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Centralize everything to EccPoint_mult_safer()

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This will make easier to add future counter-measures in a single place.

In practice this change means that:

- compute_public_key() now uses projective coordinate randomisation, which it
  should as this is a protection against Template Attacks for example.
- mult_safer() now checks that the result is not the point at infinity, which
  it can as the result is indeed never expected to be that
This commit is contained in:
Manuel Pégourié-Gonnard 2019-11-14 11:59:09 +01:00
parent 72a8c9e7dc
commit 41ab8cb6cb
1 changed files with 7 additions and 19 deletions
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26
tinycrypt/ecc.c
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@ -951,6 +951,12 @@ int EccPoint_mult_safer(uECC_word_t * result, const uECC_word_t * point,
} }
EccPoint_mult(result, point, k2[!carry], initial_Z); EccPoint_mult(result, point, k2[!carry], initial_Z);
if (EccPoint_isZero(result, curve)) {
r = 0;
goto clear_and_out;
}
r = 1; r = 1;
clear_and_out: clear_and_out:
@ -966,25 +972,7 @@ uECC_word_t EccPoint_compute_public_key(uECC_word_t *result,
uECC_word_t *private_key, uECC_word_t *private_key,
uECC_Curve curve) uECC_Curve curve)
{ {
return EccPoint_mult_safer(result, curve->G, private_key, curve);
uECC_word_t tmp1[NUM_ECC_WORDS];
uECC_word_t tmp2[NUM_ECC_WORDS];
uECC_word_t *p2[2] = {tmp1, tmp2};
uECC_word_t carry;
if (curve != uECC_secp256r1())
return 0;
/* 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);
EccPoint_mult(result, curve->G, p2[!carry], 0);
if (EccPoint_isZero(result, curve)) {
return 0;
}
return 1;
} }
/* Converts an integer in uECC native format to big-endian bytes. */ /* Converts an integer in uECC native format to big-endian bytes. */