ecmult_const_impl.h File Reference

#include "scalar.h"
#include "group.h"
#include "ecmult_const.h"
#include "ecmult_impl.h"

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Macros
#define	ECMULT_CONST_TABLE_GET_GE(r, pre, n, w)

Functions
static int	secp256k1_wnaf_const (int *wnaf, const secp256k1_scalar *scalar, int w, int size)
	Convert a number to WNAF notation. More...
static void	secp256k1_ecmult_const (secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *scalar, int size)

Macro Definition Documentation

ECMULT_CONST_TABLE_GET_GE

#define ECMULT_CONST_TABLE_GET_GE	(		r,
			pre,
			n,
			w
	)

Value:

    do { \
    int m = 0; \
    /* Extract the sign-bit for a constant time absolute-value. */ \
    int mask = (n) >> (sizeof(n) * CHAR_BIT - 1); \
    int abs_n = ((n) + mask) ^ mask; \
    int idx_n = abs_n >> 1; \
    secp256k1_fe neg_y; \
    VERIFY_CHECK(((n) & 1) == 1); \
    VERIFY_CHECK((n) >= -((1 << ((w)-1)) - 1)); \
    VERIFY_CHECK((n) <=  ((1 << ((w)-1)) - 1)); \
    VERIFY_SETUP(secp256k1_fe_clear(&(r)->x)); \
    VERIFY_SETUP(secp256k1_fe_clear(&(r)->y)); \
    /* Unconditionally set r->x = (pre)[m].x. r->y = (pre)[m].y. because it's either the correct one \
     * or will get replaced in the later iterations, this is needed to make sure `r` is initialized. */ \
    (r)->x = (pre)[m].x; \
    (r)->y = (pre)[m].y; \
    for (m = 1; m < ECMULT_TABLE_SIZE(w); m++) { \
        /* This loop is used to avoid secret data in array indices. See
         * the comment in ecmult_gen_impl.h for rationale. */ \
        secp256k1_fe_cmov(&(r)->x, &(pre)[m].x, m == idx_n); \
        secp256k1_fe_cmov(&(r)->y, &(pre)[m].y, m == idx_n); \
    } \
    (r)->infinity = 0; \
    secp256k1_fe_negate(&neg_y, &(r)->y, 1); \
    secp256k1_fe_cmov(&(r)->y, &neg_y, (n) != abs_n); \
} while(0)

Definition at line 16 of file ecmult_const_impl.h.

Function Documentation

secp256k1_ecmult_const()

static void secp256k1_ecmult_const ( secp256k1_gej *  r, const secp256k1_ge *  a, const secp256k1_scalar *  scalar, int  size  )

static

Definition at line 135 of file ecmult_const_impl.h.

secp256k1_wnaf_const()

static int secp256k1_wnaf_const ( int *  wnaf, const secp256k1_scalar *  scalar, int  w, int  size  )

static

Convert a number to WNAF notation.

The number becomes represented by sum(2^{wi} * wnaf[i], i=0..WNAF_SIZE(w)+1) - return_val. It has the following guarantees:

• each wnaf[i] an odd integer between -(1 << w) and (1 << w)
• each wnaf[i] is nonzero
• the number of words set is always WNAF_SIZE(w) + 1

Adapted from The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar Multiplications Secure against Side Channel Attacks, Okeya and Tagaki. M. Joye (Ed.) CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlag Berlin Heidelberg 2003

Numbers reference steps of Algorithm SPA-resistant Width-w NAF with Odd Scalar on pp. 335

Definition at line 55 of file ecmult_const_impl.h.

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