC_WORD_SIZE */
-/* Generates a random integer in the range 0 < random < top.
- Both random and top have num_words words. */
-uECC_VLI_API int uECC_generate_random_int(uECC_word_t *random,
- const uECC_word_t *top,
- wordcount_t num_words) {
- uECC_word_t mask = (uECC_word_t)-1;
- uECC_word_t tries;
- bitcount_t num_bits = uECC_vli_numBits(top, num_words);
-
- if (!g_rng_function) {
- return 0;
- }
-
- for (tries = 0; tries < uECC_RNG_MAX_TRIES; ++tries) {
- if (!g_rng_function((uint8_t *)random, num_words * uECC_WORD_SIZE)) {
- return 0;
- }
- random[num_words - 1] &= mask >> ((bitcount_t)(num_words * uECC_WORD_SIZE * 8 - num_bits));
- if (!uECC_vli_isZero(random, num_words) &&
- uECC_vli_cmp(top, random, num_words) == 1) {
- return 1;
- }
- }
- return 0;
-}
-
int uECC_make_key(uint8_t *public_key,
uint8_t *private_key,
uECC_Curve curve) {
