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Add ECCPoint_mult_safer() function

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This avoids the need for each calling site to manually regularize the scalar
and randomize coordinates, which makes for simpler safe use and saves 50 bytes
of code size in the library.
This commit is contained in:
Manuel Pégourié-Gonnard 2019-11-04 11:19:30 +01:00
parent c78d86b499
commit ef238283d5
4 changed files with 61 additions and 49 deletions
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14
include/tinycrypt/ecc.h
View File

@ -309,6 +309,20 @@ void EccPoint_mult(uECC_word_t * result, const uECC_word_t * point,
const uECC_word_t * scalar, const uECC_word_t * initial_Z,
bitcount_t num_bits, uECC_Curve curve);
/*
* @brief Point multiplication algorithm using Montgomery's ladder with co-Z
* coordinates. See http://eprint.iacr.org/2011/338.pdf.
* Uses scalar regularization and coordinate randomization (if a global RNG
* function is set) in order to protect against some side channel attacks.
* @note Result may overlap point.
* @param result OUT -- returns scalar*point
* @param point IN -- elliptic curve point
* @param scalar IN -- scalar
* @param curve IN -- elliptic curve
*/
int EccPoint_mult_safer(uECC_word_t * result, const uECC_word_t * point,
const uECC_word_t * scalar, uECC_Curve curve);
/*
* @brief Constant-time comparison to zero - secure way to compare long integers
* @param vli IN -- very long integer

39
tinycrypt/ecc.c
View File

@ -65,6 +65,7 @@
#if defined(MBEDTLS_USE_TINYCRYPT)
#include <tinycrypt/ecc.h>
#include "mbedtls/platform_util.h"
#include <string.h>
/* IMPORTANT: Make sure a cryptographically-secure PRNG is set and the platform
@ -936,6 +937,44 @@ uECC_word_t regularize_k(const uECC_word_t * const k, uECC_word_t *k0,
return carry;
}
int EccPoint_mult_safer(uECC_word_t * result, const uECC_word_t * point,
const uECC_word_t * scalar, uECC_Curve curve)
{
uECC_word_t tmp[NUM_ECC_WORDS];
uECC_word_t s[NUM_ECC_WORDS];
uECC_word_t *k2[2] = {tmp, s};
wordcount_t num_words = curve->num_words;
bitcount_t num_n_bits = curve->num_n_bits;
uECC_word_t carry;
uECC_word_t *initial_Z = 0;
int r;
/* 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(scalar, tmp, s, curve);
/* If an RNG function was specified, get a random initial Z value to
* protect against side-channel attacks such as Template SPA */
if (g_rng_function) {
if (!uECC_generate_random_int(k2[carry], curve->p, num_words)) {
r = 0;
goto clear_and_out;
}
initial_Z = k2[carry];
}
EccPoint_mult(result, point, k2[!carry], initial_Z, num_n_bits + 1, curve);
r = 1;
clear_and_out:
/* erasing temporary buffer used to store secret: */
mbedtls_platform_zeroize(k2, sizeof(k2));
mbedtls_platform_zeroize(tmp, sizeof(tmp));
mbedtls_platform_zeroize(s, sizeof(s));
return r;
}
uECC_word_t EccPoint_compute_public_key(uECC_word_t *result,
uECC_word_t *private_key,
uECC_Curve curve)

30
tinycrypt/ecc_dh.c
View File

@ -72,12 +72,6 @@
#include <string.h>
#include "mbedtls/platform_util.h"
#if default_RNG_defined
static uECC_RNG_Function g_rng_function = &default_CSPRNG;
#else
static uECC_RNG_Function g_rng_function = 0;
#endif
int uECC_make_key_with_d(uint8_t *public_key, uint8_t *private_key,
unsigned int *d, uECC_Curve curve)
{
@ -160,11 +154,6 @@ int uECC_shared_secret(const uint8_t *public_key, const uint8_t *private_key,
uECC_word_t _public[NUM_ECC_WORDS * 2];
uECC_word_t _private[NUM_ECC_WORDS];
uECC_word_t tmp[NUM_ECC_WORDS];
uECC_word_t *p2[2] = {_private, tmp};
uECC_word_t *initial_Z = 0;
uECC_word_t carry;
wordcount_t num_words = curve->num_words;
wordcount_t num_bytes = curve->num_bytes;
int r;
@ -186,30 +175,15 @@ int uECC_shared_secret(const uint8_t *public_key, const uint8_t *private_key,
public_key + num_bytes,
num_bytes);
/* 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, _private, tmp, 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, num_words)) {
r = 0;
r = EccPoint_mult_safer(_public, _public, _private, curve);
if (r == 0)
goto clear_and_out;
}
initial_Z = p2[carry];
}
EccPoint_mult(_public, _public, p2[!carry], initial_Z, curve->num_n_bits + 1,
curve);
uECC_vli_nativeToBytes(secret, num_bytes, _public);
r = !EccPoint_isZero(_public, curve);
clear_and_out:
/* erasing temporary buffer used to store secret: */
mbedtls_platform_zeroize(p2, sizeof(p2));
mbedtls_platform_zeroize(tmp, sizeof(tmp));
mbedtls_platform_zeroize(_private, sizeof(_private));
return r;

23
tinycrypt/ecc_dsa.c
View File

@ -114,13 +114,11 @@ int uECC_sign_with_k(const uint8_t *private_key, const uint8_t *message_hash,
uECC_word_t tmp[NUM_ECC_WORDS];
uECC_word_t s[NUM_ECC_WORDS];
uECC_word_t *k2[2] = {tmp, s};
uECC_word_t p[NUM_ECC_WORDS * 2];
uECC_word_t *initial_Z = 0;
uECC_word_t carry;
wordcount_t num_words = curve->num_words;
wordcount_t num_n_words = BITS_TO_WORDS(curve->num_n_bits);
bitcount_t num_n_bits = curve->num_n_bits;
int r;
/* Make sure 0 < k < curve_n */
if (uECC_vli_isZero(k, num_words) ||
@ -128,21 +126,8 @@ int uECC_sign_with_k(const uint8_t *private_key, const uint8_t *message_hash,
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(k, tmp, s, curve);
/* If an RNG function was specified, get a random initial Z value to
* protect against side-channel attacks such as Template SPA */
if (g_rng_function) {
if (!uECC_generate_random_int(k2[carry], curve->p, num_words)) {
return 0;
}
initial_Z = k2[carry];
}
EccPoint_mult(p, curve->G, k2[!carry], initial_Z, num_n_bits + 1, curve);
if (uECC_vli_isZero(p, num_words)) {
r = EccPoint_mult_safer(p, curve->G, k, curve);
if (r == 0 || uECC_vli_isZero(p, num_words)) {
return 0;
}